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In an ideal world, vulnerabilities would not exist. A request would be sent to a server, properly validated, and only the intended information would be provided by the server. Of course, this is not a perfect world, and vulnerabilities can be introduced unintentionally, or even found due to previously unknown weaknesses within the programming language.
One type of vulnerability that can have severe consequences if exploited, yet is not frequently talked about, is Server Side Request Forgery (SSRF).
What is Server Side Request Forgery?