Vulnerability Note VU#800829

Telnet Client Information Disclosure Vulnerability

Original Release date: 14 Jun 2005 | Last revised: 28 Jul 2005


A vulnerability in the handling of the NEW-ENVIRON command allows a malicious telnet server to gain information from a client's environment variables.


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.

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.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
Microsoft CorporationAffected-14 Jun 2005
Red Hat Inc.Affected-28 Jul 2005
Sun Microsystems Inc.Affected-14 Jun 2005
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A



GaŽl Delalleau is credited with this discovery. Thank you to iDefense for coordinating the release of information about this issue.

This document was written by Robert Mead based on information in the iDEFENSE Security Advisory

Other Information

  • CVE IDs: CAN-2005-0488
  • Date Public: 14 Jun 2005
  • Date First Published: 14 Jun 2005
  • Date Last Updated: 28 Jul 2005
  • Severity Metric: 0.17
  • Document Revision: 22


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