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.
Metrics
Affected Vendors & Products
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References
History
Sat, 21 Mar 2026 05:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | flatted: Prototype Pollution via parse() | |
| Weaknesses | CWE-1321 | |
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cvssV4_0
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MITRE
Status: PUBLISHED
Assigner: GitHub_M
Published:
Updated: 2026-03-20T23:06:48.485Z
Reserved: 2026-03-18T02:42:27.507Z
Link: CVE-2026-33228
Vulnrichment
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NVD
Status : Received
Published: 2026-03-20T23:16:46.510
Modified: 2026-03-20T23:16:46.510
Link: CVE-2026-33228
Redhat
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OpenCVE Enrichment
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