Some telnet clients may disclose sensitive information in environment variables
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.
A client machine can be "tricked" into revealing sensitive information stored in environment variables.
Thanks to Steve Bellovin of AT&T, who reported this problem to us.
This document was written by Shawn V Hernan.
|Date First Published:||2000-09-26|
|Date Last Updated:||2001-10-25 23:25 UTC|