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.
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!
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:
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.
This is to ensure that the user executing the wrapper is truly a user of the system.
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 contain a leading '/' or have a '..' backreference? These are not allowed; the target program must reside within the Apache webspace.
Does the target user exist?
Does the target group exist?
Presently, suEXEC does not allow 'root' to execute CGI/SSI programs.
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.
Presently, suEXEC does not allow the 'root' group to execute CGI/SSI programs.
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.
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.
If it doesn't exist, it can't very well contain files.
If the request is for a regular portion of the server, is the requested directory within the server's document root? If the request is for a UserDir, is the requested directory within the user's document root?
We don't want to open up the directory to others; only the owner user may be able to alter this directories contents.
If it doesn't exists, it can't very well be executed.
We don't want to give anyone other than the owner the ability to change the program.
We do not want to execute programs that will then change our UID/GID again.
Is the user the owner of the file?
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 configuration).
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 in mind.
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.
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
extra document.
The following sections describe the configuration and installation
for Apache 1.3 with the AutoConf-style interface (APACI).
APACI's suEXEC configuration options
--enable-suexec
--suexec-caller=UID
--suexec-docroot=DIR
--datadir=/home/apache" the directory
"/home/apache/htdocs" is used as document root for
the suEXEC wrapper.
--suexec-logfile=FILE
--suexec-userdir=DIR
--suexec-uidmin=UID
--suexec-gidmin=GID
--suexec-safepath=PATH
Checking your suEXEC setup
Before you compile and install the suEXEC wrapper you can check
the configuration with the --layout option.
Example output:
suEXEC setup:
suexec binary: /usr/local/apache/sbin/suexec
document root: /usr/local/apache/share/htdocs
userdir suffix: public_html
logfile: /usr/local/apache/var/log/suexec_log
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.
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.
Virtual Hosts:
One way to use the suEXEC wrapper is through the
User and
Group directives in
VirtualHost
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.
User directories:
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.
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.
NOTE! This section may not be complete. For the latest revision of this section of the documentation, see the Apache Group's Online Documentation version.
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.
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.)
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.
Again, this can cause Big Trouble if you try this without knowing what you are doing. Stay away from it if at all possible.