full-disclosure - ClamAV get_unicode_name() off-by-one buffer overflow

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Message-ID: <20081109154643.GA27847@fugu1.local>
Date: Sun, 9 Nov 2008 16:46:43 +0100
From: Moritz Jodeit <moritz@...eit.org>
To: full-disclosure@...ts.grok.org.uk
Subject: ClamAV get_unicode_name() off-by-one buffer
	overflow

-----------------------------------------------------------------
ClamAV get_unicode_name() off-by-one buffer overflow

Copyright (c) 2008 Moritz Jodeit <moritz@...eit.org> (2008/11/08)
-----------------------------------------------------------------

Application details:

	From http://www.clamav.net/:

	"Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX,
	designed especially for e-mail scanning on mail gateways. It provides
	a number of utilities including a flexible and scalable multi-threaded
	daemon, a command line scanner and advanced tool for automatic
	database updates. The core of the package is an anti-virus engine
	available in a form of shared library."

Vulnerability description:

	ClamAV contains an off-by-one heap overflow vulnerability in the
	code responsible for parsing VBA project files. Successful
	exploitation could allow an attacker to execute arbitrary code with
	the privileges of the `clamd' process by sending an email with a
	prepared attachment.

	The vulnerability occurs inside the get_unicode_name() function
	in libclamav/vba_extract.c when a specific `name' buffer is passed
	to it.

	101 static char *
	102 get_unicode_name(const char *name, int size, int big_endian)
	103 {
	104         int i, increment;
	105         char *newname, *ret;
	106
	107         if((name == NULL) || (*name == '\0') || (size <= 0))
	108                 return NULL;
	109
	110         newname = (char *)cli_malloc(size * 7);

	First the `size' of the `name' buffer multiplied by 7 is used to
	allocate the destination buffer `newname'. When the `name' buffer
	only consists of characters matching some specific criteria [1]
	and `big_endian' is set, the following loop can write exactly 7
	characters into the allocated destination buffer `newname' per
	character found in source buffer `name'.

	This effectively fills up the destination buffer completely. After
	the loop in line 143, the terminating NUL byte is written and
	overflows the allocated buffer on the heap.

	143         *ret = '\0';
	144
	145         /* Saves a lot of memory */
	146         ret = cli_realloc(newname, (ret - newname) + 1);
	147         return ret ? ret : newname;
	148 }

	[1] Every character matching the following condition results in
	    7 characters written to the destination buffer:

		(c & 0x80 || !isprint(c)) && (c >= 10 || c < 0)

	A VBA project file embedded inside an OLE2 office document send
	as an attachment can trigger the off-by-one.

Vendor response:

	2008/10/16 Initial report to vendor
	2008/10/16 Vulnerability acknowledged by acab@...mav.net
	2008/11/03 Release of version 0.94.1

Vulnerable packages:

	All versions up to 0.94 are vulnerable.
	Version 0.94.1 fixes the problem.

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