# 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.

- **Severity:** High 8.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Sep 2026
- **Updated:** 2 Sep 2026
- **Also known as:** GHSA-p95v-992w-h6c3

## 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 (Improperly Controlled Modification of Object Prototype Attributes)

## Which versions are affected by CVE-2026-82404?

2 open source packages ship an affected version, including @toon-format/toon.


| Package | Ecosystem | 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.

## Is CVE-2026-82404 being exploited?

There is no known exploitation of CVE-2026-82404 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-82404 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-82404 in the wild and no public exploit code.

Affected packages and fixed versions:
- @toon-format/toon (npm): affected all versions; fixed in 2.3.1
- @toon-format/toon (npm): affected all versions; fixed in 2.3.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-82404/ (Markdown: https://insights.sbomify.com/CVE-2026-82404/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-82404
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-82404
- OSV: https://osv.dev/vulnerability/CVE-2026-82404

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-82404/
Summaries are generated and may contain mistakes; the official description above is the record.