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�� 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
��� 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 | | disable_classes | "" | PHP_INI_SYSTEM |
For further details and definition of the PHP_INI_* constants see
ini_set().
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- safe_mode
boolean
Whether to enable PHP's safe mode. Read the
Security 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 colon (semi-colon on
Windows) separated path in a fashion similar to the
include_path directive,
rather than just a single directory.
The restriction specified is actually a prefix, not a directory name.
This means that "safe_mode_include_dir = /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: "safe_mode_include_dir = /dir/incl/"
- 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).
�`:
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, including the file itself. 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/"
�`:
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-delimited 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.
- disable_classes
string
This directive allows you to disable certain classes for
security reasons. It takes
on a comma-delimited list of class names. disable_classes
is not affected by Safe Mode.
This directive must be set in php.ini For example, you
cannot set this in httpd.conf.
Availability note:
This directive became available in PHP 4.3.2
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 |
daniel dot gaddis at tlc dot state dot tx dot us
17-Mar-2004 05:03
on windows if multiple directories are wanted for safe_mode_exec_dir and open_basedir be sure to separate the paths with a semi colon and double quote the whole path string. For example:
safe_mode = On
safe_mode_exec_dir = "F:\WWW\HTML;F:\batfiles\batch"
open_basedir = "F:\WWW\HTML;F:\batfiles\batch"
uuganbat at datacom dot mn
26-Feb-2004 07:32
users executing shell scripts by filename.php, and I take the server in safe mode but some things are not working on web server, and disabled functions by php.ini but the simple commands in Unix are executing how disable to execute shell scripts via *.php file.
example
<?php
echo `ls -l /etc/`; echo `more /etc/passwd`;
?>
matthias at remove-this dot hrz dot uni-kassel dot de
12-Nov-2003 03:38
Beware: Safe mode will not permit you to create new files in directories which have different owner than the owner of the script. This typically applies to /tmp, so contrary to Unix intuition, you will not be able to create new files there (even if the /tmp rights are set correctly).
If you need to write into files in /tmp (for example to put logfiles of your PHP application there) create them first on the command line by doing a
touch /tmp/whatever.log
as the same user who owns the PHP script. Then, provided the rest is configured correctly, the PHP script will be able to write into that file.
info at phpcoding dot net
08-Mar-2003 12:44
readfile() seems not always to take care of the safe_mode setting.
When the file you want to read with readfile() resides in the same directory as the script itself, it doesn`t matter who the owner of the file is, readfile() will work.
sj at sjaensch dot org
18-Feb-2003 11:33
safe_mode_include_dir only works when INCLUDING files, it has no effect when you access them by other means (eg. with fopen())! Your only solution then is to set the user or group (if safe_mode_gid is activated) of the file to the same as the script you are executing or disable safe mode altogether.
See bug 20054 ().
gtg782a at mail dot gatech dot edu
26-Jan-2003 03:14
zebz: The user would not be able to create a directory outside the namespace where he/she would be able to modify its contents. One can't create a directory that becomes apache-owned unless one owns the parent directory.
Another security risk: since files created by apache are owned by apache, a user could call the fputs function and output PHP code to a newly-created file with a .php extension, thus creating an apache-owned PHP script on the server. Executing that apache-owned script would allow the script to work with files in the apache user's namespace, such as logs. A solution would be to force PHP-created files to be owned by the same owner/group as the script that created them. Using open_basedir would be a likely workaround to prevent ascension into uncontrolled areas.
dizzy at roedu dot net
16-Jan-2003 05:25
For people using linux there is a very nice solution to the shared server problem. It's called vserver/ctx. Here is the URL:
10-Sep-2002 09:30
Some paranoid web managers also restrict the set_user_abort() function.
This constitutes a security issue for hosted web sites, because the hosted script cannot guarantee safe atomic operations on files in a reasonnable time (the time limit may still be in effect):
If set_user_abort() is disabled by the web admin, a user can corrupt the files of a hosted web site by simply sending many requests to the PHP script, and aborting it fast. In some cases that can be easily reproduced after a dozen of attempts, the script will be interrupted in the middle of a file or database update!
The only way for the hosted web site to protect itself in this case is to use a sleep() with a random but not null short time before performing atomic operations.
Web admins should then definitely NOT disable the set_user_abort() function which is vital to ensure atomic operations on hosted critical files or databases.
Instead they should only disable the set_time_limit() function, and set a constant but reasonnable time for any script to complete their atomic operations.
10-Sep-2002 09:19
disk_free_space($directory) is also restricted by the safe_mode ! It can also be completely forbidden as this would allow a script to test the available space in order to fill it with a giant file, preventing other scripts to use that space (notably in "/tmp").
disk_free_space() is then informational, and this does not prevent system quotas to limit the size of your files to a value lower than the available free space!
Most web server admins that propose PHP hosting, will implement quotas for your hosting account, but also on any shared resources such as temporary folders.
tom at tom420 dot com
19-Jul-2002 07:33
open_basedir only restricts file operations to files and directories under a specified directory, but you can still user system ("vi /home/somedir/somefile"), so safe_mode still has a place here as it is much more restrictive then open_basedir.
Also, to reply to someone who said that 'above' and 'below' was a matter of perspective, sure it is. Of course, a file is not under another one, etc, it just pointed by some inode. But in the common language we consider the root (/) to be above everything else, and /home is below root, and /home/myfile is below /home. There is no written standard, but most people (those I know anyway) agree on that syntax.
dw at null dot ca
13-Jun-2002 09:37
You can a vhost.conf file.
<Directory /vhosts/domain.com/httpdocs/>
php_admin_value safe_mode 0
php_admin_value open_basedir "/"
</Directory>
zebz at ihaveenoughspam_hotmail dot com
28-Apr-2002 02:42
All the filesystem-related functions (unlink, fopen, unlink, etc) seems to be restricted the same way in safe mode, at least on PHP 4.2. If the file UID is different *but* the directory (where the file is located) UID is the same, it will work.
So creating a directory in safe mode is usually a bad idea since the UID will be different from the script (it will be the apache UID) so it won't be possible to do anything with the files created on this directory.
devik at cdi dot cz
24-Jan-2002 10:45
Just to note, I created patch which allows VirtualHost to set User under which all (PHP too) runs. It is more secure than safe_mode. See luxik.cdi.cz/~devik/apache/ if you are interested
jedi at tstonramp dot com
08-Sep-2001 01:17
Many filesystem-related functions are not appropriately restricted when Safe Mode is activated on an NT server it seems. I would assume that this is due to the filesystem not making use of UID.
In all of my scripts, no matter WHO owns the script (file Ownership-wise) or WHO owns the directory/file in question; both UIDs display
(getmyuid() and fileowner()) as UID = 0
This has the rather nasty side effect of Safe Mode allowing multiple filesystem operations because it believes the script owner and file/dir owner are one and the same.
While this can be worked around by the judicious application of proper filesystem privileges, it's still a "dud" that many of Safe Mode's securities are simply not there with an NT implementation.
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