Software Engineering Institute

Web browsers can be configured to respond to certian protocol types through the use of a helper application. In this case, web browsers can respond to telnet: URLs with the use of a helper application. RFC 1572 (http://andrew2.andrew.cmu.edu/rfc/rfc1572.html) permits telnet servers to request environment variables of telnet clients prior to authentication. Quoting from that document:

Once the two hosts have exchanged a WILL and a DO, the sender of the DO NEW-ENVIRON is free to request that environment variables be sent. Only the sender of the DO may send requests (IAC SB NEW-ENVIRON SEND IAC SE) and only the sender of the WILL may transmit actual environment information (via the IAC SB NEW-ENVIRON IS ... IAC SE command). Though this option may be used at any time throughout the life of the telnet connection, the exchange of environment information will usually happen at the startup of the connection. This is because many operating systems only have mechanisms for propagating environment information at process creation, so the information is needed before the user logs in.

Thus, if a telnet server requests environment variables from a telnet client that implements RFC 1572, that client will send the environment variables prior to authentication. The CERT/CC is aware of a proof-of-concept implementation of such a server.

Further, web browsers will often use help applications to deal with non-web protocols such as telnet. If a web browser is configured to use a RFC-1572-compliant telnet client as a telnet helper, then a telnet: URL can be used to obtain sensitive information from that web browser, including such environment variables as USER, HOME, PGPPASS, SSH_CLIENT and other information commonly stored in the environment. At least one Windows telnet client, UWIN, implements RFC 1572, as well as some unix telnet clients. At the present time, we have not developed an exhaustive list of clients that implement RFC 1572.

A client machine can be "tricked" into revealing sensitive information stored in environment variables.