Bitcoin Core Hardens Transaction Security with New PSBT Safeguard
Bitcoin Core developers have introduced a critical security update to prevent a specific vulnerability that could allow funds to be redirected during the signing process. By addressing a flaw within Partially Signed Bitcoin Transactions (PSBTs), the new safeguard ensures that cryptographic signatures strictly bind funds to the user's approved destination. This update reinforces the integrity of the communication between software wallets and hardware signing devices.
Addressing the SIGHASH_SINGLE Vulnerability
The update specifically targets a technical weakness associated with the SIGHASH_SINGLE signing mode, which is designed to link a specific input to a corresponding output. In certain edge cases—specifically when a transaction lacks an output at the designated position—the cryptographic protection can fail. For legacy inputs, this flaw could result in a signature that is reusable across other outputs controlled by the same private key. While SegWit v0 transactions offer better protection by committing to the coin's value, the recipient address could still be altered without invalidating the signature. This creates a significant authorization risk where a user might unknowingly sign a transaction that sends funds to an unintended recipient.
Hardening the Signing Logic for PSBTs
To mitigate this risk, the Bitcoin Core team has integrated a mandatory check into the software’s shared signature-creation logic. While previous versions rejected these risky transactions through some interfaces, the PSBT path—widely used for coordinating with hardware wallets—remained partially exposed. The new code explicitly blocks legacy and SegWit v0 inputs from being signed under these specific, "missing-output" conditions. This change aligns with Bitcoin Improvement Proposal 174 (BIP 174), which advises signing devices to reject insecure or unacceptable signing modes. By preventing these configurations from reaching the signing stage, the software ensures that every valid signature provides a cryptographic guarantee of the transaction's destination.
Implications for Wallet Developers and Users
As of early October, this fix has been merged into Bitcoin Core’s master development branch but has not yet been included in a formal production release. This creates a transitional period where wallet providers and hardware-signing manufacturers must decide whether to wait for the next official Bitcoin Core update or implement their own safeguards. Developers are encouraged to review how their systems handle SIGHASH_SINGLE requests to ensure they are not vulnerable to this specific edge case. Ultimately, the update upholds a fundamental principle of self-custody: a cryptographic signature must always commit to the exact transaction details that the user has seen and authorized.
