Vulnerability in fastify (CVE-2026-3419)
Summary
Vulnerability in fastify (CVE-2026-3419). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 5.8.1 or later.
Original description
The description as published by the source.
Fastify incorrectly accepts malformed Content-Type headers containing trailing characters after the subtype token, in violation of RFC 9110 §8.3.1(https://httpwg.org/specs/rfc9110.html#field.content-type). For example, a request sent with Content-Type: application/json garbage passes validation and is processed normally, rather than being rejected with 415 Unsupported Media Type.
When regex-based content-type parsers are in use (a documented Fastify feature), the malformed value is matched against registered parsers using the full string including the trailing garbage. This means a request with an invalid content-type may be routed to and processed by a parser it should never have reached.
Impact:
An attacker can send requests with RFC-invalid Content-Type headers that bypass validity checks, reach content-type parser matching, and be processed by the server. Requests that should be rejected at the validation stage are instead handled as if the content-type were valid.
Workarounds:
Deploy a WAF rule to protect against this
Fix:
The fix is available starting with v5.8.1.
Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV
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-2026-3419 has been found in fastify.
Risk of unauthorized operations or information disclosure.
What to do: upgrade fastify to 5.8.1 or later.
If unsure, ask your IT team or search "fastify CVE-2026-3419" on the vendor's site.
CVE-2026-3419 (fastify) — CWE-185
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Patched: 5.8.1 — apply immediately
Workaround: Deploy a WAF rule to protect against this
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-185) exists in fastify.
Check the vendor advisory for the affected versions of fastify.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 5.8.1.
Workaround: Deploy a WAF rule to protect against this
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
Deploy a WAF rule to protect against thisパッチが適用されるまでの応急処置として、Deploy a WAF rule to protect against this を実施。回避策の副作用 (機能低下) を確認した上で。
-
3Apply patch patch
Upgrade fastify to 5.8.1ステージング環境で 5.8.1 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 github.com
- advisory nvd.nist.gov
- advisory cna.openjsf.org
- advisory github.com
- advisory github.com
- package github.com
- patch github.com
- web httpwg.org
- web cve.org