Bugtraq mailing list archives
Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass
From: Gadi Evron <ge () linuxbox org>
Date: Thu, 7 Dec 2006 06:30:49 -0600 (CST)
On Wed, 6 Dec 2006, Hendrik Weimer wrote:
Several e-mail virus scanners can be tricked into passing an EICAR test file if the following conditions are met: 1. the EICAR file is encoded in Base64 including characters not in the standard alphabet (e.g. whitespaces) and 2. the part containing the EICAR file is nested within one or several levels of multipart/mixed content.
Victor Duchovni agreed for me to post what he employs to avoid such issues. This is in some ways similar to a limited application firewall for SMTP, which is not spam specific and MIME only. Yes, I know, SMTP application firewalls are the 4th buzzword down the road, give it a couple of years. Victor's information: I have a MIME normalizer in front of the A/V engine. Non-conformant Base64 entities are made conformant or neutered (super-encoded via QP so that the user receives the base64 text itself as the entity payload). -------- In: CT: application/octet-stream CD: attachment; filename=foo.dat CTE: base64 AA AA Out: CT: application/octet-stream CD: attachment; filename=foo.dat CTE: base64 AAAA -------- In: CT: application/octet-stream CD: attachment; filename=foo.dat CTE: base64 AA<Ctrl-A>AA Out: CT: text/plain CD: attachment; filename=mime-source.txt CTE: quoted-printable =20AA=01AA -------- Solves all such problems before the vulnerability is found in the A/V engine. The MIME normalizer does more, defending other possible bypass scenarios, but I not able to describe the full feature-set at this time. It was written and deployeed in Dec 1999. --- End quote. All the above is Viktor's. Gadi Evron.
Current thread:
- Multiple Vendor Unusual MIME Encoding Content Filter Bypass Hendrik Weimer (Dec 06)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass Tomasz Kojm (Dec 07)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass Luke Borg (Dec 07)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass michele.sandrelli () katamail com (Dec 07)
- Re[2]: Multiple Vendor Unusual MIME Encoding Content Filter Bypass 3APA3A (Dec 07)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass Tomasz Kojm (Dec 07)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass Gadi Evron (Dec 07)
- Re: Multiple Vendor Unusual MIME Encoding Content Filter Bypass Tomasz Kojm (Dec 07)