source: https://www.securityfocus.com/bid/16663/info

GnuPG is affected by a detached signature verification-bypass vulnerability because it fails to properly notify scripts that an invalid detached signature was presented and that the verification process has failed.

Exploiting this issue allows attackers to bypass the signature-verification process used in some automated scripts. Depending on the use of GnuPG, this may result in a false sense of security, the installation of malicious packages, the execution of attacker-supplied code, or other attacks.

An example demonstrating this issue was provided:

fortune >x.txt
perl -e 'print "\xca"x"64"' >x.txt.sig
gpgv x.txt.sig x.txt
echo $?

This creates a file as well as an obviously invalid detached signature file. The file is then successfully validated by 'gpgv', since the exit status is '0'.