Bitcoin Core Patch Fixes Vulnerability That Could Allow Funds to Be Redirected Without Key Theft

PSBT Integrity: Why It Matters

Bitcoin Core developers have released a new update that closes a subtle weakness in the handling of partially signed transaction (PSBT) data. The patch addresses a scenario where a missing-output SIGHASH_SINGLE request could leave a transaction’s recipient unbound, allowing an attacker to alter the destination without needing to compromise the private key.

The flaw was discovered during routine security reviews of the Bitcoin Core codebase. It involved a narrow case in which a valid signature could survive modifications to the intended payment destination. If an attacker could craft a PSBT that omitted the output being signed, the transaction could be re‑oriented to a different address while the signature remained valid. This would effectively redirect funds without the need to steal or guess the recipient’s private key.

How the Fix Protects Users

The new fix is part of a broader effort to harden PSBT processing. It adds checks that ensure every output referenced by a signature is present in the transaction. When this condition is not met, the transaction is rejected or the signature is considered invalid. The patch also improves the handling of SIGHASH_SINGLE, a flag that instructs Bitcoin Core to sign only the output with the same index as the input. By tightening validation around this flag, the update eliminates the possibility of a silent output omission.

According to the release notes, the update was merged into the main branch in early September. Developers highlighted that the change does not affect normal transaction creation or signing workflows. Users and wallet software that rely on PSBTs will continue to operate as usual, but the patch provides an additional layer of security against a class of subtle attacks.

Partially signed Bitcoin transactions are a cornerstone of multi‑party payment workflows. They allow multiple signatories to contribute to a transaction without exposing private keys. The integrity of PSBTs depends on strict validation rules that ensure every signature corresponds to the correct inputs and outputs. A lapse in these rules can lead to funds being diverted to unintended addresses, a scenario that is especially dangerous in decentralized finance contexts.

What This Means for the Bitcoin Ecosystem

The recent vulnerability underscored the importance of rigorous checksum and signature verification. By allowing a valid signature to persist after an output was removed or altered, the system could be tricked into authorizing a transaction that pays a different address. This could have serious implications for users who rely on PSBTs for escrow, joint accounts, or complex smart‑contract interactions.

The patch introduces a new validation step that cross‑checks the presence of each output referenced by a signature. If an output is missing, the transaction is flagged as invalid. This ensures that any attempt to remove or modify an output will fail, even if the signature remains unchanged. Wallets that generate or process PSBTs will automatically benefit from this improvement, as the check is embedded in the core transaction parsing logic.

The update also clarifies the semantics of SIGHASH_SINGLE. Previously, the flag could be misinterpreted when the transaction had fewer outputs than inputs. The new logic treats such cases as errors, preventing accidental acceptance of malformed transactions.

Frequently Asked Questions

While the vulnerability was theoretical and required a sophisticated attack vector, its presence highlighted a gap in the Bitcoin Core code that could have been exploited by a determined adversary. The patch restores confidence in PSBT handling and reinforces the robustness of Bitcoin’s transaction validation. Users who rely on multi‑party signing, such as custodial services or decentralized exchanges, can now be assured that their transactions are protected against this specific class of manipulation.

Looking ahead, the Bitcoin Core team plans to conduct further audits of PSBT-related code and to collaborate with wallet developers to ensure that the new checks are properly integrated. The update also serves as a reminder of the ongoing need for vigilance in maintaining the security of the Bitcoin protocol.