---
title: "CVE-2026-33805: Fastify's connection header abuse enables stripping of proxy-added headers"
url: https://erroneo.us/security/cve/CVE-2026-33805/
description: "CVE-2026-33805 (@fastify/reply-from (npm)): @fastify/reply-from v12.6.1 and earlier and @fastify/http-proxy v11.4.3 and earlier process the client's…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-15
date_modified: 2026-07-15
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-33805"
kev: false
cvss: 9
epss: 0.0056
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-33805: Fastify's connection header abuse enables stripping of proxy-added headers

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.6% , percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes @fastify/reply-from (npm) <= 12.6.1, fixed in 12.6.2 ; @fastify/http-proxy (npm) <= 11.4.3, fixed in 11.4.4 . Check your lockfile, including transitive dependencies.

## What it is

@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.

## Affected packages

- npm `@fastify/reply-from` <= 12.6.1 → fixed: 12.6.2
- npm `@fastify/http-proxy` <= 11.4.3 → fixed: 11.4.4

## Affected products (CVE record)

- @fastify/reply-from @fastify/reply-from: < 12.6.2
- @fastify/reply-from @fastify/http-proxy: < 11.4.4

## Severity

- openjs CVSS 4.0: 9 `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`
- redhat-SADP CVSS 3.1: 7.4 `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub 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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact partial .
- FIRST EPSS (model of 2026-10-21): 0.56% probability of exploitation activity in the next 30 days; percentile 46.

## Sources

- [access.redhat.com](https://access.redhat.com/errata/RHSA-2026:10175)
- [GitHub Advisory GHSA-gwhp-pf74-vj37](https://github.com/advisories/GHSA-gwhp-pf74-vj37)
- [CVE.org record CVE-2026-33805](https://www.cve.org/CVERecord?id=CVE-2026-33805)
- [NVD entry CVE-2026-33805](https://nvd.nist.gov/vuln/detail/CVE-2026-33805)
- [github.com](https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37)
- [cna.openjsf.org](https://cna.openjsf.org/security-advisories.html)
- [access.redhat.com](https://access.redhat.com/security/cve/CVE-2026-33805)
- [RHBZ#2458651](https://bugzilla.redhat.com/show_bug.cgi?id=2458651)
- [security.access.redhat.com](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33805.json)

---
Source page: https://erroneo.us/security/cve/CVE-2026-33805/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt