The suEXEC feature provides
Apache users the ability
to run CGI and SSI programs
under user IDs different from the user ID of the calling
web-server. Normally, when a CGI or SSI program executes, it
runs as the same user who is running the web server.
Used properly, this feature can reduce
considerably the security risks involved with allowing users to
develop and run private CGI or SSI programs. However, if suEXEC
is improperly configured, it can cause any number of problems
and possibly create new holes in your computer's security. If
you aren't familiar with managing setuid root programs
and the security issues they present, we highly recommend that
you not consider using suEXEC.
Before jumping head-first into this document,
you should be aware of the assumptions made on the part of the
Apache Group and this document.
First, it is assumed that you are using a UNIX
derivative operating system that is capable of
setuid and setgid operations.
All command examples are given in this regard. Other platforms,
if they are capable of supporting suEXEC, may differ in their
Second, it is assumed you are familiar with
some basic concepts of your computer's security and its
administration. This involves an understanding of
setuid/setgid operations and the various
effects they may have on your system and its level of
Third, it is assumed that you are using an
unmodified version of suEXEC code. All code
for suEXEC has been carefully scrutinized and tested by the
developers as well as numerous beta testers. Every precaution
has been taken to ensure a simple yet solidly safe base of
code. Altering this code can cause unexpected problems and new
security risks. It is highly recommended you
not alter the suEXEC code unless you are well versed in the
particulars of security programming and are willing to share
your work with the Apache Group for consideration.
Fourth, and last, it has been the decision of
the Apache Group to NOT make suEXEC part of
the default installation of Apache. To this end, suEXEC
configuration requires of the administrator careful attention
to details. After due consideration has been given to the
various settings for suEXEC, the administrator may install
suEXEC through normal installation methods. The values for
these settings need to be carefully determined and specified by
the administrator to properly maintain system security during
the use of suEXEC functionality. It is through this detailed
process that the Apache Group hopes to limit suEXEC
installation only to those who are careful and determined
enough to use it.
Still with us? Yes? Good. Let's move on!
Before we begin configuring and installing
suEXEC, we will first discuss the security model you are about
to implement. By doing so, you may better understand what
exactly is going on inside suEXEC and what precautions are
taken to ensure your system's security.
suEXEC is based on a setuid
"wrapper" program that is called by the main Apache web server.
This wrapper is called when an HTTP request is made for a CGI
or SSI program that the administrator has designated to run as
a userid other than that of the main server. When such a
request is made, Apache provides the suEXEC wrapper with the
program's name and the user and group IDs under which the
program is to execute.
The wrapper then employs the following process
to determine success or failure -- if any one of these
conditions fail, the program logs the failure and exits with an
error, otherwise it will continue:
Is the user executing this wrapper a valid user of
This is to ensure that the user executing the wrapper is
truly a user of the system.
Was the wrapper called with the proper number of
The wrapper will only execute if it is given the proper
number of arguments. The proper argument format is known
to the Apache web server. If the wrapper is not receiving
the proper number of arguments, it is either being
hacked, or there is something wrong with the suEXEC
portion of your Apache binary.
Is this valid user allowed to run the
Is this user the user allowed to run this wrapper? Only
one user (the Apache user) is allowed to execute this
Does the target CGI or SSI program have an unsafe
Does the target CGI or SSI program's path contain a leading
'/' or have a '..' backreference? These are not allowed; the
target CGI/SSI program must reside within suEXEC's document
Is the target user name valid?
Does the target user exist?
Is the target group name valid?
Does the target group exist?
Is the target user NOT superuser?
Presently, suEXEC does not allow
to execute CGI/SSI programs.
Is the target userid ABOVE the minimum ID
The minimum user ID number is specified during
configuration. This allows you to set the lowest possible
userid that will be allowed to execute CGI/SSI programs.
This is useful to block out "system" accounts.
Is the target group NOT the superuser
Presently, suEXEC does not allow the
group to execute CGI/SSI programs.
Is the target groupid ABOVE the minimum ID
The minimum group ID number is specified during
configuration. This allows you to set the lowest possible
groupid that will be allowed to execute CGI/SSI programs.
This is useful to block out "system" groups.
Can the wrapper successfully become the target user
Here is where the program becomes the target user and
group via setuid and setgid calls. The group access list
is also initialized with all of the groups of which the
user is a member.
Can we change directory to the one in which the target
CGI/SSI program resides?
If it doesn't exist, it can't very well contain files. If we
can't change directory to it, it might aswell not exist.
Is the directory within the Apache
If the request is for a regular portion of the server, is
the requested directory within suEXEC's document root? If
the request is for a UserDir, is the requested directory
within the directory configured as suEXEC's userdir (see
suEXEC's configuration options)?
Is the directory NOT writable by anyone
We don't want to open up the directory to others; only
the owner user may be able to alter this directories
Does the target CGI/SSI program exist?
If it doesn't exists, it can't very well be executed.
Is the target CGI/SSI program NOT writable
by anyone else?
We don't want to give anyone other than the owner the
ability to change the CGI/SSI program.
Is the target CGI/SSI program NOT setuid or
We do not want to execute programs that will then change
our UID/GID again.
Is the target user/group the same as the program's
Is the user the owner of the file?
Can we successfully clean the process environment
to ensure safe operations?
suEXEC cleans the process' environment by establishing a
safe execution PATH (defined during configuration), as
well as only passing through those variables whose names
are listed in the safe environment list (also created
Can we successfully become the target CGI/SSI program
Here is where suEXEC ends and the target CGI/SSI program begins.
