Quake II server before R1Q2, as used in multiple products, allows remote attackers to bypass IP-based access control rules via a userinfo string that already contains an "ip" key/value pair but is also long enough to cause a new key/value pair to be truncated, which interferes with the server's ability to find the client's IP address.
https://exchange.xforce.ibmcloud.com/vulnerabilities/17895
http://www.securityfocus.com/bid/11551
http://web.archive.org/web/20041130092749/www.r1ch.net/stuff/r1q2/
http://securitytracker.com/id?1011979
http://secur1ty.net/advisories/001
http://secunia.com/advisories/13013
http://archives.neohapsis.com/archives/bugtraq/2004-10/0299.html