TLDR: Misconfigured XML parsers, highlighted in the OWASP Top Ten under “Injection” and “Security Misconfiguration,” can expose applications to vulnerabilities like XML External Entity (XXE) attacks, denial-of-service (DoS), and sensitive data exposure. Issues such as enabling external entity resolution, improper validation, and insecure parser settings compromise application security. Proper configuration of XML parsers is essential to mitigate these risks.
https://owasp.org/www-project-top-ten/
One of the most common vulnerabilities in XML parsers is the improper handling of external entities. Enabling the parsing of external entities allows attackers to inject malicious references, leading to XXE attacks that can expose sensitive server-side files or execute DoS attacks. OWASP advises disabling external entity resolution for all XML parsers by default.
https://owasp.org/www-project-cheat-sheets/cheatsheets/XML_Security_Cheat_Sheet.html
Another frequent issue is failing to validate incoming XML data. Without validation, malicious payloads can bypass defenses and execute unintended operations. OWASP recommends using strict XML schema validation to ensure that input data conforms to expected formats and values.
https://owasp.org/www-project-top-ten/
Improperly configured DTD (Document Type Definitions) processing also increases the attack surface. XML parsers that process DTDs are vulnerable to recursive entity expansion attacks, also known as “billion laughs” attacks, which can exhaust system resources and lead to DoS. Disabling DTD processing is a best practice recommended by OWASP.
https://owasp.org/www-project-cheat-sheets/cheatsheets/XML_Security_Cheat_Sheet.html
Another oversight is enabling insecure default configurations in XML parsers. Many libraries ship with permissive settings that prioritize compatibility over security. OWASP suggests reviewing and hardening default configurations to ensure secure parsing behaviors.
https://owasp.org/www-project-top-ten/
Neglecting to monitor and log XML parsing errors is another misconfiguration. Attackers can exploit error responses to infer vulnerabilities in the parser. OWASP advises sanitizing error messages and logging detailed events securely to detect potential abuse while avoiding information leakage.
https://owasp.org/www-project-cheat-sheets/cheatsheets/Error_Handling_Cheat_Sheet.html
XML parsers in shared environments, such as multi-tenant applications, can lead to data leakage if tenant isolation is not properly enforced. For example, insecure handling of namespaces or shared resources can expose sensitive information. OWASP emphasizes tenant-aware parsing and secure namespace management.
https://owasp.org/www-project-top-ten/
Improper encoding and escaping of XML data during serialization and deserialization processes create injection vulnerabilities. For instance, unescaped special characters like `<` or `>` can lead to injection attacks. OWASP recommends using secure serialization libraries that automatically escape or encode data.
https://owasp.org/www-project-top-ten/
Failing to limit XML payload sizes is another critical flaw. Large or overly complex payloads can overwhelm system resources, resulting in DoS attacks. OWASP advises setting strict limits on the size and depth of XML documents processed by parsers.
https://owasp.org/www-project-cheat-sheets/cheatsheets/XML_Security_Cheat_Sheet.html
To mitigate these risks, developers should disable external entity resolution, enforce schema validation, and implement secure configurations for XML parsers. Regular security audits, adherence to OWASP Top Ten recommendations, and testing with tools like Burp Suite or OWASP ZAP ensure robust protection against XML parser vulnerabilities.