This is a code execution bug in the combination of Apache and PHP. On Debian and Ubuntu the vulnerability is present in the default install of the php5-cgi package. When the php5-cgi package is installed on Debian and Ubuntu or php-cgi is installed manually the php-cgi binary is accessible under /cgi-bin/php5 and /cgi-bin/php. The vulnerability makes it possible to execute the binary because this binary has a security check enabled when installed with Apache http server and this security check is circumvented by the exploit. When accessing the php-cgi binary the security check will block the request and will not execute the binary. In the source code file sapi/cgi/cgi_main.c of PHP we can see that the security check is done when the php.ini configuration setting cgi.force_redirect is set and the php.ini configuration setting cgi.redirect_status_env is set to no. This makes it possible to execute the binary bypassing the Security check by setting these two php.ini settings. Prior to this code for the Security check getopt is called and it is possible to set cgi.force_redirect to zero and cgi.redirect_status_env to zero using the -d switch. If both values are set to zero and the request is sent to the server php-cgi gets fully executed and we can use the payload in the POST data field to execute arbitrary php and therefore we can execute programs on the system.