The Remote Access Service (RAS) provides dial-up connections between computers and networks over phone lines. RAS is delivered as a native system service in Windows NT 4.0, Windows 2000 and Windows XP, and also is included in a separately downloadable Routing and Remote Access Server (RRAS) for Windows NT 4.0. All of these implementations include a RAS phonebook, which is used to store information about telephone numbers, security, and network settings used to dial-up remote systems.
A flaw exists in the RAS phonebook implementation: a phonebook value is not properly checked, and is susceptible to a buffer overrun. The overrun could be exploited for either of two purposes: causing a system failure, or running code on the system with LocalSystem privileges. If an attacker were able to log onto an affected server and modify a phonebook entry using specially malformed data, then made a connection using the modified phonebook entry, the specially malformed data could be run as code by the system.