The safeguard blocks a narrow PSBT case where a valid signature could survive changes to the intended payment destination
Bitcoin Core has added a safeguard against signing transactions that may not bind funds to the payment destination a user approved
The change, merged into Bitcoin Core’s master development branch on Sept. 25, targets a narrow flaw in partially signed Bitcoin transactions, or PSBTs, that could produce a valid signature without protecting the intended output
Bitcoin Optech highlighted the update on Oct. 2. The issue does not expose a user’s private key, but creates a different risk: a signature can remain valid even when the transaction’s recipient is changed under specific conditions
The weakness involves SIGHASH_SINGLEwhich is a signing mode designed to commit an input to the output in the corresponding position. If the transaction contains no output at that position, the protection breaks down differently depending on the type of Bitcoin being spent
For legacy inputs, the missing-output case can produce a signature over a fixed hash value. Bitcoin Core developers said that signature may then be reusable against other unspent outputs controlled by the same key when the same structural conditions are present
SegWit v0 transactions retain stronger protections because the signature still commits to the specific coin being spent and its amount. The destination output, however, can remain unbound
Start with the stories that matter. Get the context behind the headlines and a clear view of what to watch next
Thanks for signing up to The Catalyst. Look out for your first daily email. If you already receive it, you’re all set
That creates an authorization problem for wallets and signing devices: software could present one payment to the user while producing a signature that does not cryptographically guarantee that the approved recipient remains unchanged
Bitcoin Core already rejected the edge case through its raw-transaction signing interface. Its PSBT path, including walletprocesspsbtcould still sign it
The new code moves the check into Bitcoin Core’s shared signature-creation logic, preventing affected legacy and SegWit v0 inputs from being signed while allowing other valid inputs in the same PSBT to proceed
PSBTs are commonly used to coordinate transactions between software wallets, hardware devices and offline signers. They allow transaction builders to pass information to a separate signer without giving that system control of the private keys
The fix therefore reinforces a boundary that wallet developers must enforce independently of key security: a valid cryptographic signature must commit to the transaction details the user actually authorized
Bitcoin Improvement Proposal 174, which defines PSBTs, already tells signers to reject unacceptable signing modes and recommends SIGHASH_ALL when no alternative is specified. The Bitcoin Core change explicitly prevents this missing-output configuration from reaching the signing stage
Users do not yet have a confirmed production release containing the safeguard. The Sept. 25 change was merged into Bitcoin Core’s development branch, while the project’s published release listings had not identified a fixed version or confirmed backport as of Oct. 4
That leaves wallet providers and hardware-signing integrations with the more immediate decision: review their own handling of SIGHASH_SINGLE requests rather than waiting for a Bitcoin Core release to enforce the same protection downstream
Oluwapelumi values Bitcoin's potential. He imparts insights on a range of topics like DeFi, hacks, mining and culture, underlining transformative power
