Vulnerability in fastify (CVE-2026-18504)
Summary
Vulnerability in fastify (CVE-2026-18504). Risk of unauthorized operations or information disclosure. Exploitable via fastify. Mitigation: upgrade to 5.12.1 or later.
Original description
The description as published by the source.
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.
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-18504 has been found in fastify.
Attackers can target a specific entry point like fastify over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.4/10.
What to do: upgrade fastify to 5.12.1 or later.
If unsure, ask your IT team or search "fastify CVE-2026-18504" on the vendor's site.
CVE-2026-18504 (fastify) — CWE-20 / CVSS v3 5.4
Attack vector: remote (network-reachable) / no user interaction
Attack surface: fastify / request.body / request.body.value
Patched: 5.12.1 — apply immediately
Workaround: Until you can upgrade, avoid relying on the validated type of a root primitive body.
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-20) exists in fastify. Attackers reach the vulnerable code path via fastify.
Check the vendor advisory for the affected versions of fastify. Attack surface: fastify / request.body / request.body.value.
Severity: Medium (CVSS 5.4/10). Risk of unauthorized operations or information disclosure.
Update to 5.12.1.
Workaround: Until you can upgrade, avoid relying on the validated type of a root primitive body.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'fastify' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
grep -r 'fastify' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
fastifyを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply temporary workaround mitigate
Until you can upgrade, avoid relying on the validated type of a root primitive body.パッチが適用されるまでの応急処置として、Until you can upgrade, avoid relying on the validated type of a root primitive body. を実施。回避策の副作用 (機能低下) を確認した上で。
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3Apply patch patch
Upgrade fastify to 5.12.1ステージング環境で 5.12.1 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
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4Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory cna.openjsf.org
- advisory github.com
- advisory nvd.nist.gov
- advisory github.com
- package github.com
- web github.com
- web github.com
- web github.com