Zebra before 4.4.0 Consensus Split via SIGHASH_SINGLE Missing-Output Handling

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Zebra before 4.4.0 contains a consensus divergence vulnerability in V5 transparent signature verification, computing a ZIP-244 digest for SIGHASH_SINGLE inputs lacking corresponding outputs instead of failing. Attackers can craft V5 transactions with fewer outputs than inputs that Zebra accepts and templates via getblocktemplate, producing blocks zcashd rejects.

Source published

2 Oct 2026 11:38:13 UTC

Source modified

6 Oct 2026 02:42:57 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS 3.1 · 7.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H

Zebra before 4.4.0 contains a consensus divergence vulnerability in V5 transparent signature verification, computing a ZIP-244 digest for SIGHASH_SINGLE inputs lacking corresponding outputs instead of failing. Attackers can craft V5 transactions with fewer outputs than inputs that Zebra accepts and templates via getblocktemplate, producing blocks zcashd rejects.

Source published

Oct 2, 2026, 11:38:13 AM (timezone not stated)

Source modified

Oct 6, 2026, 2:42:57 AM (timezone not stated)

Observed here

11 Nov 2026 05:30:18 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N

Zebra before 4.4.0 contains a consensus divergence vulnerability in V5 transparent signature verification, computing a ZIP-244 digest for SIGHASH_SINGLE inputs lacking corresponding outputs instead of failing. Attackers can craft V5 transactions with fewer outputs than inputs that Zebra accepts and templates via getblocktemplate, producing blocks zcashd rejects.

Source published

2 Oct 2026 12:17:14 UTC

Source modified

6 Oct 2026 03:17:00 UTC

Observed here

25 Oct 2026 04:17:29 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Secondary · 7.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H

# `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`: `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.

Source published

7 May 2026 21:02:30 UTC

Source modified

3 Oct 2026 03:56:05 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · 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

Publisher rating · CRITICAL