Final answer:
To test the security of XML parsing functions in a Windows application, a security analyst should request an applicable XSD file from the client to construct proper test XML files in line with the application's expected schema.
Step-by-step explanation:
A security analyst working on testing the security of parsing functions in a Windows application with XML should request an applicable XSD file from the client before proceeding. An XSD (XML Schema Definition) describes the structure of the XML file including elements and attributes used along with data types and restrictions. By having a proper XSD file, the analyst can ensure their test XML files are built in a way that conforms to the expected schema of the application, making the tests for XML parsing vulnerabilities like XML injection and entity expansion, more effective.
An XSD is the best tool for this purpose because it provides a blueprint for creating XML documents that the parser is designed to handle. It wouldn't be as helpful to request SOAP messages or REST API documentation, as these pertain to web service protocols rather than XML structure. A protocol fuzzing utility can be useful during the testing process, but it would be more of a tool to use after the XML files following the schema have been established.