Bitcoin Core developers have merged a significant security fix addressing a subtle but potentially dangerous gap in how partially signed Bitcoin transactions (PSBTs) handle incomplete output specifications. The vulnerability centered on a specific type of signing request—SIGHASH_SINGLE without corresponding outputs—that could theoretically allow an attacker to redirect funds to unintended recipients, even without compromising the private keys of the transaction signer. This distinction is crucial: the attack vector didn't require stealing cryptographic material, but rather exploiting ambiguity in transaction construction before final signing.
PSBT is a standardized format introduced in BIP 174 that allows multiple parties to collaborate on creating a single transaction without requiring any one party to hold all private keys at once. This workflow is essential for institutional custody solutions, hardware wallet ecosystems, and any multi-signature arrangement. However, the SIGHASH_SINGLE signature type—which binds a signature to a specific input-output pair—becomes problematic when the corresponding output is missing or undefined during intermediate signing stages. An attacker positioned between signing stages could theoretically modify the unbound outputs, potentially rerouting value to an address under their control while the victim believes they've signed a legitimate transaction.
The merged patch reinforces validation rules within Bitcoin Core's PSBT handling to ensure that SIGHASH_SINGLE requests always reference valid, fully-specified outputs before the transaction advances through the signing workflow. By enforcing stricter requirements at the PSBT construction stage rather than relying on downstream validation, the fix eliminates the window where ambiguity could be exploited. Notably, the patch does not yet have a confirmed release target, meaning Bitcoin Core users should expect this protection in an upcoming version rather than current stable releases. This measured approach reflects the security practice of allowing time for review and testing before pushing fixes to production systems.
The discovery underscores an ongoing tension in Bitcoin's development: as transaction construction becomes more sophisticated and multi-party workflows grow more common, the attack surface expands in subtle ways. Fixes like this one operate at the margins of wallet and protocol interaction, where most users never directly engage but where institutional and advanced operators depend on absolute correctness. The lack of a public exploit or confirmed real-world impact suggests the Bitcoin Core team identified and addressed this through rigorous code review rather than incident response—the ideal scenario for cryptocurrency security. This fix will likely become a template for how other wallet implementations handle PSBT edge cases going forward.