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Bitcoin Core merges PSBT signing safeguard to prevent unbound outputs

The update, merged into the master development branch on Sept. 25, addresses a PSBT signing mode that could leave a signature valid even if the intended recipient is changed under specific conditions.

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Bitcoin Core has added a safeguard meant to stop signing transactions where the signature does not properly bind the funds to the payment destination a user approved, a change merged into its master development branch on Sept. 25.

The fix targets a narrow PSBT issue where a valid signature could be produced without protecting the intended output. CryptoSlate notes that while the flaw does not expose private keys, it introduces a different risk, under specific conditions a signature may remain valid even if the transaction’s recipient is changed.

The weakness involves SIGHASH_SINGLE, a signing mode intended to commit an input to the output in the corresponding position. The protection can break down when the transaction has no output at that position, with Bitcoin Core developers saying legacy cases can result in a signature over a fixed hash value.

Bitcoin Optech highlighted the update on Oct. 2. The developers said this can make the signature reusable against other unspent outputs controlled by the same key when the same structural conditions occur, while SegWit v0 transactions retain stronger protections, according to the partial coverage provided by CryptoSlate.

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