GnuPG Detached Signature Verification False-Positive Vulnerability

All versions of Gnu Privacy Guard (GnuPG) have a security flaw relating to the proper checking of detached signatures.

In certain situations, changes made to signed text detached from its signature file, could be modified by an attacker. This is due to a bug in GnuPG's command-line semantics. When verifying the integrity of a signed document which has its signature in a separate file, Ggnupg can be executed from the command line in the following manner:

gpg --verify signature.sig <signed-file.txt

The problem with this format, however, is that Gnupg's command-line options used to verify "normal" signed documents is:

gpg --verify signed-file.txt

If the specified signature file is itself a valid signed document when attempting to verify a document with a detached signature, GnuPG can verify the "signature file" and will not report any errors.

Consequently, any modifications to the signed document (with the detached signature) will not be reported because it is not checked as such. For an attacker to exploit this bug, write access to the document's signature file (and signed document to be modified) is required.


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