CVE-2026-33228
CriticalCVSS 9.8Exploitation Probability (EPSS)
Elevated risk53th percentile - higher than 53% of all known CVEs
Summary
The flatted circular JSON parser before version 3.4.2 allows an attacker to inject the '__proto__' key as an array index, leaking a live reference to Array.prototype. This can lead to prototype pollution when the consumer writes to that property.
Risk Assessment
An attacker can remotely cause Array prototype pollution, enabling subsequent Cross-Site Scripting (XSS) or privilege escalation attacks in applications processing JSON data with this library.
Recommendation
Upgrade flatted to version 3.4.2 or later immediately. If an upgrade is not possible, avoid processing untrusted JSON data with the parse() function from 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.