[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
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  Antivirus Mail Scanners DoS
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SUMMARY

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.

DETAILS

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
scanning.

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

Example:
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.

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

ADDITIONAL INFORMATION

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

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