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

Type: advisorySources: OSV

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.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-84504
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 03:33 UTC · Revision 1

Source evidence

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

core-osv · Original source

Published 4 Sept 2026, 09:40 UTC

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.

osv:CVE-2026-84504
Complete source record
{
  "id": "cb9aa2c5380b4c059ce4e9ffb0aac32abab7c5c8c8f03a812cbe7a1bb04e8b1f",
  "url": "https://osv.dev/vulnerability/CVE-2026-84504",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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.",
    "evidence": "osv:CVE-2026-84504",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-84504",
      "aliases": [
        "CVE-2026-84504",
        "GHSA-667r-xxjv-c9mm"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N"
        }
      ],
      "affected": [
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              "type": "GIT",
              "events": [
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              "database_specific": {
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                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
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                  {
                    "cpe": "cpe:2.3:a:fastify:fastify:*:*:*:*:*:node.js:*:*",
                    "range": [
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                        "introduced": "0"
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      "modified": "2026-10-28T02:30:33.097147329Z",
      "provider": "osv",
      "references": [
        "https://cna.openjsf.org/security-advisories.html",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84504.json",
        "https://github.com/fastify/fastify/security/advisories/GHSA-667r-xxjv-c9mm",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-84504"
      ],
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    "related_cves": [
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        "value": "CVE-2026-84504",
        "evidence": "/id"
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    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 500049,
  "published_at": "2026-09-04T09:40:18.12+00:00"
}