This post discloses a prototype pollution vulnerability (CWE-1321) in Google’s Gemini CLI Security Extension, where a Node.js MCP server used a plain JavaScript object as a key-value map with user-controlled keys. A single standalone constructor line can trigger unsafe prototype-chain behavior and lead to runtime failure and potential escalation to prototype pollution impacts such as code execution, bypassing security findings, and supply chain compromise during code review. The report includes an attacker workflow, a CVSS 7.1 (High) breakdown, and the remediation: replacing the object map with a Map, which treats constructor as a normal key and breaks the prototype pollution path.