April 18, 2019

Using Apache with Suexec on Linux - page 2

Executing CGI Scripts as Other Users

  • January 21, 2000
  • By Ken Coar

Since suexec works by wrapping an operation up in a package executed under a different username, it's called a wrapper. In order to execute a script under the auspices of the wrapper, the Apache server creates a child process running the suexec binary and passes the particulars to it. The wrapper verifies that all the security requirements are met, edits the list of environment variables so that only the ones on its 'trusted' list are available, closes its logfile, and calls some flavour of execv(2) to load the script into the edited process environment, replacing suexec itself.

Requirements For suexec Operation
Since suexec is used to run applications on your system on behalf of arbitrary people out on the Web, it's very paranoid about doing anything that might compromise your system's security. Here is a list of the conditions suexec requires to be met before it will proceed; if any don't measure up, the wrapper will log an error and not execute the script.

  • suexec must be invoked with the correct number of arguments. If it isn't, it assumes someone is trying to penetrate your system by running it outside the Apache environment.
  • The username/UID invoking suexec must be a valid user; that is, it must be listed in the /etc/passwd file. If it isn't, something's not quite right--and when in doubt, punt.
  • The username executing the wrapper must be the one that was compiled into it when it was built. Again, a mismatch here is interpreted as someone trying to use suexec in other than the prescribed way.
  • The requested script must be a valid Web-space reference relative to the user's directory or the DocumentRoot; it cannot be an absolute filesystem path (i.e., it cannot start with a "/") and cannot include any up-level references (i.e., no "../" references either).
  • The username and group under which the script is to be run must be valid, cannot be 'root', and must be above the minimum UID and GID values (set with the --suexecuidmin and --suexec-gidmin options to the configure script, which both default to 100). In addition, the group must be a valid name, and not just a numeric GID.
  • The wrapper must be able to change its idenity to the requested username and group.
  • The script (and obviously the directory in which it lives) must actually exist and the wrapper must be able to chdir() to the directory.
  • If the script isn't from a ~username request, the script directory must be under the directory specified by DOC_ROOT (defined by the --suexec-docroot option to configure).
  • The permissions on the specified script and its parent directory must not allow write access to either the group or the other categories.
  • The script file cannot be setuid or setgid.
  • The script and the directory must be owned by the user and group as which it is to be executed.
  • The script must be executable by the user.
  • suexec must be able to allocate memory in which to reproduce the environment variable list.

As you can see, the requirements for execution are pretty stringent. The sheer number of things that can go wrong argues for the use of the wrapper only when it's really necessary.

Enabling suexec

The suexec wrapper isn't turned on or off by any particular Apache directive setting. Instead, when the Apache server is compiled, one of the constants set (SUEXEC_BIN) is a string pointing to the location of the suexec binary. When the server starts, it looks for the binary at that location; if it's found, suexec is enabled--not otherwise. This is very important.

This means that even a normal Apache build that was performed without any thought given to using the wrapper can suddenly become suexec-enabled if a properly protected suexec binary is put into place between server restarts. In the master sources, the default value of SUEXEC_BIN is set to "$HTTPD_ROOT/sbin/suexec"; the default value of HTTPD_ROOT is platform-specific:

Platform Default value of HTTPD_ROOT Resulting default SUEXEC_BIN value
OS/2 /os2httpd /os2httpd/sbin/suexec
Windows /apache /apache/sbin/suexec
BeOS /boot/home/apache /boot/home/apache/sbin/suexec
Novell NetWare sys:/apache sys:/apache/sbin/suexec
All others /usr/local/apache /usr/local/apache/sbin/suexec

You may change the values of either--or both--of the HTTPD_ROOT and SUEXEC_BIN constants when you recompile the Apache server.

If Apache does find the wrapper, it reports it in the server error log like this:

[Thu Dec 30 01:24:43 1999] [notice] suEXEC mechanism enabled (wrapper:�/usr/local/web/apache/bin/suexec)

Up until Apache version 1.3.11, there was no way to be sure where a compiled Apache server is going to be looking for the suexec binary. As of 1.3.11, though, it's part of the 'compiled modules' report displayed by the '-l' switch:

    % /usr/local/web/apache/bin/httpd -l
    Compiled-in modules:
    suexec: enabled; valid wrapper /usr/local/web/apache/bin/suexec

The 'enabled; valid' notation means that the wrapper is actually present in the indicated location, and the permissions are correct. If the wrapper isn't there, or the permissions are wrong, the output will indicate that suexec is disabled.

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