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Clam AntiVirus Win32-UPX Buffer Overflow Vulnerability



Clam Antivirus is a GPL anti-virus toolkit for Unix. It is used in
several anti-virus gateways and available for free on multiple platforms
(Unix, Windows, MacOS X, BeOS, etc.).

More information about Clam AntiVirus is available from:



Remote exploitation of an Input Validation Error in Clam AntiVirus
ClamAV allows attackers to cause a denial of service (DoS) condition.

UPX (Ultimate Executable Packer) is a compressor for small Win32 EXE
files. Remote exploitation of an unexpected condition in the
decompression of UPX files allow attackers to cause a denial of service
condition. Remote code execution is theoretically possible, however
would be non-trivial given the huge size of the buffer (nearly 4GB).

The vulnerability exists due to a signedness issue in the pefromupx()
function [libclamav/upx.c] when dealing with the number of UPX sections.

int pefromupx(...)
  char *imports, *sections, *pehdr, *newbuf;
  int sectcnt, upd=1, realstuffsz;
  int foffset=0xd0+0xf8;


  if ( ! (sectcnt = pehdr[6]+256*pehdr[7])) {


  if (!(newbuf = (char *) cli_malloc(foffset))) {


  memcpy(newbuf+0xd0, pehdr,0xf8+0x28*sectcnt);


The number of UPX sections ('sectcnt') is a signed integer calculated
using signed characters ('pehdr') from the file. By setting pehdr[6]
and/or pehdr[7] to negative values (0x80-0xff), 'sectcnt' can be forced
to be negative.

If 'foofset' is positive for the cli_malloc() to succeed (negative
malloc() is verified by ClamAV) and '0xf8+0x28* sectcnt' is negative,
the process will crash on the memcpy() call due to heap corruption.

This can happen for the following values of pehdr[6]: 0xf5, 0xf6, 0xf7,
0xf8 and 0xf9. pehdr[7] must remain at 0x00.


Successful exploitation allows attackers to overflow a buffer allocated
in the heap. Anti-virus detection functionality can be left in a
defunct state depending on the configuration (analysis process
automatically restarted or not). This can be considered as a denial of

Remote execution of arbitrary code is not easy due to the huge value of
the memcpy() size (nearly 4GB), but theoretically possible.

Remote exploitation can be achieved by sending a malicious file in an
e-mail message or during an HTTP session.


This issue was tested to work with ClamAV 0.86. The vulnerable code
was added in Clam AntiVirus 0.81. All versions between .81 and .86
are believed vulnerable.


The following patch can be applied to fix the vulnerability:

diff -ruN clamav-0.86.1/libclamav/upx.c
--- clamav-0.86.1/libclamav/upx.c       Thu Jun 23 22:03:12 2005
+++ clamav-0.86.1-upx-fix/libclamav/upx.c       Tue Jul  5 15:08:32 2005
@@ -117,7 +117,7 @@

   sections = pehdr+0xf8;
-  if ( ! (sectcnt = pehdr[6]+256*pehdr[7])) {
+  if ( ! (sectcnt = (unsigned char)pehdr[6]+256*(unsigned char)pehdr[7]))
     cli_dbgmsg("UPX: No sections? - giving up rebuildn");
     return 0;

Windows PE EXE files analysis can be disabled (--no-pe) but this can
have severe impact on the virus detection functionality.


ClamAV has released version 0.87 to correct this issue.



The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2005-2920 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.


09/08/2005   Initial vendor notification
09/19/2005   Initial vendor notification
09/19/2005   Public disclosure


The discoverer of this vulnerability wishes to remain anonymous.

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Copyright © 2005 Verisign, Inc.

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