Suexec and Apache: A Tutorial Page 2

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
“/sbin/suexec“; the default value of
HTTPD_ROOT is platform-specific:

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