TOON: Prototype pollution when decoding untrusted TOON input

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L

EPSS

• CVE-2026-82404: 0.68% estimated exploitation activity in the next 30 days; percentile 52.34 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Decoding attacker-controlled TOON containing a proto, constructor, or prototype key wrote through the object's prototype chain instead of creating an own property, polluting Object.prototype for the whole runtime. The expandPaths: 'safe' path (dotted keys such as a.proto.x) was the strongest vector; plain nested objects, tabular rows, and quoted keys were all affected. The encoder had a matching defect: it silently dropped own proto properties and could fire an inherited setter while normalizing. Impact Any service that decodes untrusted TOON is affected. Prototype pollution can escalate to denial of service or, with a suitable downstream gadget, remote code execution. Patches Upgrade to @toon-format/toon@2.3.1. Decoders now materialize proto/constructor/prototype as ordinary own data properties, matching JSON.parse semantics; the encoder preserves the same keys without invoking inherited accessors. Workarounds None. Upgrade is the only fix. Callers who cannot upgrade should…

Published by the source

3 Sep 2026, 19:52 UTC

Last modified by the source

3 Sep 2026, 19:52 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-p95v-992w-h6c3

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# Patch Next advisory brief: CVE-2026-82404

Advisory: TOON: Prototype pollution when decoding untrusted TOON input
IDs: CVE-2026-82404, GHSA-p95v-992w-h6c3
Record: https://patchnext.com/advisories/cve-2026-82404 (revision 1, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @toon-format/toon 2.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @toon-format/toon: affected < 2.3.1; fixed in 2.3.1

## Suggested inspection (read-only)
- `npm ls @toon-format/toon --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @toon-format/toon.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-p95v-992w-h6c3 (GitHub-reviewed advisory GHSA-p95v-992w-h6c3 lists affected ranges: npm @toon-format/toon >=0 <2.3.1)

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