.NET XML External Entities Guide: Examples and Prevention

XML External Entities (XXE) vulnerabilities in .NET pose significant security risks by allowing attackers to exploit XML parsing functions to access sensitive server resources. The article outlines the nature of XXE attacks, which use XML's allowance for external entities to manipulate file access, potentially leading to exposure of sensitive data or denial of service attacks. Key mitigation strategies include avoiding libraries that support entity replacement, using XML processing tools like XmlDocument or XmlReader that have built-in protections, and considering alternatives like JSON for data processing. Additionally, maintaining updated XML processing libraries, validating XML uploads with XSD, and employing security testing tools like Dynamic Application Security Testing (DAST) can help secure applications. The article emphasizes minimizing XML parsing unless necessary and provides insights into using safelist techniques for essential external entities. Authored by Juan Reyes, a professional engineer and writer, the piece also touches on the broader challenges of maintaining secure services amidst evolving threats.