The suEXEC feature provides users of the Apache HTTP Server 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 that certain assumptions are made about you and the environment in which you will be using suexec.
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 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 HTTP Server development team for consideration.
Fourth, and last, it has been the decision of the Apache HTTP Server development team to NOT make suEXEC part of the default installation of Apache httpd. 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 we hope 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 HTTP 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 httpd 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:
This is to ensure that the user executing the wrapper is truly a user of the system.
The wrapper will only execute if it is given the proper number of arguments. The proper argument format is known to the Apache HTTP 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 httpd binary.
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 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 root (see
Does the target user exist?
Does the target group exist?
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 we can't change directory to it, it might as well not exist.
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)?
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 CGI/SSI 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 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 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.
suEXEC configuration options
--with-suexec-xxxxxoption has to be provided together with the
--enable-suexecoption to let APACI accept your request for using the suEXEC feature.
suexecbinary must be hard-coded in the server for security reasons. Use this option to override the default path. e.g.
UserDirdirective (ie. one without a "*" in it) this should be set to the same value. suEXEC will not work properly in cases where the
UserDirdirective points to a location that is not the same as the user's home directory as referenced in the
passwdfile. Default value is "
UserDirfor 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!
UserDirs) that can be used for suEXEC behavior. The default directory is the
--datadirvalue 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.
suexec_log" and located in your standard logfile directory (
If you have enabled the suEXEC feature with the
--enable-suexec option the
suexec binary (together with httpd itself) is automatically built if you execute the
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. The default location is "/usr/local/apache2/bin/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.
Although the suEXEC wrapper will check to ensure that its caller is the correct user as specified with the
configure 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 httpd runs as may execute suEXEC.
If for example, your web server is configured to run as:
User www Group webgroup
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 httpd runs as can even execute the suEXEC wrapper.
Upon startup of httpd, it looks for the file
suexec in the directory defined by the
--sbindir option (default is "/usr/local/apache/sbin/suexec"). If httpd 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 HTTP Server is already running you must kill and restart httpd. Restarting it with a simple HUP or USR1 signal will not be enough.
If you want to disable suEXEC you should kill and restart httpd after you have removed the
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 is assumed.
Requests that are processed by
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
--with-suexec-userdir compile time option.
The suEXEC wrapper will write log information to the file defined with the
--with-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 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 httpd 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.
© 2017 The Apache Software Foundation
Licensed under the Apache License, Version 2.0.