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ClamAV get_unicode_name() off-by-one buffer overflow

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