Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node

Fixed Versions

This issue is fixed in Zebra 4.3.1.

The fix adds the consensus check in the caller of the C++ verification code. It also uses the raw hash type for V4 sighash computations.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Thanks Alex “Scalar” Sol for finding and reporting the issue, and to @sangsoo-osec who independently found the same issue and demonstrated that the V4 variant was also exploitable.

Am I affected

You are affected if:

1. You run any version of zebrad up to and including v4.4.1.
2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.

All default configurations are affected. No feature flags, non-default settings, or special build options are required.

Summary

Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as OP_CODESEPARATOR), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide MAX_BLOCK_SIGOPS = 20,000 threshold is crossed on one side but not the other.

An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators.

Details

The P2SH sigop counter at zebra-script/src/lib.rs:399 calls script::Code(redeemed_bytes).sig_op_count(true), which is a pure-Rust path through zcash_script-0.4.4. The legacy (non-P2SH) sigop counter at lib.rs:282-289 correctly uses the C++ FFI via interpreter.legacy_sigop_count_script(). Only the P2SH path bypasses the FFI.

The Rust parser in zcash_script-0.4.4/src/opcode/mod.rs:1247-1260 treats 16 disabled opcodes (0x7e through 0xab, including OP_CAT, OP_SUBSTR, OP_AND, OP_OR, OP_XOR, OP_2MUL, OP_2DIV, OP_MUL, OP_DIV, OP_MOD, OP_LSHIFT, OP_RSHIFT, and OP_CODESEPARATOR) as Err(Error::Disabled(...)). The sig_op_count function at iter.rs:104-115 uses try_fold, which terminates on the first Err and returns the partial sum accumulated so far.

zcashd's GetOp2 (script.h:514-562) returns true for all non-push opcodes including the disabled range. Its GetSigOpCount(true) (script.cpp:152-174) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis.

A redeem script of [0xab, OP_CHECKMULTISIG x 50] produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the MAX_BLOCK_SIGOPS = 20,000 threshold on one side only.

Patches

Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.

Workarounds

There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available.

Impact

A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by OP_CHECKSIG or OP_CHECKMULTISIG opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with bad-blk-sigops. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence.

The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.

Credit

Reported by @samsulselfut via a private GitHub Security Advisory submission.

Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output

Summary

Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with SIGHASH_SINGLE and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but zcashd rejects, creating a consensus split between Zebra and zcashd nodes.

A previous fix (GHSA-cwfq-rfcr-8hmp) addressed a closely related case in the same area of the code, but did not cover this specific one.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.4.0.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified.

ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures:

1. The signed hash type is not one of the six canonical values; and
2. The hash type is SIGHASH_SINGLE (alone or combined with ANYONECANPAY) and the input has no transparent output at the same index.

zcashd enforces both rules: its SignatureHash raises an exception, and CheckSig catches it and fails the script. A previous fix (GHSA-cwfq-rfcr-8hmp) added the first rule to Zebra's V5 sighash callback. The second rule, however, was not added — Zebra's callback forwarded the request to librustzcash's ZIP-244 implementation, which handles an out-of-range SIGHASH_SINGLE output index by hashing an empty output set rather than refusing to produce a digest. As a result, Zebra would compute a well-defined sighash for the missing-output case and accept any signature that verified against it.

An attacker could exploit this by:

• Constructing a V5 transaction with two or more transparent inputs and fewer transparent outputs;
• Signing an input whose index has no matching vout entry with SIGHASH_SINGLE (0x03) or SIGHASH_SINGLE|ANYONECANPAY (0x83), using the digest Zebra computes;
• Broadcasting the transaction, or a block containing it, to the network.

Zebra would verify the transaction's transparent script and accept the transaction (and any block containing it), while zcashd would reject both, splitting Zebra nodes from the rest of the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.1.

Mitigation

Users should upgrade to Zebra 4.4.1 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec, @zmanian, and @fivelittleducks for finding and reporting the issue.

CVE-2026-44497: Consensus Divergence in Transparent Sighash Hash-Type Handling due to Stale Buffer

Summary

The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj introduced a separate issue due to insuficient error handling of the case where the sighash type is invalid, during sighash computation. Instead of returning an error, the normal flow would resume, and the input sighash buffer would be left untouched. In scenarios where a previous signature validation could leave a valid sighash in the buffer, an invalid hash-type could be incorrectly accepted, which would create a consensus split between Zebra and zcashd nodes.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

Zebra 4.3.1.

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++, called from Zebra through a foreign function interface (FFI) with a Rust callback that computes the sighash. The fix for https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-8m29-fpq5-89jj added the missing V5 hash-type consensus check on the Rust side, returning None for undefined hash types. However, the FFI bridge only writes to the C++ sighash buffer when the callback returns Some, and the C++ checker reads that buffer unconditionally, so the failure signal is lost.

An attacker could exploit this by:

• Constructing a transparent output spent by a script that runs a valid OP_CHECKSIGVERIFY immediately before an OP_CHECKSIG with an undefined hash type.
• The first opcode primes the C++ sighash buffer with a valid digest; the second causes Zebra's callback to return None while the C++ checker verifies the invalid signature against the stale digest.
• Zebra accepts the spend, zcashd rejects it, creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any affected Zebra node, and any miner or template pipeline that relies on Zebra's validation result.

Fixed Versions

This issue is fixed in 4.4.0.

The fixes uses a workaround where the input buffer is filled with random bytes on validation failure, which makes signature validation fail (as expected) with overwhelming probability. This avoids a breaking release of the zcash_script crate. A future release will propagate the error correctly for a direct fix.

Mitigation

Users should upgrade to 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains on the correct consensus path and is protected against malicious chain forks.

Credits

Zebra thanks @sangsoo-osec for finding and reporting the issue.

CVE-2026-41583: Consensus Divergence in Transparent Sighash Hash-Type Handling

Summary

After a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes.

In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split.

Severity

Critical - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes.

Note that the impact is currently alleviated by the fact that currently most miners run zcashd.

Affected Versions

All Zebra versions prior to version 4.3.1. (Some older versions are not impacted but are no longer supported by the network.)

Description

Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code was called from Zebra through foreign function interface (FFI). That interface was clunky because it required parsing the whole transaction in C++ code, which would then pull Rust libraries which could get in conflict with Zebra code. A refactoring was done so that only the verification itself was done in C++, and the rest done by Rust code, using a callback. However, in this refactoring, it was not noticed that a particular consensus rule - only accepting known hash types in transparent transaction signatures - was being enforced in C++ code and thus had to be enforced by the Rust caller.

An attacker could exploit this by:

• Submitting a V4 or V5 transaction with an invalid hash type
• The V5 transaction would be accepted by Zebra nodes but not by zcashd nodes (and vice-versa for V4), creating a consensus split in the network.

Impact

Consensus Failure

• Attack Vector: Network.
• Effect: Network partition/consensus split.
• Scope: Any Zebra affected Zebra node