full-disclosure - [ TZO-012005 ] F-Prot/Frisk Anti Virus bypass

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Date: Wed Nov  2 19:34:47 2005
From: Thierry at sniff-em.com (Thierry Zoller)
Subject: [ TZO-012005 ] F-Prot/Frisk Anti Virus bypass -
	ZIP Version Header

_______________________________________________________________________

         F-Prot/Frisk Anti Virus bypass - ZIP Version Header
_______________________________________________________________________

Ref     : TZO-012005-Fprot
Author  : Thierry Zoller / Security Engineer
WWW     : http://thierry.sniff-em.com
Article : http://thierry.sniff-em.com/research/fprot.html

I. Background
~~~~~~~~~~~~~

http://www.f-prot.com/products/corporate_users/

FRISK Software International has, since it was first established in 1993, 
consistently maintained its position as one of the world's leading companies
in antivirus research and product development.

FRISK Software produces the hugely popular F-Prot Antivirus products range
offering unrivalled neural network and heuristic detection capabilities. 
In addition to this, the F-Prot AVES managed online e-mail security service
filters away the nuisance of spam e-mail as well as viruses, worms and other
malware that increasingly clog up inboxes and threaten data security. 

F-Prot Antivirus for Windows
F-Prot Antivirus for Microsoft Exchange
F-Prot Antivirus for Linux x86 / BSD x86
F-Prot Antivirus for AIX
F-Prot Antivirus for DOS
F-Prot Antivirus for Solaris SPARC / Solaris x86
F-Prot Antivirus for AIX

II. Description
~~~~~~~~~~~~~~~

The F-prot engines fails to decompress ZIP files that have a version
header greater then 15. The consequence is that the F-prot Engine 
is unable to scan the virus/malware inside and consequently  flags
it as harmless. If used as an Email Gateway solution the offending
Emails will slip through.

Local ZIP file header:
local file header signature     4 bytes  (0x04034b50)
version needed to extract       2 bytes

Winzip, Winrar, MS Zip engine decompress fine.

Tested offset :
Offset      0  1  2  3  4  5  6  7   8  9 10 11 12 13 14 15
00000000   50 4B 03 04 15 00 00 00  00 00 88 80 38 33 3C CF
00000016   51 68 44 00 00 00 44 00  00 00 09 00 00 00 65 69

In this example byte 4 has the version header value 15. F-prot fails to
decompress the ZIP files with a version header greater then 15.

Solution:
The ZIP decompression engine should ignore the Version header of the
ZIP file and nonetheless decompress the file whatever the version
field indicates.

III. Summary
~~~~~~~~~~~~~~~
Vendor contact :  30/10/2005
Vendor Response : 01/11/2005

        Thank you very much for notifying us of this bug in the current version of
        F-Prot Antivirus. A fix for this bug will be included in future versions 
        of F-Prot Antivirus.

IV. Thanks
~~~~~~~~~~~~~~~ 
http://virusscan.jotti.org/
http://www.virustotal.com

_______________________________________________________________________

Reference : TZO-012005-Fprot
Author    : Thierry Zoller / Security Engineer
WWW       : http://thierry.sniff-em.com