How severe is CVE-2026-82404?

CVE-2026-82404 has a CVSS score of 8.3/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-82404?

CVE-2026-82404 affects the following packages: @toon-format/toon (npm). Ecosystems affected: npm.

How do I fix CVE-2026-82404?

Update @toon-format/toon to 2.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-82404 is resolved across your whole dependency graph.

How do I detect CVE-2026-82404 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @toon-format/toon, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-82404 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-82404 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-82404 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-82404?

CVE-2026-82404 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 51th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-82404?

CVE-2026-82404 is classified as CWE-1321 (CWE-1321). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-82404 published, and has it been updated?

CVE-2026-82404 was published on September 2, 2026 and was last updated on September 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

HIGH severity

CVE-2026-82404 — @toon-format/toon

HIGHFix: toon-format/toon@94a2b75

CVE-2026-82404 is a high-severity (CVSS 8.3) CWE-1321 vulnerability in @toon-format/toon. A fix is available for @toon-format/toon — see the affected versions and patch details below.

TOON: Prototype pollution when decoding untrusted TOON input

Also known asGHSA-p95v-992w-h6c3

Published

Sep 2, 2026

Updated

Sep 11, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 17, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs51th percentile — riskier than 51% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-82404 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 390,807 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

654other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

@toon-format/toonnpm

1.3Mdownloads / week

Description

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/[email protected]. 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 reject input whose keys include __proto__, constructor, or prototype before decoding.

Port maintainers

The same bug shape can exist in any implementation that assigns decoded keys with obj[key] = value. Rust, Swift, Java, Python, and C# ports should audit their object-construction and path-expansion paths for the three prototype keys.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`@toon-format/toon`	all versions	2.3.1`npm install @toon-format/toon@2.3.1`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @toon-format/toon, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @toon-format/toon to 2.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-82404 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### 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 aff