Suexec and Apache: A Tutorial
The Apache Web server, like most if not all of the others in common use today, lets you execute arbitrarily complex operations through the use of CGI scripts. These can involve database lookups, system administration functions, real-time control of machinery, online payments, or almost anything else you can think of.When you're running an Apache Web server for yourself, you don't think about the user that's running the Apache server itself (typically nobody on Linux systems). But what if you're an ISP with multiple companies being hosted on your system? Or an educational institution with faculty who want to be able to execute their own scripts?
Ordinarily, all of these things occur in the context of the user running
the Apache server itself (typically
nobody on Linux systems). This
is fine when you're using a system that is owned and used by a single
entity...but what if you're an ISP with multiple companies being hosted on your
system? Or an educational institution with faculty who want to be able to
execute their own scripts? Either everything has to be accessible to the Apache
nobody user, or you have to run multiple instances of Apache on
multiple ports and IP addresses, one of each per user, with the
concomitant confusion of configuration files.
On the other hand, if the server is to be allowed to change its identity,
it needs to be done in a controlled manner, so that the chance of compromising
your system's security is kept to a minimum. (Remember, Apache is usually
root and only changes to
suexec (pronounced 'SUE-ex-Ek') tool helps
make this possible. It's found in the
src/support/ directory under
your Apache source tree.
Assumptions in This Article
For the rest of this article, I'm going to make the following assumptions:
your Apache source tree starts at
your Apache ServerRoot is
your Apache DocumentRoot is
the username under which Apache runs (the value of the
Userdirective in your
All of the
cd and other shell commands in this article that
refer to directories use these locations.
How Does Suexec Work?
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,
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.
suexecmust 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
suexecmust be a valid user; that is, it must be listed in the
/etc/passwdfile. 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
suexecin other than the prescribed way.