XML External Entity Attack and .NET Core

Improving the security of the systems I help design and build for my clients is a continuous effort where every day new vulnerabilities are discovered. That being said, the OWASP Top 10 hasn't change that much since it's original introduction.

This means that using the OWASP Top 10 remains a good starting point and the first important step towards more secure code.

During a security audit, we identified that one of our older (.NET) applications had an XML External Entities(XXE) vulnerability.

What is the XXE vulnerability?

An XML eXternal Entity injection (XXE) is an attack against applications that parse XML input. An XXE attack can happen when untrusted XML input with a reference to an external entity is processed by a weakly configured XML parser.

It is a high risk vulnerability as it can lead to:

• A denial of service attack on the system
• A Server Side Request Forgery (SSRF) attack
• The ability to scan ports from the machine where the parser is located

One of the things that can be done through this attack is to retrieve files from the server. For example, if your application is using ASP.NET (Core) and running in IIS you probably have a web.config and maybe an appsettings.json file. By referring to this file as an external entity in the provided XML, the content can be read and returned in the response.

More information: payloadbox/xxe-injection-payload-list: XML External Entity (XXE) Injection Payload List (github.com)

How can we safeguard against this vulnerability in .NET (Core)?

The question is then how can we prevent this attack from happening. The good news is that if you are using a .NET Framework version ≥4.5.2, you are protected against this type of attack by default. Also .NET Core gives you protection against this out-of-the-box.

For this older ASP.NET application, we fixed the problem by updating the the <httpRuntime targetFramework="..." /> setting in the Web.config. Although the application was compiled against .NET 4.6 and the .NET 4.8 runtime was installed, it is not sufficient. The used .NET Framework Version for an ASP.NET applications is either the .NET version the application was build with or the httpRuntime's targetFramework (Web.config), whichever is lower(!).

If you cannot change the .NET version, it is recommended to explicitly disable external entity support for the xml parser:

More information

XML External Entity Prevention - OWASP Cheat Sheet Series

XML Denial of Service Attacks and Defenses | Microsoft Learn