qpopper timing analysis on to determine if a username exists on a system

From: Dennis Lubert (plasmahhat_private-bremen.de)
Date: Sat Mar 15 2003 - 11:13:43 PST

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    Hello,

during development of a pop3 tool I found an issue that makes it possible 
for any user to check the validity of a user on a target system. If a user 
is valid and an invalid password has been supplied, then the system waits 
~10 seconds until it sends a disconnect message and disconnect. If the 
username was not correct, then it disconnect immediately after the wrong 
password.

This makes it possible to scan a server for valid users, to generate spam 
sending lists, or to check a username for another kind of attack.

Tested against qpopper 3.1 and 4.0.4, others might be affected as well.

Attached is the source code for a program that will do a simple check on a 
pop3 server. Additionally qpopper will also return an answer if the 
username supplied has a UID < 100 (< 10 for 3.1), which will also been checked.

The fix should be simple, there must be a usleep() call or similar that 
should either be deleted, or added also to the part where the username was 
not correct.

greets

Dennis

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