An XML parser is software that reads XML documents and turns their tags, attributes, and text into structured data an application can use. Parsers are built into libraries and frameworks, so they often sit deep inside servers, middleware, appliances, and development tools.
In cyber security, parsers matter because they process attacker-controlled input before the rest of the application can inspect it. A bug in the parser can cause crashes, memory corruption, or excessive resource use, turning a simple document into an availability problem. Defenders reduce risk by patching parser libraries quickly, rebuilding bundled software when needed, and limiting exposure to untrusted XML. Some deployments also disable risky features such as external entity handling when they are not required.