Using PHP as a CGI binary is an option for
setups that for some reason do not wish to integrate PHP as a
module into server software (like Apache), or will use PHP with
different kinds of CGI wrappers to create safe chroot and setuid
environments for scripts. This setup usually involves installing
executable PHP binary to the web server cgi-bin directory. CERT
advisory CA-96.11 recommends
against placing any interpreters into cgi-bin. Even if the PHP
binary can be used as a standalone interpreter, PHP is designed
to prevent the attacks this setup makes possible:
Accessing system files: http://my.host/cgi-bin/php?/etc/passwd
The query information in a URL after the question mark (?) is
passed as command line arguments to the interpreter by the CGI
interface. Usually interpreters open and execute the file
specified as the first argument on the command line.
When invoked as a CGI binary, PHP refuses to interpret the
command line arguments.
Accessing any web document on server: http://my.host/cgi-bin/php/secret/doc.html
The path information part of the URL after the PHP binary name,
conventionally used to specify the name of the file to be
opened and interpreted by the CGI program.
Usually some web server configuration directives (Apache:
Action) are used to redirect requests to documents like
http://my.host/secret/script.php to the
PHP interpreter. With this setup, the web server first checks
the access permissions to the directory /secret, and after that creates the
redirected request http://my.host/cgi-bin/php/secret/script.php.
Unfortunately, if the request is originally given in this form,
no access checks are made by web server for file /secret/script.php, but only for the
/cgi-bin/php file. This way
any user able to access /cgi-bin/php is able to access any
protected document on the web server.
In PHP, compile-time configuration option --enable-force-cgi-redirect
and runtime configuration directives doc_root and user_dir can be used to prevent
this attack, if the server document tree has any directories
with access restrictions. See below for full the explanation
of the different combinations.