Description
PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML. When PHP is used in a CGI-based setup (such as Apache's mod_cgid), the php-cgi receives a processed query string parameter as command line arguments which allows command-line switches, such as -s, -d or -c to be passed to the php-cgi binary, which can be exploited to disclose source code and obtain arbitrary code execution.
An example of the -s command, allowing an attacker to view the source code of index.php is below:
http://localhost/index.php?-s
Remediation
An alternative is to configure your web server to not let these types of requests with query strings starting with a "-" and not containing a "=" through. Adding a rule like this should not break any sites. For Apache using mod_rewrite it would look like this:
RewriteCond %{QUERY_STRING} ^(%2d|-)[^=]+$ [NC]
RewriteRule ^(.*) $1? [L]