Apache suEXEC Support
- What is suEXEC?
- Before we begin.
- suEXEC Security Model.
- Configuring & Installing suEXEC
- Enabling & Disabling suEXEC
- Using suEXEC
- Debugging suEXEC
- Beware the Jabberwock: Warnings &
The suEXEC feature -- introduced in Apache 1.2 -- 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.
BACK TO CONTENTS
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 derivate 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 configuration.
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 security.
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!
BACK TO CONTENTS
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:
- 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
is not receiving the proper number of arguments, it is either being hacked,
there is something wrong with the suEXEC portion of your Apache binary.
- Is the user executing this wrapper a valid user of this
This is to ensure that the user executing the wrapper is truly a user of the
- Is this valid user allowed to run the wrapper?
Is this user the user allowed to run this wrapper? Only one user (the
Apache user) is allowed to execute this program.
- Does the target program have an unsafe hierarchical
Does the target program contain a leading '/' or have a '..' backreference?
These are not allowed; the target program must reside within the Apache
- 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 'root' to execute CGI/SSI programs.
- Is the target userid ABOVE the minimum ID
The minimum user ID number is specified during configuration. This allows
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 group?
Presently, suEXEC does not allow the 'root' group to execute CGI/SSI
- Is the target groupid ABOVE the minimum ID
The minimum group ID number is specified during configuration. This allows
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 and
Here is where the program becomes the target user and group via setuid and
calls. The group access list is also initialized with all of the groups
the user is a member.
- Does the directory in which the program resides exist?
If it doesn't exist, it can't very well contain files.
- Is the directory within the Apache webspace?
If the request is for a regular portion of the server, is the requested
within the server's document root? If the request is for a UserDir, is
directory within the user's document root?
- Is the directory NOT writable by anyone else?
We don't want to open up the directory to others; only the owner user
may be able
to alter this directories contents.
- Does the target program exist?
If it doesn't exists, it can't very well be executed.
- Is the target program NOT writable by anyone
We don't want to give anyone other than the owner the ability to
change the program.
- Is the target program NOT setuid or setgid?
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 during
- Can we successfully become the target program and
Here is where suEXEC ends and the target program begins.
This is the standard operation of the 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
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 document.
BACK TO CONTENTS
Here's where we begin the fun. If you use Apache 1.2 or prefer to configure
Apache 1.3 with the "
src/Configure" script you have to edit
the suEXEC header file and install the binary in its proper location
manually. This procedure is described in an
The following sections describe the configuration and installation
for Apache 1.3 with the AutoConf-style interface (APACI).
APACI's suEXEC configuration options
- This option enables the suEXEC feature which is never installed or
activated by default. At least one --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 username under which
Apache normally runs.
This is the only user allowed to execute this program.
- 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 --datadir value with
the suffix "/htdocs", e.g. if you configure with
--datadir=/home/apache" the directory
"/home/apache/htdocs" is used as document root for
the suEXEC wrapper.
- 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 (--logfiledir).
- 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 "public_html".
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 not work!
- 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.
- Define a safe PATH environment to pass to CGI executables.
Default value is "/usr/local/bin:/usr/bin:/bin".
Checking your suEXEC setup
Before you compile and install the suEXEC wrapper you can check
the configuration with the --layout option.
suexec binary: /usr/local/apache/sbin/suexec
document root: /usr/local/apache/share/htdocs
userdir suffix: public_html
safe path: /usr/local/bin:/usr/bin:/bin
caller ID: www
minimum user ID: 100
minimum group ID: 100
Compiling and installing the suEXEC wrapper
If you have enabled the suEXEC feature with the --enable-suexec option
the suexec binary (together with Apache itself) is automatically built
if you execute the command "make".
After all components have been built you can execute the command
"make install" to install them.
The binary image "suexec" is installed in the directory defined by
the --sbindir option. Default location is "/usr/local/apache/sbin/suexec".
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 owner
root and must have the
setuserid execution bit set for file modes.
BACK TO CONTENTS
Upon startup of Apache, it looks for the file "suexec" in the "sbin"
directory (default is "/usr/local/apache/sbin/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 "suexec" file.
BACK TO CONTENTS
One way to use the suEXEC wrapper is through the
Group directives in
definitions. By setting these directives 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 only one or
neither of these directives are specified for a
<VirtualHost> then the main
server userid is assumed.
The suEXEC wrapper can also be used to execute CGI programs as
the user to which the request is being directed. This is accomplished by
using the "
~" character prefixing the user
ID for whom execution is desired.
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.
BACK TO CONTENTS
The suEXEC wrapper will write log information to the file defined
with the --suexec-logfile 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.
BACK TO CONTENTS
NOTE! This section may not be complete. For the latest
revision of this section of the documentation, see the Apache Group's
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
- Hierarchy limitations
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.
BACK TO CONTENTS