XML
Extensible Markup Language
A structured data format; XXE and injection target its parsers.
XML is a structured, tag-based format widely used for configuration, documents, and data exchange between applications and web services. Its security relevance comes from how parsers handle it: XML external entity processing can be abused to read local files, reach internal systems, or cause denial of service. Improper input handling also enables injection into XML-based queries. The standard defense is to disable external entity resolution and validate or sanitize input, a frequent exam pairing with the XXE attack.
Looking XML up is step one. Getting tested on it is step two.
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