Vulnerability Note VU#291924

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.

Many Telnet client implementations may be vulnerable to a flaw which may allow arbitrary code to be executed on the connected client. The Telnet server may supply a specially crafted reply containing a larger number of RFC1184 LINEMODE "Set Local Character" (SLC) suboption commands, which are not checked for proper length before being stored into a fixed length buffer. Affected Telnet clients possibly include the BSD Telnet implementation and the MIT Kerberos distribution.

The Telnet LINEMODE mode is enabled by default in a majority of modern Telnet clients and servers, and is often negotiated automatically before user input is required. Therefore, an attacker may be able to launch a vulnerable client, for example, through commands embedded in web pages such as an IFRAME with a "telnet:" URL, and exploit this flaw requiring only minimal or no user interaction.

A remote server may be able to execute arbitrary code under the permissions of the user running the Telnet client on the local host.

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Patches, updates, and fixes are available from multiple vendors.

As a workaround, the client may explicitly disable the LINEMODE mode before connecting in order to prevent LINEMODE command processing. In addition, as a best practice clients should never connect to unknown servers.