Summary
TOON: Prototype pollution when decoding untrusted TOON input
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.
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.
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.
CVE-2026-82404 has a CVSS score of 8.3 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.3.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-82404? CVE-2026-82404 is a high-severity security vulnerability in @toon-format/toon (npm), affecting versions < 2.3.1. It is fixed in 2.3.1.
- How severe is CVE-2026-82404? CVE-2026-82404 has a CVSS score of 8.3 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of @toon-format/toon are affected by CVE-2026-82404? @toon-format/toon (npm) versions < 2.3.1 is affected.
- Is there a fix for CVE-2026-82404? Yes. CVE-2026-82404 is fixed in 2.3.1. Upgrade to this version or later.
- Is CVE-2026-82404 exploitable, and should I be worried? Whether CVE-2026-82404 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-82404 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-82404? Upgrade
@toon-format/toonto 2.3.1 or later.