Vulnerability-Lookup

CVE-2026-82404 (GCVE-0-2026-82404)

Vulnerability from cvelistv5 – Published: 2026-09-02 16:30 – Updated: 2026-09-02 18:59

Title

TOON: Prototype pollution when decoding untrusted TOON input

Summary

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.

Severity

8.3 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-09-02 18:58 UTC

CWE

CWE-1321 - Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/toon-format/toon/security/advi…	x_refsource_CONFIRM
https://github.com/toon-format/toon/pull/316	x_refsource_MISC
https://github.com/toon-format/toon/commit/94a2b7…	x_refsource_MISC
https://github.com/toon-format/toon/releases/tag/v2.3.1	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
toon-format	toon	Affected: < 2.3.1	guessed

Show details on NVD website

JSON

To clipboard

{
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              "content": {
                "id": "CVE-2026-82404",
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        }
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      "title": "TOON: Prototype pollution when decoding untrusted TOON input"
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  }
}

FKIE_CVE-2026-82404

Vulnerability from fkie_nvd - Published: 2026-09-02 17:18 - Updated: 2026-09-02 19:18

Severity

8.3 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/toon-format/toon/commit/94a2b7560b2b5ed903a4d466a3bce8b13daa2660
	security-advisories@github.com	https://github.com/toon-format/toon/pull/316
	security-advisories@github.com	https://github.com/toon-format/toon/releases/tag/v2.3.1
	security-advisories@github.com	https://github.com/toon-format/toon/security/advisories/GHSA-p95v-992w-h6c3

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "toon",
          "vendor": "toon-format",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 2.3.1"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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: \u0027safe\u0027 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."
    }
  ],
  "id": "CVE-2026-82404",
  "lastModified": "2026-09-02T19:18:06.453",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "LOW",
          "baseScore": 8.3,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "NONE",
          "scope": "CHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 3.7,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
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          "options": [
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            },
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            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-09-02T18:58:59.419694Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-09-02T17:18:00.223",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/toon-format/toon/commit/94a2b7560b2b5ed903a4d466a3bce8b13daa2660"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/toon-format/toon/pull/316"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/toon-format/toon/releases/tag/v2.3.1"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/toon-format/toon/security/advisories/GHSA-p95v-992w-h6c3"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
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GHSA-P95V-992W-H6C3

Vulnerability from github – Published: 2026-09-03 19:52 – Updated: 2026-09-03 19:52

Summary

TOON: Prototype pollution when decoding untrusted TOON input

Details

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 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.

Severity

8.3 (High)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "@toon-format/toon"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.3.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-82404"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-03T19:52:08Z",
    "nvd_published_at": "2026-09-02T17:18:00Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nDecoding attacker-controlled TOON containing a `__proto__`, `constructor`, or `prototype` key wrote through the object\u0027s prototype chain instead of creating an own property, polluting `Object.prototype` for the whole runtime. The `expandPaths: \u0027safe\u0027` 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.\n\n### Impact\n\nAny service that decodes untrusted TOON is affected. Prototype pollution can escalate to denial of service or, with a suitable downstream gadget, remote code execution.\n\n### Patches\n\nUpgrade 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.\n\n### Workarounds\n\nNone. Upgrade is the only fix. Callers who cannot upgrade should reject input whose keys include `__proto__`, `constructor`, or `prototype` before decoding.\n\n### Port maintainers\n\nThe 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.",
  "id": "GHSA-p95v-992w-h6c3",
  "modified": "2026-09-03T19:52:08Z",
  "published": "2026-09-03T19:52:08Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/toon-format/toon/security/advisories/GHSA-p95v-992w-h6c3"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-82404"
    },
    {
      "type": "WEB",
      "url": "https://github.com/toon-format/toon/pull/316"
    },
    {
      "type": "WEB",
      "url": "https://github.com/toon-format/toon/commit/94a2b7560b2b5ed903a4d466a3bce8b13daa2660"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/toon-format/toon"
    },
    {
      "type": "WEB",
      "url": "https://github.com/toon-format/toon/releases/tag/v2.3.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "TOON: Prototype pollution when decoding untrusted TOON input"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

CVE-2026-82404 (GCVE-0-2026-82404)

FKIE_CVE-2026-82404

GHSA-P95V-992W-H6C3

Summary

Impact

Patches

Workarounds

Port maintainers

Tags

Sightings

Nomenclature