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Multiple Unix/Linux Vendor Xpdf makeFileKey2 Stack Overflow

01.18.05

BACKGROUND

Xpdf is an open-source viewer for PDF files. More information is
available at the following site:

http://www.foolabs.com/xpdf/

DESCRIPTION

Remote exploitation of a buffer overflow vulnerability in the xpdf PDF
viewer included in multiple Unix and Linux distributions could allow for
arbitrary code execution as the user viewing a PDF file.

The vulnerability specifically exists due to insufficient bounds
checking while processing a PDF file that provides malicious values in
the /Encrypt /Length tag. The offending code can be found in the
Decrypt::makeFileKey2 function in the source file xpdf/Decrypt.cc.

GBool Decrypt::makeFileKey2(int encVersion, int encRevision,
                            int keyLength, GString *ownerKey,
                            GString *userKey, int permissions,
                            GString *fileID, String *userPassword,
                            Guchar *fileKey) {
      Guchar *buf;
      Guchar test[32];
      Guchar fState[256];
      Guchar tmpKey[16];
      Guchar fx, fy;
      int len, i, j;
      GBool ok;
      ...

            memcpy(test, userKey->getCString(), 32);
            for (i = 19; i >= 0; --i) {
                  for (j = 0; j < keyLength; ++j) {
[overflow]               tmpKey[j] = fileKey[j] ^ i;
                  }
            ...
      }
      ...
}

In this piece of code, the keyLength value is ultimately supplied by the
PDF file. This allows an attacker to specify an arbitrarily large value
and overwrite portions of stack memory. As a consequence, arbitrary code
execution is possible.

ANALYSIS

Successful exploitation of this vulnerability leads to arbitrary code
execution as the user who opened the malicious file. An attacker would
have to convince a target to open the provided file in order to exploit
this vulnerability, thus lessening the impact.

Exploitation can be performed reliably, especially with knowledge of the
target system.

DETECTION

iDEFENSE has confirmed the existence of this vulnerability in version
3.00 of xpdf. It is suspected previous versions are vulnerable.

The following Linux vendors may be affected by this vulnerability:

 Novell Inc. (SUSE)
 Red Hat Inc.
 The Fedora Project
 Debian Project
 Gentoo Foundation Inc.
 The FreeBSD Project
 OpenBSD

WORKAROUND

Only open PDF files from trusted individuals.

VENDOR RESPONSE

A patch to address this issue is available at:

    ftp://ftp.foolabs.com/pub/xpdf/xpdf-3.00pl3.patch

Updated binaries (ver. 3.00pl3) to address this issue are available at:

    http://www.foolabs.com/xpdf/download.html

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2005-0064 to these issues. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

DISCLOSURE TIMELINE

01/06/2005   Initial vendor notification
01/12/2005   Initial vendor response
01/18/2005   Coordinated public disclosure

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

LEGAL NOTICES

Copyright © 2005 Verisign, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDEFENSE. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically, please
email customerservice@idefense.com for permission.

Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
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 arising from use of, or reliance on,
this information.