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Public proof of conceptFix available

CVE-2026-33228

A public proof of concept exists; no exploitation attested.

  • Published2026-03-20
  • Fixed in3.4.2
  • WeaknessCWE-1321 · CWE-915

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2026-03-2023:16CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-33228?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-33228?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Webreflection flatted
BranchAffectedFixed in
3.4< 3.4.23.4.2

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • RHSA-2026:86618 Red Hat Product Security4 products fixed

    Red Hat Security Advisory: New container image: rhceph-9.2

  • RHSA-2026:34342 Red Hat Product Security12 products fixed

    Red Hat Security Advisory: Cluster Observability Operator 1.5.0

  • RHSA-2026:13826 Red Hat Product Security1 product fixed

    Red Hat Security Advisory: Red Hat Developer Hub 1.9.4 release.

  • RHSA-2026:9742 Red Hat Product Security1 product fixed

    Red Hat Security Advisory: Red Hat Developer Hub 1.8.6 release.

Full record on NVD

Fix

  1. Update Webreflection flatted to 3.4.2
  2. Vendor fix Red Hat Product Security RHSA-2026:86618 · 4 productsThe container images provided by this update can be downloaded from the Red Hat container registry at registry.redhat.io using the "podman pull" command.
  3. Vendor fix Red Hat Product Security RHSA-2026:34342 · 12 productsBefore applying this update, make sure all previously released errata relevant to your system have been applied. For details on how to apply this update, refer to: https://access.redhat.com/articles/11258
  4. Vendor fix Red Hat Product Security RHSA-2026:13826 · 1 productFor more about Red Hat Developer Hub, see References links

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • CISA-ADPSSVC decisionpoc2026-03-24
  • OpenCVESSVC decisionpoc—
  • CNACVSS 4.08.9read 2026-10-23
  • CNACVSS 3.19.8read 2026-10-23
  • NVDCVSS 3.19.8read 2026-10-23
  • CNAweakness (CWE)CWE-1321, CWE-915read 2026-10-23
  • FIRST EPSSEPSS score1.3 %2026-10-22
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS 4.0 vector
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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Published
2026-03-20
Last Modified
2026-09-04
NVD status
Modified

Frequently asked questions

What is 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.

Is CVE-2026-33228 actively exploited?

A proof-of-concept exploit exists for CVE-2026-33228, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-33228?

CVE-2026-33228 has a CVSS v3 base score of 9.8 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

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