Tuesday, 28 September 2010

Full path disclosure tutorial

Full path disclosure(FPD) is the revelation of the full operating path of a vulnerable script. Full Path Disclosure vulnerabilities enable the attacker to see the path to the webroot/file. e.g.: /home/samar/public_html/. FPD bugs are executed by providing unexpected characters to the vulnerable functions that will in return output the full path of the vulnerable script.

FPD bugs are often overlooked and are not considered as the security threat by many webmasters but that's not true. FPD might be useful for the hackers to determine the structure of the server and they can utilize it to perform other attacks such as file inclusion attacks or load_file() attacks via sql injection.

How to execute FPD
a) Nulled session cookie
Nulled session injection or illegal session injection is done by changing the value of session cookie to an invalid or illegal character.
Illegal Session Injection is made possible via changing the value of the session cookie to an invalid, or illegal character. The most common method is by injecting the NULL character to the PHPSESSID cookie. To inject a PHPSESSID cookie, use JavaScript injection via the URL bar:

On setting the PHPSESSID cookie value to NULL, we can see the result like:

Warning: session_start() [function.session-start]: The session id contains illegal characters,
valid characters are a-z, A-Z, 0-9 and '-,' in /home/samar/public_html/includes/functions.php on line 3

b) Array parameter injection(Empty array)
This is another common method of executing the full path disclosure vulnerabilities and usually works for me in many sites. There are different PHP functions which will output warning message along with the full path of the script such as htmlentities(), mysql_num_rows(), opendir(), etc.
We can exploit the $_GET variables... Lets take a simple example:


Now, lets exploit the $_GET['page'] variable which will look as below:


The full path disclosure can be prevented by turning off the display of errors either in php.ini configuration file or in the script itself:

display_errors = 'off'

in php scripts


ini_set('display_errors', false);