CVE-2026-33228
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk49th percentile - higher than 49% of all known CVEs
Summary
The flatted library (circular JSON parser) before version 3.4.2 has a vulnerability where the parse() function does not validate that array index keys are numeric. An attacker can use the '__proto__' key in parsed JSON, leaking a reference to Array.prototype into the output object.
Risk Assessment
If the application writes to a property derived from this leak, global prototype pollution can occur, allowing an attacker to modify the behavior of all objects in the application and potentially achieve remote code execution.
Recommendation
Upgrade flatted to version 3.4.2 or later immediately. If an upgrade is not possible, avoid parsing untrusted JSON data with flatted.
Original NVD description (English source)
flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "__proto__" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. This issue has been patched in version 3.4.2.