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VulnerabilityPublished Updated

CVE-2026-33228: WebReflection flatted · Prototype Pollution

Affects WebReflection flattedSource CVE Database V5

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Public exploit available · CVSS 8.9 High : upgrade to 3.4.2 as a priority.

Data as of 1 Nov 2026, 22:58 UTC

CVSS 4.0 base score
8.9High
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.8% chance of exploitation in 30 days – higher than 57% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 3.4.2
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 20 Mar 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: evidence found.

    CVSS exploit maturity: proof of concept public

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CVSS vector, the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

What to do

Upgrade WebReflection flatted to 3.4.2.

Affected software

ProductAffectedFixed in
WebReflectionflattedAffected
  • <3.4.2
Fixed in
  • 3.4.2
npmflattedpkg:npm/flattedOther threatsAffected
  • <3.4.2
Fixed in
  • 3.4.2

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available in flatted version 3.4.2. Users and vendors should upgrade to version 3.4.2 or later to remediate this vulnerability. Red Hat has issued advisories and is working on fixes for affected products that incorporate the flatted library. Until patched, avoid processing untrusted JSON data with vulnerable versions or apply input sanitization to prevent prototype pollution.

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.

References

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