Bean Stalking: Growing Java beans into Remote Code Execution

The analysis of CVE-2018-16621 in Nexus Repository Manager 3 highlights a critical Expression Language (EL) Injection vulnerability that was mitigated initially by sanitizing user input, but not by preventing the injection or sandboxing the EL engine. This oversight allowed for potential bypasses, as the root cause was linked to user-controlled data being reflected in validation error messages, which could lead to Remote Code Execution (RCE) if certain conditions were met. The vulnerability was explored using Java Bean Validation (JSR 380) and demonstrated how interpolation issues within custom validators could expose applications to RCE threats. The analysis further delves into various mitigation strategies, such as disabling EL interpolation, using parameterized templates, and employing robust sanitization logic, while highlighting the role of CodeQL in identifying such vulnerabilities. The exploration revealed multiple vulnerable applications, including Sonatype Nexus and Netflix Conductor, and discussed the exploitation tactics and challenges faced with different EL engines and limitations, such as incomplete EL implementations and OSGi module constraints. The study underscores the importance of proper implementation of custom validators and the risks associated with untrusted bean validation and default EL expression evaluations, suggesting a potential rise in similar vulnerabilities in open-source and proprietary applications.

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