full-disclosure - Re: Filtering sobig with postfix

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From: craig at strong-box.net (Craig Pratt)
Subject: Re: Filtering sobig with postfix

On Wednesday, Aug 20, 2003, at 20:51 US/Pacific, Bojan Zdrnja wrote:
>
>> -----Original Message-----
>> From: full-disclosure-admin@...ts.netsys.com
>> [mailto:full-disclosure-admin@...ts.netsys.com] On Behalf Of
>> martin f krafft
>> Sent: Wednesday, 20 August 2003 10:43 p.m.
>> To: full-disclosure@...ts.netsys.com
>> Subject: [Full-Disclosure] Re: Filtering sobig with postfix
>>
>>
>> also sprach vogt@...senet.com <vogt@...senet.com>
>> [2003.08.20.1017 +0200]:
>>> in main.cf, enable "body_checks = (filename)". In that (filename)
>>> file, write a regular expression matching sobig, e.g. something
>>> like
>>>
>>> /see attached file for details/	REJECT
>>
>> this incurs a factor 2-4 performance drop, and it could also elicit
>> false positives. you should definitely do more than just REJECT
>> (i.e. write out a message: s/REJECT/554 Suspected virus/).
>
> Yep, as the OP is using postfix, he could use the header_checks 
> directive,
> which can identify MIME headers, so he can easily stop this worm.
> Just check for Content-Disposition header and block everything with 
> .pif in
> filename.
>
> Regards,
>
> Bojan Zdrnja

You'd better check for a lot more than just .pif files. .scr and .exe 
files are critical as well.

And filtering on this stuff is problematic - since there are lots of 
ways to play with MIME headers - such as escaping characters and 
playing with the type fields. Check out Nessus's e-mail tests for some 
examples of the ways to subvert the kind of checks described here. 
You'd better be ready to write a few thousand REs.

If you want to really filter stuff, look at something like MailScanner 
(http://www.mailscanner.info) which has file-type/mime-type checking as 
just the first line of defense. The power and configurability of this 
system is amazing. It works with postfix and sendmail, and lots of 
virus scanners - if you choose to integrate one.

There's definitely a performance cost. But it's very smart about how it 
works - batching scans into a single job. Even if you scan during the 
SMTP exchange, there's going to be a cost. Such is life.

Craig

---
Craig Pratt
Strongbox Network Services Inc.
mailto:craigATstrong-box.net

-- 
This message checked for dangerous content by MailScanner on StrongBox.