Safe Mode

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# Chapter 22. Safe Mode

The PHP safe mode is an attempt to solve the shared-server security problem. It is architecturally incorrect to try to solve this problem at the PHP level, but since the alternatives at the web server and OS levels aren't very realistic, many people, especially ISP's, use safe mode for now.

# Security and Safe Mode

Table 22-1. Security and Safe Mode Configuration Directives

Name | Default | Changeable
--- | --- | ---
safe_mode | "0" | PHP_INI_SYSTEM
safe_mode_gid | "0" | PHP_INI_SYSTEM
safe_mode_include_dir | NULL | PHP_INI_SYSTEM
safe_mode_exec_dir | "" | PHP_INI_SYSTEM
safe_mode_allowed_env_vars | PHP_ | PHP_INI_SYSTEM
safe_mode_protected_env_vars | LD_LIBRARY_PATH | PHP_INI_SYSTEM
open_basedir | NULL | PHP_INI_SYSTEM
disable_functions | "" | PHP_INI_SYSTEM

For further details and definition of the PHP_INI_* constants see ini_set().

Here is a short explanation of the configuration directives.

`safe_mode` boolean

  Whether to enable PHP's safe mode. Read the Security and chapter for more information.

`safe_mode_gid` boolean

  By default, Safe Mode does a UID compare check when opening files. If you want to relax this to a GID compare, then turn on safe_mode_gid. Whether to use `UID` (`FALSE`) or `GID` (`TRUE`) checking upon file access.

`safe_mode_include_dir` string

  `UID`/`GID` checks are bypassed when including files from this directory and its subdirectories (directory must also be in include_path or full path must including).

  As of PHP 4.2.0, this directive can take a semi-colon separated path in a similar fashion to the include_path directive, rather than just a single directory.

`safe_mode_exec_dir` string

  If PHP is used in safe mode, system() and the other functions executing system programs refuse to start programs that are not in this directory.

`safe_mode_allowed_env_vars` string

  Setting certain environment variables may be a potential security breach. This directive contains a comma-delimited list of prefixes. In Safe Mode, the user may only alter environment variables whose names begin with the prefixes supplied here. By default, users will only be able to set environment variables that begin with PHP_ (e.g. PHP_FOO=BAR).

  > Note: If this directive is empty, PHP will let the user modify ANY environment variable!

`safe_mode_protected_env_vars` string

  This directive contains a comma-delimited list of environment variables that the end user won't be able to change using putenv(). These variables will be protected even if safe_mode_allowed_env_vars is set to allow to change them.

`open_basedir` string

  Limit the files that can be opened by PHP to the specified directory-tree. This directive is NOT affected by whether Safe Mode is turned On or Off.

  When a script tries to open a file with, for example, fopen or gzopen, the location of the file is checked. When the file is outside the specified directory-tree, PHP will refuse to open it. All symbolic links are resolved, so it's not possible to avoid this restriction with a symlink.

  The special value . indicates that the directory in which the script is stored will be used as base-directory.

  Under Windows, separate the directories with a semicolon. On all other systems, separate the directories with a colon. As an Apache module, open_basedir paths from parent directories are now automatically inherited.

  The restriction specified with open_basedir is actually a prefix, not a directory name. This means that "open_basedir = /dir/incl" also allows access to "/dir/include" and "/dir/incls" if they exist. When you want to restrict access to only the specified directory, end with a slash. For example: "open_basedir = /dir/incl/"

  > Note: Support for multiple directories was added in 3.0.7.

  The default is to allow all files to be opened.

`disable_functions` string

  This directive allows you to disable certain functions for security reasons. It takes on a comma-dilimited list of function names. disable_functions is not affected by Safe Mode.

  This directive must be set in `php.ini` For example, you cannot set this in `httpd.conf`.

See also: register_globals, display_errors, and log_errors

When safe_mode is on, PHP checks to see if the owner of the current script matches the owner of the file to be operated on by a file function. For example:

    -rw-rw-r--    1 rasmus   rasmus       33 Jul  1 19:20 script.php
    -rw-r--r--    1 root     root       1116 May 26 18:01 /etc/passwd

Running this script.php

    <?php
     readfile('/etc/passwd');
    ?>

results in this error when safe mode is enabled:

    Warning: SAFE MODE Restriction in effect. The script whose uid is 500 is not
    allowed to access /etc/passwd owned by uid 0 in /docroot/script.php on line 2

However, there may be environments where a strict `UID` check is not appropriate and a relaxed `GID` check is sufficient. This is supported by means of the safe_mode_gid switch. Setting it to `On` performs the relaxed `GID` checking, setting it to `Off` (the default) performs `UID` checking.

If instead of safe_mode, you set an open_basedir directory then all file operations will be limited to files under the specified directory For example (Apache httpd.conf example):

    <Directory /docroot>
      php_admin_value open_basedir /docroot
    </Directory>

If you run the same script.php with this open_basedir setting then this is the result:

    Warning: open_basedir restriction in effect. File is in wrong directory in
    /docroot/script.php on line 2

You can also disable individual functions. Note that the disable_functions directive can not be used outside of the `php.ini` file which means that you cannot disable functions on a per-virtualhost or per-directory basis in your httpd.conf file. If we add this to our `php.ini` file:

    disable_functions readfile,system

Then we get this output:

    Warning: readfile() has been disabled for security reasons in
    /docroot/script.php on line 2

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