[NEWS] ClamAV get_unicode_name() Off-By-One Buffer Overflow

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ClamAV get_unicode_name() Off-By-One Buffer Overflow
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SUMMARY

" <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."

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.

DETAILS

Vulnerable Systems:
* ClamAV version 0.94

Immune Systems:
* ClamAV version 0.94.1

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@xxxxxxxxxx
2008/11/03 - Release of version 0.94.1


ADDITIONAL INFORMATION

The information has been provided by <mailto:moritz@xxxxxxxxxx> Moritz
Jodeit.



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