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

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