Rails XML External Entities (XXE) Guide: Examples and Prevention

XML External Entities (XXE) pose a significant security risk for systems that process XML files, allowing attackers to exploit vulnerabilities by accessing sensitive server resources through malicious XML payloads. The article discusses the nature of XXE attacks, which can disclose sensitive data or lead to denial-of-service (DoS) attacks, and provides guidance on safeguarding Ruby on Rails applications against these vulnerabilities. It emphasizes using the default REXML library in Rails to prevent entity replacement and advises against using libraries like LibXML unless necessary precautions are taken. To further secure systems, the article suggests safelisting known external entities and recommends avoiding XML parsing unless essential for the application. For enhanced security, the use of Dynamic Application Security Testing (DAST) tools is advocated to identify vulnerabilities in real-time, while also encouraging developers to stay informed and leverage community resources. The discussion is rooted in the expertise of Juan Reyes, whose diverse experiences inform his insights into the complexities of securing modern applications.