The ATOMIC.TCP signature engine in the Intrusion Prevention System (IPS) feature for Cisco IOS 12.4XA, 12.3YA, 12.3T, and other trains allows remote attackers to cause a denial of service (IPS crash and traffic loss) via unspecified manipulations that are not properly handled by the regular expression feature, as demonstrated using the 3123.0 (Netbus Pro Traffic) signature.