bugtraq - Re: Decompression Bombs

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Date: Mon, 09 Feb 2004 08:44:53 -0800
From: Brian Dessent <brian@...sent.net>
To: bugtraq@...urityfocus.com
Subject: Re: Decompression Bombs


Myron Davis wrote:

> This as far as I know is fairly well known as we had a problem with this a
> while back (by accident).
> 
> We put a little check in like this:
> 
> unzip -l $SANITIZED_ZIP_FILE|tail -n 1|cut -f4 -d' '
> 
> then checked the size .. if it was larger then oohh.. 400 megs, then drop
> it  w/ an error for it being too large.

This check will fail for all but the most naive of bombs.  For example,
consider the file located at <http://www.unforgettable.dk/42.zip>.  This
file contains a number of recursively nested ZIP files, to a depth of
5.  Compressed it is only 41kB, yet unpacks to 4.5 PB
(4,503,599,626,321,920 bytes) in total.

$ unzip -l 42.zip 
Archive:  42.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
    34902  03-28-00 21:40   lib 3.zip
    34902  03-28-00 21:40   lib 1.zip
    34902  03-28-00 21:40   lib 2.zip
    34902  03-28-00 21:40   lib 0.zip
    34902  03-28-00 21:40   lib 4.zip
    34902  03-28-00 21:40   lib 5.zip
    34902  03-28-00 21:40   lib 6.zip
    34902  03-28-00 21:40   lib 7.zip
    34902  03-28-00 21:40   lib 8.zip
    34902  03-28-00 21:40   lib 9.zip
    34902  03-28-00 21:40   lib a.zip
    34902  03-28-00 21:40   lib b.zip
    34902  03-28-00 21:40   lib c.zip
    34902  03-28-00 21:40   lib d.zip
    34902  03-28-00 21:40   lib e.zip
    34902  03-28-00 21:40   lib f.zip
 --------                   -------
   558432                   16 files

Your virus scanner will probably try to descend each of those archives,
and will croak if it does not recognise this as malware.

Brian