Hilgraeve HyperTerminal is a communications/telnet application shipped and installed with every copy of Microsoft Windows 98, ME, NT 4.0, and 2000. It is the default telnet client in Microsoft 98 and ME but not in Windows NT 4.0 / 2000.
A buffer overflow condition exists when a user attempts to access a telnet address over 153 characters long. Depending on the data entered, a denial of service attack or arbitrary code could be launched by a malicious third party. A specially malformed telnet address could be launched on a remote system if it were embedded in a HTML page or email message.
Although HyperTerminal is also shipped with NT 4.0, it is not susceptible to this vulnerability because it cannot be configured to be the default telnet client.
telnet://<string consisting of over 153 characters>