VULNERABILITY INTELLIGENCE

CVE-2026-33228

CVSS score9.8 CRITICAL
EPSS probability0.84%
CISA KEVNot currently listed
Published2026-03-20
Last modified2026-09-04
CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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.

Weakness classification

  • security-advisories@github.com: CWE-1321
  • 0b0ca135-0b70-47e7-9f44-1890c2a1c46c: CWE-915

Primary references