RFI
Although not as common in modern applications, RFI vulnerabilities do still pop up from time to time. RFI was popular back in the early days of the web and PHP. PHP was notorious for allowing developers to implement features that were inherently dangerous. The include() and require() functions essentially allowed code to be included from other files, either on the same disk or over the wire. This makes web applications more powerful and dynamic but comes at a great cost. As you can imagine, allowing user data to pass to include() unsanitized can result in application or server compromise.
The danger of allowing remote files to be included in server-side code is pretty obvious. PHP will download the remote text and interpret it as code. If the remote URL is controlled by the attacker, they could easily feed the application a shell.
In the following example, the RFI vulnerability can be exploited using a simple system() passthrough shell. On the attacker-controlled c2.spider.ml server...
