XXE Vulnerabilities Explained: How XML Parsers Can Leak Server Files and Internal Data

XML External Entity (XXE) vulnerabilities exploit a built-in XML feature that allows parsers to load content from external files or URLs, rather than injecting malicious code in the traditional sense. When an XML parser processes untrusted input without disabling external entity resolution, attackers can instruct it to read sensitive server files such as /etc/passwd and return their contents. Beyond file disclosure, XXE can be leveraged for server-side request forgery, theft of cloud provider credentials, and in rare cases, remote code execution. The vulnerability hinges on the parser behaving exactly as designed — processing the SYSTEM keyword to fetch external resources — making it a configuration and hardening issue rather than a code flaw. Security testers are advised to understand XML entity declarations and DTD structure to fully assess XXE exposure, including blind and out-of-band attack variations.