FSC Internet / SecureXpert Labs SecureXpert Labs Advisory [SX-20000620-1] - Denial of Service vulnerability in Microsoft Windows 2000 Telnet Server Summary Microsoft Windows 2000 Server is supplied with a Telnet server for remote console access. A Denial of Service vulnerability exists in this server which may be exploited by a local or remote attacker. Details A remote user can cause the telnet server to stop responding to requests by sending a stream of binary zeros to the telnet server. This can easily be reproduced from a Linux system using netcat with an input of /dev/zero, with a command such as "nc target.host 23 < /dev/zero". The Windows 2000 Telnet Server stops responding to requests after a few seconds. If the Telnet Server is set to restart upon failure, it will restart and immediately fail. This will occur repeatedly until the Telnet Server exceeds its restart count, at which point the service remains down. Status Microsoft Corp. has been informed of this vulnerability, and has assigned it incident ID# [MSRC 290]. As of Tuesday June 2000, Microsoft has successfully reproduced the vulnerability and SecureXpert Labs staff are working with Microsoft to prepare a fix. Credits Mike Murray, SecureXpert Labs Max Degtyar, SecureXpert Labs Richard Reiner, SecureXpert Labs About SecureXpert DIRECT SecureXpert DIRECT is an advance security advisory service provided by SecureXpert Labs. Subscriptions are free of charge and may be obtained online at http://www.securexpert.com/services.html.