Multiple Vendor chmlib CHM File Handling Buffer Overflow Vulnerability

Credit: iDEFENSE Labs
Risk: High
Local: No
Remote: Yes

CVSS Base Score: 5.1/10
Impact Subscore: 6.4/10
Exploitability Subscore: 4.9/10
Exploit range: Remote
Attack complexity: High
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

Multiple Vendor chmlib CHM File Handling Buffer Overflow Vulnerability iDefense Security Advisory 10.28.05 October 28, 2005 I. BACKGROUND chmlib is a library for dealing with Microsoft ITSS/CHM format files. II. DESCRIPTION Remote exploitation of a stack overflow vulnerability in chmlib as included in various Linux distributions allows attackers to execute arbitrary code. The vulnerability specifically exists due to an unchecked memory copy while processing a CHM file. The vulnerability exists in the following code, which is found in chm_lib.c: static UChar *_chm_find_in_PMGL(UChar *page_buf, UInt32 block_len, const char *objPath) { [...] char buffer[CHM_MAX_PATHLEN+1]; /* figure out where to start and end */ cur = page_buf; hremain = _CHM_PMGL_LEN; if (! _unmarshal_pmgl_header(&cur, &hremain, &header)) return NULL; end = page_buf + block_len - (header.free_space); /* now, scan progressively */ while (cur < end) { /* grab the name */ temp = cur; strLen = _chm_parse_cword(&cur); if (! _chm_parse_UTF8(&cur, strLen, buffer)) return NULL; [..] _chm_parse_cword can be forced to return a value larger than CHM_MAX_PATHLEN. This value is then used while copying user controlled data into a CHM_MAX_PATHLEN sized stack buffer. This allows the attacker full control over execution flow by overwriting the saved return address on the stack. III. ANALYSIS Exploitation could allow attackers to execute arbitrary code with the privileges of the user processing the CHM file. Remote exploitation can be achieved by sending a malicious file in an e-mail message to the target user. IV. DETECTION iDefense has confirmed the existence of this vulnerability in chmlib 0.35. It is suspected that all versions of chmlib are vulnerable. The following vendors distribute susceptible chmlib packages within their respective operating system distributions: FreeBSD Project: FreeBSD 5.4 and prior Gentoo Foundation Inc.: Gentoo Linux 1.1a, 1.2, 1.4, 2004.0, 2004.1 and 2004.2 V. WORKAROUND Do not open CHM files sent from an untrusted source. VI. VENDOR RESPONSE chmlib-0.36 addresses this vulnerability and is available for download at: VII. CVE INFORMATION The Common Vulnerabilities and Exposures (CVE) project has assigned the name CAN-2005-2930 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems. VIII. DISCLOSURE TIMELINE 09/08/2005 Initial vendor notification 09/09/2005 Initial vendor response 10/28/2005 Public disclosure IX. CREDIT The discoverer of this vulnerability wishes to remain anonymous. Get paid for vulnerability research Free tools, research and upcoming events 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 (at) iDefense (dot) com [email concealed] 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.

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