[NEWS] Antivirus Mail Scanners DoS

From: support@securiteam.com
Date: 02/27/02

From: support@securiteam.com
To: list@securiteam.com
Date: Wed, 27 Feb 2002 18:13:20 +0100 (CET)

The following security advisory is sent to the securiteam mailing list, and can be found at the SecuriTeam web site: http://www.securiteam.com
- - promotion

When was the last time you checked your server's security?
How about a monthly report?
http://www.AutomatedScanning.com - Know that you're safe.
- - - - - - - - -

  Antivirus Mail Scanners DoS


Most Antivirus applications uncompress archives before scanning them for
viruses. A specially constructed compressed file containing only NULL
values (or any other repeating one character value) will be a size of only
a few bytes when compressed, but when uncompressed it will be a few
megabytes of "information". This could cause a denial of service attack
against the Antivirus scanning engine, by exhausting all available memory
or disk space.


An antivirus mail scanner should check the file sizes inside a compressed
file like .tar.gz, .zip, .bz2, etc, before they open the file for

All products that do not do this type of checking are vulnerable to a
denial of service attack.

See the following procedure:

# dd if=/dev/zero of=/tmp/file count=200000
# ls -l /tmp/file
-rw-r--r-- 1 root root 102400000 Feb 24 22:13 file

# bzip2 -z file
# ls -l /tmp/file.bz2
rw-r--r-- 1 root root 113 Feb 24 22:14 file

Since the file has only NULL (Numerical zeros, not the ASCII kind)
characters, the size of the compressed file was reduced to a almost
insignificant value. Sending several emails with these compressed files
may cause a machine to become out of memory or disk space.

The mail scanner should check the file sizes of files included inside a
compressed file.


The information has been provided by <mailto:maciel@inetd.com.br> Eduardo
R. Maciel.


This bulletin is sent to members of the SecuriTeam mailing list.
To unsubscribe from the list, send mail with an empty subject line and body to: list-unsubscribe@securiteam.com
In order to subscribe to the mailing list, simply forward this email to: list-subscribe@securiteam.com


The information in this bulletin is provided "AS IS" without warranty of any kind.
In no event shall we be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages.