CapÃtulo 42. Modo Seguro (Safe Mode)

Tabla de contenidos
Funciones restringidas/inhabilitadas por Modo Seguro

El Modo Seguro de PHP es un intento para resolver el problema de la seguridad en un servidor compartido. Tratar de resolver este problema al nivel de PHP es arquitectÃ³nicamente incorrecto, pero ya que las alternativas en un servidor web y a niveles de sistemas operativos no son tan realistas, mucha gente, especialmente la de proveedores de Internet (ISP), usa el Modo Seguro por ahora.

Tabla 42-1. Las directivas de ConfiguraciÃ³n que controlan el Modo Seguro son:

Directiva	Valor por OmisiÃ³n
safe_mode	`Off`
safe_mode_gid	`0`
safe_mode_include_dir	`""`
safe_mode_exec_dir	`1`
open_basedir	`""`
`safe_mode_allowed_env_vars`	`PHP_`
`safe_mode_protected_env_vars`	`LD_LIBRARY_PATH`
`disable_functions`	`""`

Cuando safe_mode estÃ¡ en On, el PHP verifica si el dueÃ±o del script actual coincide con el dueÃ±o del fichero a ser operado por una funciÃ³n de fichero. Por ejemplo:
-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
Corriendo este script.php

<?php readfile('/etc/passwd'); ?>
resulta in este error cuando Modo Seguro estÃ¡ habilitado:
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

Sin embargo, pueden haber ambientes donde una estricta verificaciÃ³n del UID no es apropiada, y una relajada verificaciÃ³n del GID es suficiente. Esto es soportado por medio del switch safe_mode_gid. SeteÃ¡ndolo a On hace la verificaciÃ³n relajada GID, seteÃ¡ndolo a Off (el valor por omisiÃ³n) hace la verificaciÃ³n del UID.

Si en vez del safe_mode, Ud. setea un directorio open_basedir, entonces todas las operaciones de fichero estarÃ¡n limitadas a los ficheros bajo ese directorio especificado. Por ejemplo (ejemplo de httpd.conf de Apache):
<Directory /docroot> php_admin_value open_basedir /docroot </Directory>
Si Ud. corre el mismo script.php con este seteo open_basedir, entonces este es el resultado:
Warning: open_basedir restriction in effect. File is in wrong directory in /docroot/script.php on line 2

Ud. tambiÃ©n puede inhabilitar funciones individuales. Note que la directiva disable_functions no puede ser usada fuera del fichero php.ini lo que significa que Ud. no puede inhabilitar funciones en los principios per-virtualhost o per-directory en su fichero httpd.conf. Si agregamos esto a nuestro fichero php.ini:
disable_functions readfile,system
Entonces obtenemos esta salida:
Warning: readfile() has been disabled for security reasons in /docroot/script.php on line 2

Funciones restringidas/inhabilitadas por Modo Seguro

Esta es una lista probablemente incompleta y posiblemente incorrecta de las funciones limitadas por safe mode.

Tabla 42-2. Funciones limitadas por Modo Seguro

FunciÃ³n	Limitaciones
dbmopen()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
dbase_open()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
filepro()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
filepro_rowcount()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
filepro_retrieve()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
*ifx_()**	restricciones sql_safe_mode, (!= safe mode)
*ingres_()**	restricciones sql_safe_mode, (!= safe mode)
*mysql_()**	restricciones sql_safe_mode, (!= safe mode)
pg_loimport()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
posix_mkfifo()	Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
putenv()	Obecede las ini-directivas safe_mode_protected_env_vars y safe_mode_allowed_env_vars. Vea tambiÃ©n la documentaciÃ³n de putenv()
move_uploaded_file()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
chdir()	Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
dl()	Esta funciÃ³n no estÃ¡ habilitada en safe-mode (modo-seguro)
backtick operator	Esta funciÃ³n no estÃ¡ habilitada en safe-mode (modo-seguro)
shell_exec() (equivalencia funcional de backticks)	Esta funciÃ³n no estÃ¡ habilitada en safe-mode (modo-seguro)
exec()	Ud. puede correr sÃ³lo ejecutables dentro delsafe_mode_exec_dir. Por razones prÃ¡cticas, no estÃ¡ actualmente permitido tener componentes `..` en la ruta del fichero ejecutable.
system()	Ud. puede correr sÃ³lo ejecutatables dentro delsafe_mode_exec_dir. Por razones prÃ¡cticas, no estÃ¡ actualmente permitido tener componentes `..` en la ruta del fichero ejecutable.
passthru()	Ud. puede correr sÃ³lo ejecutatables dentro delsafe_mode_exec_dir. Por razones prÃ¡cticas, no estÃ¡ actualmente permitido tener componentes `..` en la ruta del fichero ejecutable.
popen()	Ud. puede correr sÃ³lo ejecutatables dentro delsafe_mode_exec_dir. Por razones prÃ¡cticas, no estÃ¡ actualmente permitido tener componentes `..` en la ruta del fichero ejecutable.
mkdir()	Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
rmdir()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
rename()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
unlink()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
copy()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado. (en `source` y `target`)
chgrp()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
chown()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado.
chmod()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. AdemÃ¡s, Ud. no puede setear los bits de SUID, SGID y sticky
touch()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado.
symlink()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado. (Nota: sÃ³lo el target es comprobado)
link()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado. (Nota: sÃ³lo the target es comprobado)
getallheaders()	En Modo Seguro, las cabeceras que empiezan con 'authorization' (insensitivo al tipo de letra) no serÃ¡n retornadas. Advertencia: esto estÃ¡ roto por la implementaciÃ³n de aol-server de getallheaders()!
header()	En Modo Seguro, el UID del script estÃ¡ agregado a la parte `realm` de la cabecera `WWW-Authenticate` si Ud. setea esta cabecera (usado por HTTP Authentication).
highlight_file(), show_source()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado. (Nota: sÃ³lo afectado desde PHP 4.2.1)
parse_ini_file()	Comprueba que los archivos/directorios que va a utilizar, tengan la misma UID que el script que estÃ¡ siendo ejecutado. Comprueba si el directorio que va a utilizar, tiene la misma UID que el script que estÃ¡ siendo ejecutado. (Nota: sÃ³lo afectado desde PHP 4.2.1)
Cualquier funciÃ³n que usa `php4/main/fopen_wrappers.c`	??

add a note User Contributed Notes
Modo Seguro (Safe Mode)

mailrinke at yahoo dot com
14-Feb-2006 02:20


Note that safemode restricts setting the sticky bits, so this may not be an option

martin at example dot com
01-Nov-2005 04:07


On the note of php security 



have a look at: http://www.suphp.org/Home.html



 suPHP is a tool for executing PHP scripts with the permissions of their owners. It consists of an Apache module (mod_suphp) and a setuid root binary (suphp) that is called by the Apache module to change the uid of the process executing the PHP interpreter.

hunter+phpnet at spysatcentral dot net
07-Aug-2005 10:25


I use mkdir just fine. You just have to make sure you set sticky bits on the directory you are creating the files in. Look at "man chmod" clipping: 



4000    (the setuid bit).  Executable files with this bit set will run with effective uid set to the uid of the file owner. Directories with this bit set will force all files and sub-directories created in them to be owned by the directory owner and not by the uid of the creating process, if the underlying file system supports this feature: see chmod(2) and the suiddir option to mount(8).

mordae at mordae dot net
18-Jul-2005 07:19


It is possible to patch PHP/4 so it checks if just any directory in path is owned by proper user, e.g.

/home/mordae/www/dir/file

(where /home/mordae/www is mine and dir and file Apache's) will be readable if proper permissions set.

Read more at http://titov.net/safemodepatch/

jo at durchholz dot org
18-Jul-2005 01:18


Note that safe mode is largely useless. Most ISPs that offer Perl also offer other scripting languages (mostly Perl), and these other languages don't have the equivalent of PHP.

In other words, if PHP's safe mode won't allow vandals into your web presence, they will simply use Perl.

Also, safe mode prevents scripts from creating and using directories (because they will be owned by the WWW server, not by the user who uploaded the PHP script). So it's not only useless, it's also a hindrance.

The only realistic option is to bugger the Apache folks until they run all scripts as the user who's responsible for a given virtualhost or directory.

03-May-2005 09:37


To separate distinct open_basedir use : instead of on , or ; on unix machines.

plyrvt at mail dot ru
29-Apr-2005 03:14


[In reply to jedi at tstonramp dot com]

Safe mode is used "since the alternatives at the web server and OS levels aren't very realistic". Manual says about UNIX OS level and UNIX web-servers by design (Apache). It's not realistic for unix-like server, but for NT (IIS) each virtual host can run from different user account, so there is no need in Safe Mode restrictions at all, if proper NTFS rights are set.

bertrand dot gorge at epistema dot com
23-Sep-2004 05:58


Beware that when in safe mode, mkdir creates folders with apache's UID, though the files you create in them are of your script's UID (usually the same as your FTP uid).



What this means is that if you create a folder, you won't be able to remove it, nor to remove any of the files you've created in it (because those operations require the UID to be the same).



Ideally mkdir should create the folder with the script's UID, but this has already been discussed and will not be fixed in the near future. 



In the meantime, I would advice NOT to user mkdir in safe mode, as you may end up with folders you can't remove/use.

Russ
31-Aug-2004 08:52


Sometimes you're stuck on a system you don't run and you can't control the setting of safe mode.  If your script needs to run on different hosts (some with safe mode, some without it), you can code around it with something like:



<?php 

// Check for safe mode

if( ini_get('safe_mode') ){

    // Do it the safe mode way

}else{

    // Do it the regular way

}



?>

daniel dot gaddis at tlc dot state dot tx dot us
17-Mar-2004 11:03


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 01:32


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 09:38


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 06:44


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.

gtg782a at mail dot gatech dot edu
26-Jan-2003 09:14


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 11:25


For people using linux there is a very nice solution to the shared server problem. It's called vserver/ctx. Here is the URL: http://www.solucorp.qc.ca/miscprj/s_context.hc

10-Sep-2002 03:30


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 03:19


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 01:33


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.

zebz at ihaveenoughspam_hotmail dot com
28-Apr-2002 08:42


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 04:45


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
07-Sep-2001 07:17


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.

add a note