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High severityNVD Advisory· Published Mar 20, 2026· Updated Mar 24, 2026

flatted: Prototype Pollution via parse()

CVE-2026-33228

Description

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.

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Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
flattednpm
< 3.4.23.4.2

Affected products

19

Patches

Vulnerability mechanics

References

5

News mentions

0

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