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

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.

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N

Attack vector

Network

Complexity

Low

Privileges

None

User interaction

None

Confidentiality

None

Integrity

High

Availability

None

Scope

Changed

Exploitation

Is it being used?

Exploit prediction (EPSS)0.4% · higher than 38% of CVEs

Exploited in the wildNo evidence yet

CISA KEVNot listed

Ransomware campaign useNot known

Affected products

Which versions

FA

@fastify/http-proxy · @fastify/reply-from

before 11.4.4 · source: vendor

FA

@fastify/reply-from · @fastify/reply-from

before 12.6.2 · source: vendor

FA

Fastify/http-proxy · Fastify

before 11.4.4 · source: NVD

RE

Reply-from · Fastify

before 12.6.2 · source: NVD

RH

Red Hat OpenShift AI (RHOAI) · Red Hat

All versions, or not specified · source: CISA

RH

Red Hat OpenShift Dev Spaces 3.27 · Red Hat

All versions, or not specified · source: CISA