XML External Entity (XXE) is a vulnerability that arises when an application parses XML input without properly restricting the use of external entities. Attackers can exploit this by including malicious entities in the XML payload that reference external files or resources on the server. When the parser processes the XML, it may inadvertently access local files, make HTTP requests, or even cause denial of service through recursive entity expansion (a variant known as a Billion Laughs attack). This can lead to the disclosure of sensitive files, server-side request forgery (SSRF), or other unexpected behavior.

Remediation

To remediate XXE vulnerabilities, applications should disable the ability to resolve external entities and DTDs (Document Type Definitions) within the XML parser. Developers should use secure libraries or configurations that explicitly turn off this functionality. Additionally, where possible, applications should avoid using XML altogether in favor of less complex data formats like JSON. Input validation and enforcing strict content-type checks can also reduce the risk. Keeping XML parsers and dependencies up to date ensures that known vulnerabilities are patched.