The ios-authproxy implementation in Cisco IOS before 15.1(1)SY3 allows remote attackers to cause a denial of service (webauth and HTTP service outage) via vectors that trigger incorrectly terminated HTTP sessions, aka Bug ID CSCtz99447.
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-1SY/release_notes.pdf