Multiple Unix/Linux Vendor Xpdf makeFileKey2 Stack Overflow 

iDEFENSE Security Advisory 01.18.05
www.idefense.com/application/poi/display?id=186&type=vulnerabilities
January 18, 2005

I. BACKGROUND

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

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

II. 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.

III. 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.

IV. 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 

V. WORKAROUND

Only open PDF files from trusted individuals.

VI. 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

VII. 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.

VIII. DISCLOSURE TIMELINE

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

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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

X. LEGAL NOTICES

Copyright (c) 2005 iDEFENSE, 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.