CVE-2026-82404 TOON: Prototype pollution when decoding untrusted TOON input
Attackers can achieve remote code execution or denial of service by sending crafted TOON input that pollutes the object prototype.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-82404?
CVE-2026-82404 is a high-severity unsafe deserialization vulnerability in toon-format toon, published on 2 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
TOON is a compact, human-readable serialization of JSON data for LLM prompts. Prior to 2.3.1, decoding attacker-controlled TOON with a __proto__, constructor, or prototype key wrote through the object prototype chain instead of creating an own property, polluting Object.prototype for the runtime. In packages/toon/src/decode/expand.ts, the expandPaths: 'safe' path and insertPathSafe function made dotted keys such as a.__proto__.x the strongest vector, while plain nested objects, tabular rows, quoted keys, and streaming decode were also affected. The encoder also dropped own __proto__ properties and could invoke an inherited setter during normalization. Services that decode untrusted TOON could experience denial of service or, when a suitable downstream gadget is present, remote code execution. This issue is fixed in version 2.3.1.
Weakness: CWE-1321
Which versions are affected by CVE-2026-82404?
2 open source packages ship an affected version, including @toon-format/toon.
| Package | Affected | Fixed in |
|---|---|---|
@toon-format/toon npm | All versions | 2.3.1 |
@toon-format/toon npm | All versions | 2.3.1 |
How do I fix CVE-2026-82404?
Upgrade @toon-format/toon to 2.3.1 or later.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2026-82404 being exploited?
There is no known exploitation of CVE-2026-82404 in the wild and no public exploit code.
Sources
Also known as GHSA-p95v-992w-h6c3.