Vulnerability-Lookup

GHSA-667R-XXJV-C9MM

Vulnerability from github – Published: 2026-09-30 23:45 – Updated: 2026-09-30 23:45

Summary

fastify vulnerable to request body replacement via an async validation result collision

Details

Impact

Fastify runs a route's validator and, for a result shaped like { value, error }, unwraps it: an error becomes a validation failure and value replaces the request part. This convention is intended for synchronous custom compilers (for example Joi). A JSON Schema $async validator, however, resolves with the validated data itself, so Fastify applied the same unwrapping to it. If a request part validated by an $async schema contains a value property, Fastify replaced the whole request part with that nested value before the handler ran, so a value or error property in the payload was attacker-controlled. An application that dispatches operations from the validated request body could then act on data that never satisfied the route schema, leading to unauthorized state changes or disclosure. Reaching the vulnerable path requires the route to use an $async request schema.

Patches

Fastify no longer treats an asynchronous validation result as a { value, error } wrapper: an async validator's resolved value is used only to determine pass or fail, and it can no longer replace the request part or inject an error. The synchronous custom-compiler contract is unchanged. Patched in fastify 5.12.2 and 6.0.0.

Workarounds

If upgrading is not immediately possible, avoid $async request schemas, or perform the security-sensitive check in an onRequest or preHandler hook rather than relying on the schema-validated request part. Custom async validator compilers should signal failure by throwing (rejecting) rather than returning an { error } object.

Severity

8.1 (High)


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

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JSON

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fastify"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.12.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-84504"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-30T23:45:45Z",
    "nvd_published_at": "2026-09-04T10:17:13Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nFastify runs a route\u0027s validator and, for a result shaped like `{ value, error }`, unwraps it: an `error` becomes a validation failure and `value` replaces the request part. This convention is intended for synchronous custom compilers (for example Joi). A JSON Schema `$async` validator, however, resolves with the validated data itself, so Fastify applied the same unwrapping to it. If a request part validated by an `$async` schema contains a `value` property, Fastify replaced the whole request part with that nested value before the handler ran, so a `value` or `error` property in the payload was attacker-controlled. An application that dispatches operations from the validated request body could then act on data that never satisfied the route schema, leading to unauthorized state changes or disclosure. Reaching the vulnerable path requires the route to use an `$async` request schema.\n\n### Patches\n\nFastify no longer treats an asynchronous validation result as a `{ value, error }` wrapper: an async validator\u0027s resolved value is used only to determine pass or fail, and it can no longer replace the request part or inject an error. The synchronous custom-compiler contract is unchanged. Patched in fastify `5.12.2` and `6.0.0`.\n\n### Workarounds\n\nIf upgrading is not immediately possible, avoid `$async` request schemas, or perform the security-sensitive check in an `onRequest` or `preHandler` hook rather than relying on the schema-validated request part. Custom async validator compilers should signal failure by throwing (rejecting) rather than returning an `{ error }` object.",
  "id": "GHSA-667r-xxjv-c9mm",
  "modified": "2026-09-30T23:45:45Z",
  "published": "2026-09-30T23:45:45Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify/security/advisories/GHSA-667r-xxjv-c9mm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-84504"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify/commit/f02d8d4ab1ab208f10819c4dc213595b06497833"
    },
    {
      "type": "WEB",
      "url": "https://cna.openjsf.org/security-advisories.html"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/fastify/fastify"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify/releases/tag/v5.12.2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fastify vulnerable to request body replacement via an async validation result collision"
}

CVE-2026-84504 (GCVE-0-2026-84504)

Vulnerability from cvelistv5 – Published: 2026-09-04 09:40 – Updated: 2026-09-04 16:34

Title

fastify vulnerable to request body replacement via an async validation result collision

Summary

fastify versions before 5.12.2 treat the object resolved by a successful Ajv async validator as the value result protocol used by custom validator compilers. If a request that passes its route schema contains a property named value at the root, fastify replaces the entire request body with that property's value before the handler runs, so the handler receives a different object than the one that satisfied the schema. An authenticated low-privilege caller can use this to make nested data replace the validated body and trigger an operation the route schema did not authorize, leading to unauthorized state changes and data disclosure. Users should upgrade to fastify 5.12.2 or later.

Severity

8.1 (High)


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

SSVC

Exploitation: none Automatable: no Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-09-04 16:34 UTC

CWE

CWE-20 - Improper Input Validation

Assigner

openjs CNA under the redhat root

CNA scorecard C 65/100 over 121 records in the last 180 days details

References

2 references

URL	Tags
https://github.com/fastify/fastify/security/advis…
https://cna.openjsf.org/security-advisories.html

Impacted products

1 product

Vendor	Product	Version
fastify	fastify	Affected: 0 , < 5.12.2 (semver) Unaffected: 5.12.2 (semver)

Credits

velgusgus599 UlisesGascon climba03003 mcollina Eomm

Show details on NVD website

JSON

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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.

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