Modern applications rarely exist as their own unit in today’s connected world. Instead, a single application may be compromised of multiple services, expanding across different networks and machines. Large databases generally operate on their own systems, while the application logic happens on another. Now, with the widespread adoption of cloud services, certain components may be operated not only on different networks and machines but by entirely independent organizations.

Given this, it is not uncommon for applications to offer capabilities that allow them to generate requests and retrieve data from separate systems. While this operation can be used to access and initiate complex processes on a cloud service provider, it can also provide users with basic features designed to improve the overall application experience.

Think, for example, a profile picture. We could ask users to download a picture they like and upload it to our application. However, if it’s already on the web, why not just let them give us a URL? With careful tuning, we can create an easy-to-use feature to streamline user customization!

However, as with everything application-wise, we need to be conscious of the security of such functions. What happens if a user makes a request to a file that isn’t a picture? What happens if they enter a URL designed to access a restricted resource such as an administrative panel?

If we’re not careful, we can easily introduce a server-side request forgery (SSRF) vulnerability within our application. SSRF is an incredibly serious vulnerability that occurs when an attacker can force a server-side application to make arbitrary requests on our behalf. Depending on how an application is configured, the impacts can range from wasted bandwidth to a full system compromise!

In the coming examples, we’ll see the potential impact of such issues and several basic solutions we can use to prevent this vulnerability within our software.

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