Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.

CVE-2026-34377: Consensus Failure via Crafted V5 Authorization Data

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By matching a valid transaction's txid while providing invalid authorization data, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network. To be clear, this would not allow invalid transactions to be accepted but could result in a consensus split between vulnerable Zebra nodes and invulnerable Zebra and Zcashd nodes.

Severity

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

Affected Versions

All Zebra versions supporting V5 transactions (Network Upgrade 5 and later) prior to version 4.3.0.

Description

The vulnerability exists in the find_verified_unmined_tx function within transaction.rs. This function was designed to optimize block verification by checking if a transaction was already verified in the mempool.

The lookup mechanism used the ZIP-244 txid as the unique key. However, for V5 transactions, the txid specifically excludes the Authorization Data Root (signatures and proofs). Because Zebra returned a "verified" status based solely on the txid, it skipped the essential check_v5_auth() call for the transaction version provided in the block.

An attacker (specifically a malicious miner) could exploit this by:

1. Observing a valid V5 transaction broadcast to the network and entering a Zebra node's mempool.
2. Creating a block containing a modified version of that transaction. The modified version has the same txid but contains invalid signatures or proofs.
3. The affected Zebra node identifies the txid in its mempool and incorrectly assumes the block's version of the transaction is already verified.
4. The node commits the block with the invalid transaction data.
5. Other nodes (like zcashd or zebra nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

• Attack Vector: Network (specifically via a malicious miner).
• Effect: Network partition/consensus split.
• Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.0.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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.

Resources

• Zebra 4.3.0 Release Announcement
• ZIP-244: Transaction Identifier

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVE-2026-40880: Cached Mempool Verification Bypasses Consensus Rules for Ahead-of-Tip Blocks

Summary

A logic error in Zebra's transaction verification cache could allow a malicious miner to induce a consensus split. By carefully submitting a transaction that is valid for height H+1 but invalid for H+2 and then mining that transaction in a block at height H+2, a miner could cause vulnerable Zebra nodes to accept an invalid block, leading to a consensus split from the rest of the Zcash network.

Severity

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

Affected Versions

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

Description

The vulnerability exists due to a performance optimization whose goal is to improve performance of transaction validation if the transaction was previously accepted into the mempool (and thus was verified as valid). That however did not take into account that a transaction can be valid for a specific height, but invalid at higher heights; for example, it can contain an expiry height, a lock time, and it is always bound to a network upgrade, all of which are height-dependant.

An attacker (specifically a malicious miner) could exploit this by (e.g. in the expiry height case):

• Submitting a transaction with expiry height H+1 (where H is the height of the current tip)
• Mining a block H+1, and a block H+2 that contains that same transaction, and submitting block H+2 before H+1
• Zebra nodes would accept H+2 as valid (pending contextual verification) and wait for block H+1; when it arrives, Zebra would commit both blocks even if H+2 contains an expired transaction
• Other nodes (like zcashd or zebrad nodes without that transaction in their mempool) reject the block, resulting in a chain fork where the poisoned Zebra node is isolated.

Impact

Consensus Failure

Attack Vector: Network (specifically via a malicious miner). Effect: Network partition/consensus split. Scope: Any Zebra node utilizing the transaction verification cache optimization for V5 transactions.

Fixed Versions

This issue is fixed in Zebra 4.3.1.

We removed the performance optimization altogether, since we deemed it too risky.

The fix ensures that verification is only skipped if the transaction's full integrity—including authorization data—is validated against the mempool entry.

Mitigation

Users should upgrade to Zebra 4.3.0 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

Thanks to @sangsoo-osec for a thorough advisory submission that noticed the lock time issue, and to @shieldedonly with an also thorough advisory (that was submitted while we were working on the first one) who noticed that the issue applied to other aspects of the transaction validation.