XML External Entity (XXE) prevention addresses a class of vulnerabilities in applications that parse XML input. XXE attacks exploit the XML specification's entity feature to read local files, perform server-side request forgery, execute denial of service, and in some cases achieve remote code execution. Prevention focuses on disabling dangerous XML parser features and implementing safe parsing configurations.
XML documents can define entities -- shortcuts that reference internal or external content. External entities instruct the XML parser to fetch content from URIs, including local file paths and remote URLs. When applications parse user-supplied XML without disabling external entity processing, attackers craft malicious XML documents that exfiltrate server files through entity expansion, scan internal networks through server-side requests, or crash applications through recursive entity expansion (the billion laughs attack). Prevention is straightforward: disable Document Type Definition (DTD) processing entirely when DTDs are not required. When DTD support is necessary, disable external entity resolution and external DTD loading specifically. Each XML parsing library has different configuration methods -- libxml2, Java SAXParser, .NET XmlReader, and Python lxml all require library-specific settings. Alternative data formats like JSON that lack entity processing features eliminate XXE risk entirely. Input validation should reject XML documents containing DTD declarations when external entities are not expected. Web Application Firewalls can detect and block XXE payloads in incoming requests as a defense-in-depth measure.
XXE vulnerabilities are deceptively simple to exploit and appear wherever applications process XML -- API endpoints, file uploads (DOCX, SVG, XLSX are XML-based formats), SAML authentication, SOAP web services, and configuration file parsing. The attack surface is broader than many developers realize because XML parsing occurs in unexpected contexts.
CDA addresses XXE within VSD domain operations. Theater missions audit XML parsing configurations across application stacks, identify hidden XML processing in file upload handlers and authentication flows, and standardize secure parser configurations that development teams apply consistently across all projects.