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Zebra's Transparent SIGHASH_SINGLE Handling Diverges from zcashd for Corresponding Outputs

Critical severity GitHub Reviewed Published May 2, 2026 in ZcashFoundation/zebra • Updated May 7, 2026

Package

cargo zebrad (Rust)

Affected versions

< 4.4.0

Patched versions

4.4.0

Description

Zebra Transparent SIGHASH_SINGLE Corresponding-Output Handling Diverges From zcashd

Summary

For V5+ transparent spends, Zebra and zcashd disagree on the same consensus rule: SIGHASH_SINGLE must fail when the input index has no corresponding output. zcashd treats this as consensus-invalid under ZIP-244, while Zebra's transparent verification path computes a digest for the missing-output case instead of failing.

The result is a direct block-validity split. A malformed V5 transparent transaction can be accepted by Zebra, retained in Zebra's mempool, selected into Zebra getblocktemplate, mined into a block, and then rejected by zcashd.

Details

Validated code revisions used during analysis:

  • zcashd: 2c63e9aa08cb170b0feb374161bea94720c3e1f5
  • Zebra: a905fa19e3a91c7b4ead331e2709e6dec5db12cb

Scope note:

  • earlier triage material grouped pre-V5 and V5 behavior together;
  • re-execution on the pinned revisions did not reproduce the claimed pre-V5 / V4 reject-side behavior;
  • this advisory therefore covers the V5+ / ZIP-244 variant only.

zcashd side:

  • Transparent scripts in blocks are checked through TransactionSignatureChecker::CheckSig() and SignatureHash(): zcash/src/script/interpreter.cpp.
  • In the ZIP-244 branch, SignatureHash() explicitly throws when SIGHASH_SINGLE or SIGHASH_SINGLE|ANYONECANPAY is used with nIn >= txTo.vout.size(): zcash/src/script/interpreter.cpp.
  • CheckSig() catches that exception and returns false, causing the transparent script to fail.

Zebra side:

Why this is exploitable:

  • the malformed transaction only needs fewer transparent outputs than inputs;
  • the attacker signs the digest that Zebra computes for the missing-output case;
  • Zebra then sees a valid transparent signature, while zcashd never reaches the same digest because it fails first.

Ordinary path viability:

PoC

Validated commits:

  • zcashd: 2c63e9aa08cb170b0feb374161bea94720c3e1f5
  • Zebra: a905fa19e3a91c7b4ead331e2709e6dec5db12cb

Manual reproduction steps:

  1. Build an otherwise-valid V5 transaction with at least two transparent inputs and only one transparent output.
  2. Sign input 0 normally.
  3. Sign input 1 with canonical SIGHASH_SINGLE or SIGHASH_SINGLE|ANYONECANPAY.
  4. Use the digest returned by Zebra's ZIP-244 path, where the missing output contributes transparent_outputs_hash([]).
  5. Submit the transaction to Zebra and to zcashd.
  6. Observe:
    • Zebra accepts it into the mempool;
    • Zebra selects it into getblocktemplate;
    • Zebra can mine and accept a block containing it;
    • zcashd rejects it in the ordinary mempool path.

Impact

This is a direct V5+ transparent consensus split.

Who can trigger it:

  • an ordinary transaction author can craft the malformed V5 transparent transaction;
  • the accept-side stock path is Zebra's mempool and block-template path;
  • an external miner still has to include the transaction in a block for the split to materialize.

Who is impacted:

  • Zebra can accept and template a transaction / block that zcashd rejects;
  • this makes the issue both a consensus-divergence problem and a practical Zebra block-template safety problem.

References

@mpguerra mpguerra published to ZcashFoundation/zebra May 2, 2026
Published to the GitHub Advisory Database May 7, 2026
Reviewed May 7, 2026
Last updated May 7, 2026

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:N

EPSS score

Weaknesses

Improper Validation of Integrity Check Value

The product does not validate or incorrectly validates the integrity check values or checksums of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission. Learn more on MITRE.

Improper Following of Specification by Caller

The product does not follow or incorrectly follows the specifications as required by the implementation language, environment, framework, protocol, or platform. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-cwfq-rfcr-8hmp

Source code

Credits

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