Janos Pasztor

Security: What is Server Side Request Forgery?

CSRF is, while still prevalent, no longer an unknown type of security vulnerability. It’s brother, SSRF, however, is. Most developers, or even DevOps engineers don’t know about it. So how does it work?

SSRF explained

SSRF is, at it’s core, very simple. Let’s say you have a web application that needs to download some data from external sources. So we trick this web application to, instead of accessing the external site, download the data from an internal server that is publicly not accessible.

Sounds unimpressive, right? However, keep in mind that more and more database, like redis, etcd, or ElasticSearch can be accessed over HTTP. What if I trick the application to download some data from etcd? Maybe your AWS API keys are stored in there and I can just grab them?

PlantUML SVG diagram

Figure: How SSRF works

Or, if I can trick the application to make a PUT or POST request instead of a GET request, I may even be able to change data.

Where SSRF happens

Now, you may be asking in what kind of scenarios SSRF can even happen. After all, not many web applications download data from external sources at the request of the user?

You may think that, but just a few obscure examples include:

  • Downloading an RSS feed or other data source from a remote server.
  • Displaying snippets or previews of links.
  • Partial code execution vulnerabilities in the application.

Even if your application initially doesn’t download data from external sources, later on there may be a business requirement and you may not think of it.

Additionally, even a developer could be tricked into opening a link that, under certain circumstances, could load data from the internal service.

A practical example

Let’s look at a practical example. Many social networks nowadays integrate OpenGraph as a method to render previews such as this:

Image: How Facebook renders previews

Now, think about how this works: the user pastes a link and some server side component needs to download the information from the server. Assuming that your etcd database lives on 10.2.0.1. What is the user pastes this link?

http://10.2.0.1:2379/v2/keys/amazon-api-key

If your etcd doesn’t use authentication, your application will happily download the super secret API key from the etcd server and display it to the user.

Or, let’s say you are running on AWS. What if the user enters the address http://169.254.169.254/latest/user-data/? (This is the AWS metadata API that lets you access various things about the instance the requesting code is running on.) Or, even more devious, the user sets up a domain that points to this IP address? This will let the user access your user data, which may contain sensitive data.

Defense against SSRF

I’ve talked about layered security before, so my recommendations follow that principle. You will want to have more than method of defense.

First of all, all internal services, no matter if they are behind a firewall or not, should be put behind some sort of authentication. Sometimes that will be usernames and passwords, sometimes it will be TLS client certificates. (TLS certificates will also enable encryption of the traffic.) If you use authentication, that will prevent clients that don’t have access to the authentication data to connect the service.

Second, you should (of course) set up an internal firewall between your application servers and internal services. If you are using Kubernetes, you should set up a network policy.

And last, your application should vet the URL. At the very least it should check what IP address the URL resolves to.

Conclusion

SSRF is a tricky vulnerability because it abuses a trusted component of your system: the application code. It’s hard to think of our own components as a potential threat.

Janos Pasztor

I'm a DevOps engineer with a strong background in both backend development and operations, with a history of hosting and delivering content.

I run an active DevOps and development community on Discord, and if you like what I do and would like me to do more, you can also support me on Patreon.

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