Fastify's connection header abuse enables stripping of proxy-added headers

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.0 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:L/SI:H/SA:N · score source

EPSS

• CVE-2026-33805: 0.44% estimated exploitation activity in the next 30 days; percentile 38.35 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary @fastify/reply-from and @fastify/http-proxy 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 (like access control or identification headers) from upstream requests by listing them in the Connection header value. This affects applications using these plugins with custom header injection for routing, access control, or security purposes. Details The vulnerability exists in @fastify/reply-from/lib/request.js at lines 128-136 (HTTP/1.1 handler) and lines 191-200 (undici handler). The processing flow is: 1. Client headers are copied including the connection header (@fastify/reply-from/index.js line 91) 2. The proxy adds custom headers via rewriteRequestHeaders (line 151) 3. During request construction, the transport handlers read the client's Connection header and strip any headers listed in it 4. This stripping happens after rewriteRequestHeaders, allowing…

Published by the source

16 Apr 2026, 01:02 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:53 UTC

1. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-gwhp-pf74-vj37

2. Revision 3 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-gwhp-pf74-vj37

4. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-gwhp-pf74-vj37

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# Patch Next advisory brief: CVE-2026-33805

Advisory: Fastify's connection header abuse enables stripping of proxy-added headers
IDs: CVE-2026-33805, GHSA-gwhp-pf74-vj37
Record: https://patchnext.com/advisories/cve-2026-33805 (revision 4, source checked 26 Oct 2026, 12:53 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @fastify/http-proxy 11.4.4; @fastify/reply-from 12.6.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @fastify/http-proxy: affected <= 11.4.3; fixed in 11.4.4
- npm @fastify/reply-from: affected <= 12.6.1; fixed in 12.6.2

## Suggested inspection (read-only)
- `npm ls @fastify/http-proxy --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @fastify/reply-from --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @fastify/http-proxy, @fastify/reply-from.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-gwhp-pf74-vj37 (GitHub-reviewed advisory GHSA-gwhp-pf74-vj37 lists affected ranges: npm @fastify/reply-from >=0 =0 <=11.4.3)
- Verified: https://osv.dev/vulnerability/GHSA-gwhp-pf74-vj37 (OSV record GHSA-gwhp-pf74-vj37 lists affected ranges: npm @fastify/reply-from >=0 =0 <11.4.4 (fixed))

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