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CVE-2026-33228 critical CVSS 9.8

Vulnerability in flatted (CVE-2026-33228)

Summary

Vulnerability in flatted (CVE-2026-33228). Successful exploitation can lead to full system takeover. Mitigation: upgrade to 3.4.2 or later.

Original description

The description as published by the source.

Summary

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.

Root Cause

File: esm/index.js:29 (identical in cjs/index.js)

  const resolver = (input, lazy, parsed, $) => output => {
    for (let ke = keys(output), {length} = ke, y = 0; y < length; y++) {
      const k = ke[y];
      const value = output[k];    
      if (value instanceof Primitive) {
        const tmp = input[value];      // Bug is here

No validation that value is a safe numeric index input is built as a plain Array. JavaScript's property lookup on arrays traverses the prototype chain for non-numeric keys. The key "__proto__" resolves to Array.prototype, which:

  • has type "object" → passes the typeof tmp === object guard at line 30
  • is not in the parsed Set yet → passes the !parsed.has(tmp) guard.
  • The reference to Array.prototype is then enqueued in lazy and later unconditionally assigned to the output object.

**Replication St…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-33228 has been found in flatted.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade flatted to 3.4.2 or later.
If unsure, ask your IT team or search "flatted CVE-2026-33228" on the vendor's site.

CVE-2026-33228 (flatted) — CWE-915 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 3.4.2 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-915) exists in flatted.

📍 Affected scope

Check the vendor advisory for the affected versions of flatted.

🔥 Severity

Severity: Critical (CVSS 9.8/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 3.4.2.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'flatted' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'flatted' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で flatted を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade flatted to 3.4.2

    ステージング環境で 3.4.2 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

npm flatted
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"3.4.2"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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