Server Side Request Forgery (SSRF)

Server Side Request Forgery or SSRF is where an attacker is able to cause a web application to send a request that the attacker defines.

For example, say there is a website that lets you take a screenshot of any site on the internet.

Under normal usage a user might ask it to take a screenshot of a page like Google, or The New York Times. But what if a user does something more nefarious? What if they asked the site to take a picture of http://localhost ? Or perhaps tries to access something more useful like http://localhost/server-status ?

Note (also known as localhost or loopback) represents the computer itself. Accessing localhost means you are accessing the computer's own internal network. Developers often use localhost as a way to access the services they have running on their own computers.

Depending on what the response from the site is the attacker may be able to gain additional information about what's running on the computer itself.

In addition, the requests originating from the server would come from the server's IP not the attackers IP. Because of that, it is possible that the attacker might be able to access internal resources that he wouldn't normally be able to access.

Another usage for SSRF is to create a simple port scanner to scan the internal network looking for internal services.