A Server-Side Request Forgery (a.k.a. SSRF) is a web vulnerability allowing attackers to make the server-side application do certain requests. This vulnerability can lead to unauthorized actions, Sensitive Information Disclosure and even RCE (Remote Code Execution).
SSRF is very similar to file inclusion since both vulnerabilities can be exploited to access external or internal content. The difference resides in the fact that file inclusion vulnerabilities rely on code inclusion functions (e.g. include() in PHP) while SSRF ones on functions that only handle data (e.g. fopen() in PHP). RFI vulnerabilities will lead to RCE much more often and easily that SSRF ones.
Testers need to find input vectors and fields that could be used for publishing or importing data from a URL (e.g. GET and POST parameters).
With http://some.website/index.php?url=https://someother.website/index.php, and url being the vulnerable parameter, the following basic payloads can help a tester fetch content of files, scan ports, access filtered resources and so on.
SSRFMap (Python) is a tool used to ease the exploitation of SSRFs. Gopherus (Python) can be used to gain RCE (Remote Code Execution) by generating Gopher payloads.
What is SSRF (Server-side request forgery)? Tutorial & Examples | Web Security Academy
Server Side Request Forgery Software Attack | OWASP Foundation
PayloadsAllTheThings/Server Side Request Forgery at master · swisskyrepo/PayloadsAllTheThings