VulnerabilityPublished 7 May 2026

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

Affects zebradSource GCVE Database

CVSS 9.2 Critical : upgrade to 4.4.0 as a priority.

Data as of 27 Oct 2026, 00:07 UTC

CVSS 4.0 base score

9.2Critical

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 4.4.0

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

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: High
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the Subsequent System. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the Subsequent System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

What to do

Upgrade zebrad to 4.4.0.

Affected software

Product	Affected	Fixed in
crates.iozebrad	Affected <4.4.0	Fixed in 4.4.0

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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`](https://github.com/zcash/zcash/blob/2c63e9aa08cb170b0feb374161bea94720c3e1f5/src/script/interpreter.cpp#L1386-L1407). - 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`](https://github.com/zcash/zcash/blob/2c63e9aa08cb170b0feb374161bea94720c3e1f5/src/script/interpreter.cpp#L1221-L1259). - `CheckSig()` catches that exception and returns `false`, causing the transparent script to fail. `Zebra` side: - V5 transparent inputs route into the same FFI-based transparent script verifier used for block validation: [`zebra/zebra-consensus/src/transaction.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebra-consensus/src/transaction.rs#L989-L1098). - `Zebra` converts the decoded hash type and asks its Rust sighash engine for a digest without adding the corresponding-output pre-check that `zcashd` enforces first: [`zebra/zebra-script/src/lib.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebra-script/src/lib.rs#L160-L175), [`zebra/zebra-chain/src/primitives/zcash_primitives.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebra-chain/src/primitives/zcash_primitives.rs#L307-L343). - `Zebra` forwards canonical `SIGHASH_SINGLE` into the Rust ZIP-244 implementation. - In that implementation, when `input.index() >= bundle.vout.len()`, the code uses `transparent_outputs_hash::<TxOut>(&[])` instead of erroring: [`zcash_primitives/src/transaction/sighash_v5.rs`](https://github.com/zcash/librustzcash/blob/c3425f9c3c7f6deb20720bb78b18f35fbbed8edd/zcash_primitives/src/transaction/sighash_v5.rs#L101-L107), [`zcash_primitives/src/transaction/sighash_v5.rs`](https://github.com/zcash/librustzcash/blob/c3425f9c3c7f6deb20720bb78b18f35fbbed8edd/zcash_primitives/src/transaction/sighash_v5.rs#L131-L139). 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: - `zcashd` ordinary mempool admission is not the practical trigger path, because the same ZIP-244 `SignatureHash()` checks fail there first: [`zcash/src/main.cpp`](https://github.com/zcash/zcash/blob/2c63e9aa08cb170b0feb374161bea94720c3e1f5/src/main.cpp#L1981-L1995), [`zcash/src/script/interpreter.cpp`](https://github.com/zcash/zcash/blob/2c63e9aa08cb170b0feb374161bea94720c3e1f5/src/script/interpreter.cpp#L1221-L1259). - `Zebra` ordinary mempool admission is viable because `Zebra` uses the same transparent verifier for mempool and block validation and does not have a separate "one output per input" standardness rule here: [`zebra/zebra-consensus/src/transaction.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebra-consensus/src/transaction.rs#L414-L519), [`zebra/zebrad/src/components/mempool/storage.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebrad/src/components/mempool/storage.rs#L255-L376). - `Zebra` is a block-template producer, so the realistic stock path is `Zebra` mempool -> `Zebra` `getblocktemplate` -> external miner: [`zebra/zebra-rpc/src/methods/types/get_block_template/zip317.rs`](https://github.com/ZcashFoundation/zebra/blob/a905fa19e3a91c7b4ead331e2709e6dec5db12cb/zebra-rpc/src/methods/types/get_block_template/zip317.rs#L72-L105). ### PoC Validated commits: - `zcashd`: `2c63e9aa08cb170b0feb3741

CWE-354Improper Validation of Integrity Check Value (opens in a new tab)
Base weaknessLikelihood of exploit: Medium
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.
Implementation
Ensure that the checksums present in messages are properly checked in accordance with the protocol specification before they are parsed and used.
OWASP Cheat Sheets
XML Security (opens in a new tab)
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
OWASP ASVS 4.0.3
V11.1.7 (opens in a new tab)
V13.3.2 (opens in a new tab)
V3.5.3 (opens in a new tab)
Attack patterns
Checksum Spoofing (opens in a new tab)
Padding Oracle Crypto Attack (opens in a new tab)
Manipulating Writeable Configuration Files (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-573Improper Following of Specification by Caller (opens in a new tab)
Class weakness
The product does not follow or incorrectly follows the specifications as required by the implementation language, environment, framework, protocol, or platform.

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What to do

Affected software

Description

WeaknessesCWE-354, CWE-573

Technical details

References

Community reviews

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