CVE-2026-33228 - flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-co

CVE-2026-33228 + flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-co + Critical + 2026-03-20

One-sentence business risk

Server-side request forgery can bridge from the public app into cloud metadata and private network services.

Research notes

  • Root cause: 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.
  • Affected versions: Prior to version 3.
  • Fixed / safe versions: version 3.4.2.
  • Public exploit / PoC signal: the consulted NVD/GHSA/vendor sources describe the trigger class; treat it as reproducible until patched.
  • CVSS: 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

Exact vulnerable code pattern

setPath(target, key.split("."), value)  # no segment denylist

Fixed / mitigated code pattern

reject segments in {"__proto__", "constructor", "prototype"} before walking the object

Dependency or runtime update:

npm ls affected component || true
npm install affected component@latest --save-exact
npm audit --omit=dev

Step-by-step integration guide

  1. Inventory affected component across source, lockfiles, SBOMs, container images, CI images, and deployment manifests.
  2. Upgrade to version 3.4.2 or apply the exact vendor patch referenced below.
  3. Replace the vulnerable prototype-pollution pattern with the fixed pattern and fail closed on malformed input.
  4. Add a regression test that exercises the advisory trigger and proves the request is rejected, bounded, or authorized.
  5. Deploy through canary, monitor security logs and resource metrics, then remove temporary edge blocks only after all runtimes are fixed.

Alternative mitigations

  • Disable the vulnerable feature path while the package/runtime update rolls out.
  • Add WAF or reverse-proxy rules for traversal tokens, prototype-polluting keys, oversized frames, shell metacharacters, or unauthenticated tool calls matching the trigger class.
  • Restrict internal egress, rotate exposed secrets, and revoke affected tokens/keys if the vulnerability can disclose config, logs, JWT secrets, proxy credentials, or PKI material.
  • For admin/API authorization issues, temporarily require elevated roles and explicit ownership checks at the gateway.

Detection signature

id: cve-2026-33228-flatted-is-a-circular-json-parser-prior-to-versi
source: NVD
dependency_or_product: "affected component"
affected: "Prior to version 3"
fixed: "version 3.4.2"
signals:
  - "prototype-pollution"
  - "../"
  - "__proto__"
  - "tools/call"
  - "wp_ajax_nopriv"
  - "pickle.loads"
  - "shell command construction"
action: "upgrade, add regression test, and verify deploy artifact"

Copy-paste skill

You are remediating CVE-2026-33228 (flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-co) in this repository.

Find every affected dependency, runtime, image, service configuration, and reachable code path. Upgrade or patch to version 3.4.2. Replace the vulnerable prototype-pollution pattern with a fail-closed implementation, add a regression test for the trigger shape, and update deployment/SBOM artifacts. If this repository does not control the affected runtime, create TRIAGE.md naming the owner, affected version, fixed version, and blocking decision.

Keywords, affected tech stack, and revenue tags

  • Keywords: CVE-2026-33228, prototype-pollution, affected component, NVD.
  • Affected tech stack: javascript/npm.
  • Revenue tags: sellable_to_fintech, enterprise_blocker, zero_day_gold.

References