CVE-2026-33228
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedflatted 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe flatted library's parse() function allows attacker-controlled string values like '__proto__' to be used as array index keys without validation. Since the internal buffer is a JavaScript Array, accessing it with '__proto__' returns Array.prototype via inheritance, leaking a live reference to the output object and enabling global prototype pollution.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 3.4.2CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
-
Identify flatted versionRun 'npm list flatted' or check package.json dependencies to find the installed version of the flatted libraryAffected if The installed version is lower than 3.4.2
-
Locate flatted import in codebaseSearch source files for 'require("flatted")' or 'import ... from "flatted"' statements to find where flatted is importedAffected if Flatted is used in the project and the version is below 3.4.2
-
Identify parse() usage with untrusted inputSearch for calls to flatted.parse() and trace whether the input comes from untrusted sources such as user input, HTTP request bodies, or external APIsAffected if flatted.parse() processes data from untrusted sources and the flatted version is below 3.4.2
-
Inspect parsed data structureWhen debugging or logging enabled, examine the output of flatted.parse() to see if object keys include '__proto__' or 'constructor' stringsAffected if Parsed objects contain '__proto__' as a direct property key indicating potential exploitation
A user is affected if they have flatted version lower than 3.4.2 installed AND use flatted.parse() to process untrusted JSON data containing '__proto__' or similar special property names as object keys.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped3.4.2
Update flatted to version 3.4.2 or later to patch the prototype pollution vulnerability. Audit existing code for dependencies on flatted and verify no other similar unvalidated key access patterns exist.
flatted version 3.4.2
- Check the current installed version of flatted in your project by running: npm list flatted or yarn list flatted
- Update the flatted dependency in your package.json file to version 3.4.2 or higher
- Run npm install or yarn install to install the updated package
- Verify the installation was successful by checking the installed version again
- Run your existing test suite to ensure the upgrade does not break any functionality
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,224.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-33228 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-33228 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.
A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data