CVE-2026-33805 CRITICAL

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

Vendor @Fastify/Reply-From
Product @fastify/reply-from
Weakness CWE-644
Published April 15, 2026
Last update June 30, 2026
View on NVD All CVEs

CVSS base score

9.0/10
Attack vector Network
Attack complexity Low
Privileges required None
User interaction None
Confidentiality —
Integrity —

CVSS 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

What the vulnerability does

01Description

@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.

Key dates

02Disclosure timeline

April 15, 2026 CVE published
June 30, 2026 Record updated