This is the standard operation of the
suEXEC wrapper's security model. It is somewhat stringent and
can impose new limitations and guidelines for CGI/SSI design,
but it was developed carefully step-by-step with security in
For more information as to how this security
model can limit your possibilities in regards to server
configuration, as well as what security risks can be avoided
with a proper suEXEC setup, see the "Beware the Jabberwock" section of this
Here's where we begin the fun.
- This option enables the suEXEC feature which is never
installed or activated by default. At least one
--with-suexec-xxxxx option has to be provided
together with the
--enable-suexec option to let
APACI accept your request for using the suEXEC feature.
- The path to the
suexec binary must be hard-coded
in the server for security reasons. Use this option to override
the default path. e.g.
- The username under which
Apache normally runs. This is the only user allowed to
execute this program.
- Define to be the subdirectory under users' home
directories where suEXEC access should be allowed. All
executables under this directory will be executable by suEXEC
as the user so they should be "safe" programs. If you are
using a "simple" UserDir directive (ie. one without a "*" in
it) this should be set to the same value. suEXEC will not
work properly in cases where the UserDir directive points to
a location that is not the same as the user's home directory
as referenced in the passwd file. Default value is
If you have virtual hosts with a different UserDir for each,
you will need to define them to all reside in one parent
directory; then name that parent directory here. If
this is not defined properly, "~userdir" cgi requests will
- Define as the DocumentRoot set for Apache. This will be
the only hierarchy (aside from UserDirs) that can be used for
suEXEC behavior. The default directory is the
value with the suffix "/htdocs", e.g. if you configure
--datadir=/home/apache" the directory
"/home/apache/htdocs" is used as document root for the suEXEC
- Define this as the lowest UID allowed to be a target user
for suEXEC. For most systems, 500 or 100 is common. Default
value is 100.
- Define this as the lowest GID allowed to be a target
group for suEXEC. For most systems, 100 is common and
therefore used as default value.
- This defines the filename to which all suEXEC
transactions and errors are logged (useful for auditing and
debugging purposes). By default the logfile is named
"suexec_log" and located in your standard logfile directory
- Define a safe PATH environment to pass to CGI
executables. Default value is
Compiling and installing the suEXEC
If you have enabled the suEXEC feature with the
--enable-suexec option the
(together with Apache itself) is automatically built if you execute
After all components have been built you can execute the
make install to install them. The binary image
suexec is installed in the directory defined by the
--sbindir option. The default location is
Please note that you need root
privileges for the installation step. In order
for the wrapper to set the user ID, it must be installed as
root and must have the setuserid
execution bit set for file modes.
Setting paranoid permissions
Although the suEXEC wrapper will check to ensure that its
caller is the correct user as specified with the
option, there is
always the possibility that a system or library call suEXEC uses
before this check may be exploitable on your system. To counter
this, and because it is best-practise in general, you should use
filesystem permissions to ensure that only the group Apache
runs as may execute suEXEC.
If for example, your web-server is configured to run as:
suexec is installed at
"/usr/local/apache2/bin/suexec", you should run:
chgrp webgroup /usr/local/apache2/bin/suexec
chmod 4750 /usr/local/apache2/bin/suexec
This will ensure that only the group Apache runs as can even
execute the suEXEC wrapper.
Upon startup of Apache, it looks for the file
suexec in the directory defined by the
--sbindir option (default is
"/usr/local/apache/bin/suexec"). If Apache finds a properly
configured suEXEC wrapper, it will print the following message
to the error log:
[notice] suEXEC mechanism enabled (wrapper: /path/to/suexec)
If you don't see this message at server startup, the server is
most likely not finding the wrapper program where it expects
it, or the executable is not installed setuid root.
If you want to enable the suEXEC mechanism for the first time
and an Apache server is already running you must kill and
restart Apache. Restarting it with a simple HUP or USR1 signal
will not be enough.
If you want to disable suEXEC you should kill and restart
Apache after you have removed the
Requests for CGI programs will call the suEXEC wrapper only if
they are for a virtual host containing a
SuexecUserGroup directive or if
they are processed by
One way to use the suEXEC
wrapper is through the
SuexecUserGroup directive in
VirtualHost definitions. By
setting this directive to values different from the main server
user ID, all requests for CGI resources will be executed as the
User and Group defined for that
<VirtualHost>. If this
directive is not specified for a
<VirtualHost> then the main server userid
Requests that are
mod_userdir will call the suEXEC
wrapper to execute CGI programs under the userid of the requested
user directory. The only requirement needed for this feature to
work is for CGI execution to be enabled for the user and that the
script must meet the scrutiny of the security
checks above. See also the
The suEXEC wrapper will write log information
to the file defined with the
option as indicated above. If you feel you have configured and
installed the wrapper properly, have a look at this log and the
error_log for the server to see where you may have gone astray.
NOTE! This section may not be
complete. For the latest revision of this section of the
documentation, see the Apache Group's Online
There are a few points of interest regarding
the wrapper that can cause limitations on server setup. Please
review these before submitting any "bugs" regarding suEXEC.
- suEXEC Points Of Interest
For security and efficiency reasons, all suEXEC requests
must remain within either a top-level document root for
virtual host requests, or one top-level personal document
root for userdir requests. For example, if you have four
VirtualHosts configured, you would need to structure all
of your VHosts' document roots off of one main Apache
document hierarchy to take advantage of suEXEC for
VirtualHosts. (Example forthcoming.)
suEXEC's PATH environment variable
This can be a dangerous thing to change. Make certain
every path you include in this define is a
trusted directory. You don't want to
open people up to having someone from across the world
running a trojan horse on them.
Altering the suEXEC code
Again, this can cause Big Trouble if you
try this without knowing what you are doing. Stay away
from it if at all possible.