AI Coding Tools Like Cursor Can Introduce Prototype Pollution in Merge Functions

AI code editors such as Cursor frequently generate recursive object-merge helpers that contain a prototype pollution vulnerability, a developer warned in a recent post. The flaw allows a crafted JSON payload using a "__proto__" key to silently modify every object in a running application, potentially enabling privilege escalation. The vulnerability goes undetected during normal testing because it only triggers when malicious input is supplied. The root cause is that AI models learn from legacy blog posts and Stack Overflow answers that historically omit guards for special keys like "__proto__", "constructor", and "prototype". Developers can fix the issue by explicitly skipping those keys in merge loops, using Object.create(null) as the merge target, or catching the pattern early with static analysis tools such as Semgrep.