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

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.8% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-20 · updated 2026-09-04
Weakness: CWE-1321

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.

Affected products

WebReflection flatted: before 3.4.2 — fixed in 3.4.2
Npm Flatted: before 3.4.2 — fixed in 3.4.2

How to fix

Upgrade to a fixed version: WebReflection flatted 3.4.2; Npm Flatted 3.4.2. Check the vendor advisory for mitigations.

References

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