Vulnerability-Lookup

GHSA-W2QP-RPH6-63G4

Vulnerability from github – Published: 2026-09-02 15:13 – Updated: 2026-09-02 15:13

Summary

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Details

Impact

fastify before 5.12.1, when a route uses a root-level primitive body schema (for example an integer with a minimum and maximum) and the default type coercion, validates the coerced value but exposes the original, uncoerced value to the route handler. For example, a JSON body "10" is coerced to the number 10 and passes an integer 1 to 10 schema, but request.body stays the string "10". An application that trusts the validated type is handed a value that did not satisfy the schema, which can bypass limits the application enforces on that typed value. Object and array body schemas are not affected, they coerce their members in place.

Patches

Upgrade to fastify 5.12.1.

Workarounds

Until you can upgrade, avoid relying on the validated type of a root primitive body. Wrap the value in an object schema (object properties are coerced in place), for example accept { "value": 10 } and read request.body.value, or re-check the type in the handler.

Severity

5.4 (Medium)


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

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "fastify"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.12.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-18504"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-02T15:13:23Z",
    "nvd_published_at": "2026-08-18T21:16:34Z",
    "severity": "MODERATE"
  },
  "details": "### Impact\n\n`fastify` before 5.12.1, when a route uses a root-level primitive body schema (for example an integer with a minimum and maximum) and the default type coercion, validates the coerced value but exposes the original, uncoerced value to the route handler. For example, a JSON body `\"10\"` is coerced to the number `10` and passes an integer 1 to 10 schema, but `request.body` stays the string `\"10\"`. An application that trusts the validated type is handed a value that did not satisfy the schema, which can bypass limits the application enforces on that typed value. Object and array body schemas are not affected, they coerce their members in place.\n\n### Patches\n\nUpgrade to `fastify` 5.12.1.\n\n### Workarounds\n\nUntil you can upgrade, avoid relying on the validated type of a root primitive body. Wrap the value in an object schema (object properties are coerced in place), for example accept `{ \"value\": 10 }` and read `request.body.value`, or re-check the type in the handler.",
  "id": "GHSA-w2qp-rph6-63g4",
  "modified": "2026-09-02T15:13:23Z",
  "published": "2026-09-02T15:13:23Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-18504"
    },
    {
      "type": "WEB",
      "url": "https://github.com/fastify/fastify/commit/1ef8a60c87e94f42f666aa15ad68957772a83655"
    },
    {
      "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/v5.12.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "fastify vulnerable to schema validation bypass via root primitive coercion mismatch"
}

CVE-2026-18504 (GCVE-0-2026-18504)

Vulnerability from cvelistv5 – Published: 2026-08-18 20:19 – Updated: 2026-08-19 13:15

Title

fastify vulnerable to schema validation bypass via root primitive coercion mismatch

Summary

fastify is a fast and low overhead web framework for Node.js. Versions of fastify before 5.12.1 are affected by a schema validation bypass when a request body schema targets a root primitive value. When the schema validates a top-level primitive such as an integer, Ajv can coerce a JSON string into the expected type during validation, but Fastify does not replace the root request body with the coerced value, so the route handler receives the original unvalidated string. As a result, a request that should have failed validation can reach application logic with a value that does not satisfy the schema, which can undermine integrity and access-control checks that rely on the validated type. Users should upgrade to fastify 5.12.1, which fixes the mismatch. No known workarounds are available.

Severity

5.4 (Medium)


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

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-19 13:14 UTC

CWE

CWE-20 - Improper Input Validation

Assigner

openjs redhat root

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 , < 5.12.1 (semver) Unaffected: 5.12.1 (semver)

Credits

velgusgus599 mcollina UlisesGascon

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.

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.