SMTP attacks, anyone ?

[Andrew Daviel <advax () TRIUMF CA>]

In the last few months on two occasions we've had a user's email 
credentials compromised and used to send spam via SMTP.
We have a Postfix mail relay where users can authenticate via SASL to 
send mail from offsite, and this was what was used.

There was no obvious trace of a dictionary attack; it seems the attackers 
knew a password somehow and then proceeded to use it from a couple of 
hundred different client addresses around the world (which themselves 
appear to be SMTP servers, rather than home PCs).

Both the users in question deny "risky network behaviour" and are fairly 
clueful - would not fall for phishing, do not frequent cybercafes etc.
Their passwords (now changed of course) were robust enough not to fall to 
a few hours of "John the Ripper" so I doubt they were trivially guessed.

I wondered if anyone else had seen this kind of abuse.

Right now it's not a serious problem, but of course if we've got 
unexplained compromises I want to understand. I'll probably write some 
kind of filter to flag/block excessive offsite logins, or impossibly 
short travel times like the credit card companies do.

--
Andrew Daviel, TRIUMF, Canada
Tel. +1 (604) 222-7376  (Pacific Time)
Network Security Manager