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

Description (CNA)

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.

Affected (CNA)

Vendor / product	Versions
WebReflection / flatted	affected < 3.4.2

CWE

CWE-1321

References

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.

redhat-SADP

A flaw was found in flatted, a JavaScript Object Notation (JSON) parser designed for handling circular data structures. A remote attacker can exploit this vulnerability by providing specially crafted JSON input. The parse() function in flatted fails to properly validate string values used as array index keys, allowing an attacker to manipulate internal JavaScript object prototypes. This prototype pollution can enable an attacker to execute arbitrary code or cause a denial of service, impacting the availability and integrity of affected systems.