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9.8Critical

Webreflection Flatted

CVE-2026-33228

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.

What this means for your business

  • It affects Webreflection Flatted. It matters if your company, or a supplier that handles your data, runs it.
  • Code showing how to exploit it has been published, but real attacks have not been confirmed.
  • An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
  • FIRST's prediction model gives it a 0.8% chance of attack attempts being seen in the next 30 days, ranking above 54% of all known flaws.

What to do

  1. 1Check whether your company or your suppliers run Webreflection Flatted, and which version. The affected versions are listed further down this page.
  2. 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Scoring

CVSS
9.8 (v3.1)
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS v4
8.9
Weakness
CWE-1321
Assigned by
security-advisories@github.com

Dates

Published
2026-03-20
Last modified
2026-09-04
Sources
NVD

Affected products

  • Webreflection Flatted- 3.4.2

As listed in the NVD configuration data. Not a statement about your estate.

References