GHSA-pvmv-cwg8-v6c8

# Consensus Divergence in V5 Transparent SIGHASH_SINGLE With No Corresponding Output ## Summary Zebra failed to enforce a ZIP-244 consensus rule for V5 transparent transactions: when an input is signed with `SIGHASH_SINGLE` and there is no transparent output at the same index as that input, validation must fail. Zebra instead asked the underlying sighash library to compute a digest, and that library produced a digest over an empty output set rather than failing. An attacker could craft a V5 transaction with more transparent inputs than outputs that Zebra accepts but `zcashd` rejects, creating a consensus split between Zebra and `zcashd` nodes. A previous fix ([`GHSA-cwfq-rfcr-8hmp`](https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-cwfq-rfcr-8hmp)) addressed a closely related case in the same area of the code, but did not cover this specific one. ## Severity **Critical** - This is a Consensus Vulnerability that could allow a malicious party to induce network partitioning, service disruption, and potential double-spend attacks against affected nodes. Note that the impact is currently alleviated by the fact that currently most miners run `zcashd`. ## Affected Versions Zebra 4.4.0. ## Description Verification of transparent transactions inherits the Bitcoin Script verification code in C++. Since it is consensus-critical, this code is called from Zebra through a foreign function interface (FFI), with a Rust callback that computes the sighash for each input being verified. ZIP-244 §S.2a marks two situations as consensus failure for V5 transparent signatures: 1. The signed hash type is not one of the six canonical values; and 2. The hash type is `SIGHASH_SINGLE` (alone or combined with `ANYONECANPAY`) and the input has no transparent output at the same index. `zcashd` enforces both rules: its `SignatureHash` raises an exception, and `CheckSig` catches it and fails the script. A previous fix (`GHSA-cwfq-rfcr-8hmp`) added the first rule to Zebr