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CVE-2026-33805 high CVSS 8.6

Vulnerability in @fastify/reply-from (CVE-2026-33805)

Summary

Vulnerability in @fastify/reply-from (CVE-2026-33805). Data can be tampered with by attackers. Exploitable via Connection. Mitigation: upgrade to 12.6.2 or later.

Original description

The description as published by the source.

@fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's Connection header after the proxy has added its own headers via rewriteRequestHeaders. This allows attackers to retroactively strip proxy-added headers from upstream requests by listing them in the Connection header value. Any header added by the proxy for routing, access control, or security purposes can be selectively removed by a client. @fastify/http-proxy is also affected as it delegates to @fastify/reply-from.

Upgrade to @fastify/reply-from v12.6.2 or @fastify/http-proxy v11.4.4 or later.

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-2026-33805 has been found in @fastify/reply-from.
Attackers can target a specific entry point like Connection over the network to misuse the product.
Data can be tampered with by attackers. CVSS score: 8.6/10.
What to do: upgrade @fastify/reply-from to 12.6.2 or later.
If unsure, ask your IT team or search "@fastify/reply-from CVE-2026-33805" on the vendor's site.

CVE-2026-33805 (@fastify/reply-from) — CWE-644 / CVSS v3 8.6
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Connection / rewriteRequestHeaders / connection / index.js
Patched: 12.6.2 — apply immediately
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.

❓ What is the problem

A vulnerability (CWE-644) exists in @fastify/reply-from. Attackers reach the vulnerable code path via Connection without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of @fastify/reply-from. Attack surface: Connection / rewriteRequestHeaders / connection / index.js.

🔥 Severity

Severity: High (CVSS 8.6/10). Data can be tampered with by attackers.

🔧 How to fix

Update to 12.6.2.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r '@fastify/reply-from' . 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

  1. 1
    Identify exposure identify 
    grep -r '@fastify/reply-from' . | 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/reply-from を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade @fastify/reply-from to 12.6.2

    ステージング環境で 12.6.2 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。

  3. 3
    Post-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

npm @fastify/reply-from
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"12.6.2"}]}]
npm @fastify/http-proxy
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"11.4.4"}]}]

References

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