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:

• V5 transparent inputs route into the same FFI-based transparent script verifier used for block validation: zebra/zebra-consensus/src/transaction.rs.
• 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, zebra/zebra-chain/src/primitives/zcash_primitives.rs.
• 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, zcash_primitives/src/transaction/sighash_v5.rs.

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, zcash/src/script/interpreter.cpp.
• 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, zebra/zebrad/src/components/mempool/storage.rs.
• 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.

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.