XXE: XML External Entity Injection
Published: 19 October 2020 Last Updated: 03 November 2022
XML Entity Injection is a powerful vulnerability that can allow for confidential data theft and in rare cases command execution. It was also often overlooked for a while - but now it features in the OWASP Top 10 as A4 it's a lot more well known. The issue comes about within XML parsers where external entities are processed which can allow for URIs to be loaded.
Wait, back up. What's an entity? An easy way to think of entities is like a variable. It can hold strings, so an entity can be used in XML to hold text content - or it can be used with a URI to load remote content.