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Re: qpopper timing analysis on to determine if a username exists on a system
From: Waldo Nell <pwnell () iafrica com>
Date: Mon, 17 Mar 2003 16:09:02 +0200


I have tested this on my qpopper 4.0.5 - and I get this response no matter 
from which host I test (even localhost):

sun waldo # ./poptest mail.XXX.net gert
Validating username gert , please stand by..
Disconnected after 119.993 seconds.
User "gert" is probably a valid user

But that user is not a valid user. I have APOP authentication on and required, 
thus the pop server responded with

You must use TLS/SSL or stronger authentication such as APOP to connect to 
this server

Maybe this is a temporary solution? Or maybe the issue was fixed in 4.0.5?

- Waldo

On Saturday 15 March 2003 21:13, Dennis Lubert wrote:

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

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

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.



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