CVE-2026-33228

CVE-2026-33228 · flatted: Prototype Pollution via parse()

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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.

CVSS 8.9 · High · EPSS 0.8% · Fix available

Published

2026-03-20T23:06:48.485Z

Updated

2026-09-07T12:05:12.907Z

Stack: system · node

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