Software Engineering Institute

The Telnet network protocol is described in RFC854 and RFC855 as a general, bi-directional communications facility. The Telnet protocol is commonly used for command-line login sessions between Internet hosts.

The vulnerability is in the NEW-ENVIRON sub-command that is the mechanism to used for passing environment information between a telnet client and server. Use of this mechanism enables a telnet user to propagate configuration information to a remote host when connecting. Please see RFC1572 for more information. As specified in section 3 of RFC1572 the expected default behavior should be "that there will not be any exchange of environment information".

In order to exploit this vulnerability, a malicious server can send a connected client the following telnet command:

SB NEW-ENVIRON SEND ENV_USERVAR <name of environment variable> SE

Vulnerable telnet clients will send the value of the referenced environment variable. Environment variables may contain a variety of the information such as local username, executable file search paths, locations of sensitive data, and other potentially sensitive information about the client computer.

Please note telnet functionality has been embedded in many applications and not just underlying operating systems distributions.

The iDefense Security Advisory contains additional information about affected and unaffected vendors.

An attacker may be able to gather information about remote systems and users who connect to attackers malicious telnet server. An attacker would have to trick a victim into initiating a telnet connection using a vulnerable client. This may be accomplished with an HTML rendered email or web page, using the TELNET:// URI handler, however further user interaction may be required.

Apply an update from your vendor

Patches, updates, and fixes should be available from multiple vendors.

Workarounds
Disable access to telnet, limit the use of telnet to trusted sites and/or encourage the use more secure remote connection clients.

On Unix systems it might be viable to remove execute permission from telnet and other binaries that perform telnet.

On Windows systems changing or removing the registry key entry: HKEY_CLASSES_ROOT\telnet\shell\open\command
should reduce the likelihood of successful automatic exploitation attempts such as those using telnet URLs.

Note these workarounds do not address the underlying vulnerability.