Apache suEXEC Support

  1. What is suEXEC?
  2. Before we begin.
  3. suEXEC Security Model.
  4. Configuring & Installing suEXEC
  5. Enabling & Disabling suEXEC
  6. Debugging suEXEC
  7. Beware the Jabberwock: Warnings & Examples

What is suEXEC?

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 we begin.

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 is a manual process requiring of the administrator careful attention to details. It is through this process that the Apache Group hopes to limit suEXEC installation only to those who are determined to use it.

Still with us? Yes? Good. Let's move on!


suEXEC Security Model

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:

  1. Was the wrapper called with the proper number of arguments?
    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.
  2. Is the user executing this wrapper a valid user of this system?
    This is to ensure that the user executing the wrapper is truly a user of the system.
  3. 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.
  4. Does the target program have an unsafe hierarchical reference?
    Does the target program contain a leading '/' or have a '..' backreference? These are not allowed; the target program must reside within the Apache webspace.
  5. Is the target user name valid?
    Does the target user exist?
  6. Is the target group name valid?
    Does the target group exist?
  7. Is the target user NOT superuser?
    Presently, suEXEC does not allow 'root' to execute CGI/SSI programs.
  8. Is the target userid ABOVE the minimum ID number?
    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.
  9. Is the target group NOT the superuser group?
    Presently, suEXEC does not allow the 'root' group to execute CGI/SSI programs.
  10. Is the target groupid ABOVE the minimum ID number?
    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.
  11. Can the wrapper successfully become the target user and group?
    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.
  12. Does the directory in which the program resides exist?
    If it doesn't exist, it can't very well contain files.
  13. Is the directory within the Apache webspace?
    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?
  14. 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.
  15. Does the target program exist?
    If it doesn't exists, it can't very well be executed.
  16. Is the target program NOT writable by anyone else?
    We don't want to give anyone other than the owner the ability to change the program.
  17. Is the target program NOT setuid or setgid?
    We do not want to execute programs that will then change our UID/GID again.
  18. Is the target user/group the same as the program's user/group?
    Is the user the owner of the file?
  19. 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 configuration).
  20. Can we successfully become the target program and execute?
    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.


Configuring & Installing suEXEC

Here's where we begin the fun. The configuration and installation of suEXEC is a four step process: edit the suEXEC header file, compile suEXEC, place the suEXEC binary in its proper location, and configure Apache for use with suEXEC.

- From the top-level of the Apache source tree, type:   cd support [ENTER]

Edit the suexec.h file and change the following macros to match your local Apache installation.

From support/suexec.h /* * HTTPD_USER -- Define as the username under which Apache normally * runs. This is the only user allowed to execute * this program. */ #define HTTPD_USER "www" /* * UID_MIN -- Define this as the lowest UID allowed to be a target user * for suEXEC. For most systems, 500 or 100 is common. */ #define UID_MIN 100 /* * GID_MIN -- Define this as the lowest GID allowed to be a target group * for suEXEC. For most systems, 100 is common. */ #define GID_MIN 100 /* * USERDIR_SUFFIX -- 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. * * If you have VirtualHosts 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! * See the suEXEC documentation for more detailed * information. */ #define USERDIR_SUFFIX "public_html" /* * LOG_EXEC -- Define this as a filename if you want all suEXEC * transactions and errors logged for auditing and * debugging purposes. */ #define LOG_EXEC "/usr/local/etc/httpd/logs/cgi.log" /* Need me? */ /* * DOC_ROOT -- Define as the DocumentRoot set for Apache. This * will be the only hierarchy (aside from UserDirs) * that can be used for suEXEC behavior. */ #define DOC_ROOT "/usr/local/etc/httpd/htdocs" /* * SAFE_PATH -- Define a safe PATH environment to pass to CGI executables. * */ #define SAFE_PATH "/usr/local/bin:/usr/bin:/bin"

You now need to compile the suEXEC wrapper. At the shell command prompt, type:  cc suexec.c -o suexec [ENTER]. This should create the suexec wrapper executable.

By default, Apache is compiled to look for the suEXEC wrapper in the following location.

From src/httpd.h /* The path to the suEXEC wrapper */ #define SUEXEC_BIN "/usr/local/etc/httpd/sbin/suexec"

If your installation requires location of the wrapper program in a different directory, edit src/httpd.h and recompile your Apache server. See Compiling and Installing Apache for more info on this process.

Copy the suexec executable created in the exercise above to the defined location for SUEXEC_BIN.

cp suexec /usr/local/etc/httpd/sbin/suexec [ENTER]

In order for the wrapper to set the user ID, it must me installed as owner root and must have the setuserid execution bit set for file modes. If you are not running a root user shell, do so now and execute the following commands.

chown root /usr/local/etc/httpd/sbin/suexec [ENTER]
chmod 4711 /usr/local/etc/httpd/sbin/suexec [ENTER]


Enabling & Disabling suEXEC

After properly installing the suexec wrapper executable, you must kill and restart the Apache server. A simple kill -1 `cat httpd.pid` will not be enough. Upon startup of the web-server, if Apache finds a properly configured suexec wrapper, it will print the following message to the console:

Configuring Apache for use with suexec wrapper.

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. Check your installation and try again.

One way to use suEXEC 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.

suEXEC can also be used to 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.


Debugging suEXEC

The suEXEC wrapper will write log information to the location defined in the suexec.h 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.


Beware the Jabberwock: Warnings & Examples

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.