CVE-2025-47934: OpenPGP.js's message signature verification can be spoofed

A maliciously modified message can be passed to either openpgp.verify or openpgp.decrypt, causing these functions to return a valid signature verification result while returning data that was not actually signed.

This flaw allows signature verifications of inline (non-detached) signed messages (using openpgp.verify) and signed-and-encrypted messages (using openpgp.decrypt with verificationKeys) to be spoofed, since both functions return extracted data that may not match the data that was originally signed. Detached signature verifications are not affected, as no signed data is returned in that case.

In order to spoof a message, the attacker needs a single valid message signature (inline or detached) as well as the plaintext data that was legitimately signed, and can then construct an inline-signed message or signed-and-encrypted message with any data of the attacker’s choice, which will appear as legitimately signed by affected versions of OpenPGP.js.

In other words. any inline-signed message can be modified to return any other data (while still indicating that the signature was valid), and the same is true for signed+encrypted messages if the attacker can obtain a valid signature and encrypt a new message (of the attacker’s choice) together with that signature.

Both OpenPGP.js v6 and v5 are affected. OpenPGP.js v4 is not affected.