Multiple Vendor xpdf PDF Viewer Buffer Overflow Vulnerability iDEFENSE Security Advisory 12.21.04 www.idefense.com/application/poi/display?id=172&type=vulnerabilities December 21, 2004 I. BACKGROUND Xpdf is an open-source viewer for Portable Document Format (PDF) files. II. DESCRIPTION Remote exploitation of a buffer overflow vulnerability in the xpdf PDF viewer, as included in multiple Linux distributions, could allow attackers to execute arbitrary code as the user viewing a PDF file. The offending code can be found in the Gfx::doImage() function in the source file xpdf/Gfx.cc. void Gfx::doImage(Object *ref, Stream *str, GBool inlineImg) { Dict *dict; int width, height; int bits; GBool mask; GBool invert; GfxColorSpace *colorSpace; GfxImageColorMap *colorMap; Object maskObj; GBool haveMask; int maskColors[2*gfxColorMaxComps]; Object obj1, obj2; int i; ... // get the mask haveMask = gFalse; dict->lookup("Mask", &maskObj); if (maskObj.isArray()) { for (i = 0; i < maskObj.arrayGetLength(); ++i) { maskObj.arrayGet(i, &obj1); [!] maskColors[i] = obj1.getInt(); obj1.free(); } haveMask = gTrue; } ... } Due to the fact that the loop boundaries are not less than the storage area, the maskColors array is eventually filled up. After that, local variables and other stack memory is overwritten. This ultimately leads to control of program flow and arbitrary code execution. III. ANALYSIS The severity of this issue is mitigated by the fact that several of the local overwritten variables in doImage() are referenced prior to EIP being restored; therefore, before the attack gains control of the target process. However, an attacker with knowledge of the remote operating system can construct and validate a malicious payload before attempting exploitation, thus increasing the chances of success. An attacker must convince a target user to open the malicious file to exploit this vulnerability. IV. DETECTION iDEFENSE has confirmed the existence of this vulnerability in version 3.00 of xpdf. It is suspected previous versions are also vulnerable. The following Linux distributions are affected by this vulnerability: SUSE Linux Redhat Linux Fedora Linux Debian Linux Gentoo Linux FreeBSD (ports) OpenBSD V. WORKAROUND Only open PDF files from trusted individuals. VI. VENDOR RESPONSE A patch to address this vulnerability is available from: ftp://ftp.foolabs.com/pub/xpdf/xpdf-3.00pl2.patch Updated binaries (version 3.00pl2) are available from: http://www.foolabs.com/xpdf/download.html VII. CVE INFORMATION The Common Vulnerabilities and Exposures (CVE) project has assigned the names CAN-2004-1125 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 11/23/2004 Initial vendor notification 11/29/2004 Initial vendor response 12/21/2004 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) 2004 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. _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.netsys.com/full-disclosure-charter.html