The OpenPGP PGP standard allows an attacker to determine the private signature key via a cryptanalytic attack in which the attacker alters the encrypted private key file and captures a single message signed with the signature key.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6558
http://www.securityfocus.com/bid/2673
http://www.redhat.com/support/errata/RHSA-2001-063.html
http://archives.neohapsis.com/archives/bugtraq/2001-03/0311.html
http://archives.neohapsis.com/archives/bugtraq/2001-03/0274.html
http://archives.neohapsis.com/archives/bugtraq/2001-03/0252.html