Apache suEXEC Support
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.
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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 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!
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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:
- 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.
- 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.
- 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 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.
- 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 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.
- Is the target group NOT the superuser group?
Presently, suEXEC does not allow the ‘root’ group to execute CGI/SSI programs.
- 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.
- 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.
- 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 directory within the server’s document root? If the request is for a UserDir, is the requested 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 else?
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 user/group?
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 configuration).
- 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.
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Configuring & Installing suEXEC
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 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.
--suexec-caller=UID The username under which Apache normally runs. This is the only user allowed to execute this program.
--suexec-docroot=DIR 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.
--suexec-logfile=FILE 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).
--suexec-userdir=DIR 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!
--suexec-uidmin=UID 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.
--suexec-gidmin=GID 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.
--suexec-safepath=PATH 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 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.
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Enabling & Disabling suEXEC
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.
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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 . If only one or neither of these directives are specified for a 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.
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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.
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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.
- 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.
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