CVE-2026-33805CWE-644

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

Critical · published April 15, 2026

CVSS v4.0
9.0
EPSS
1%
Percentile
45.8
In the wild
Unconfirmed
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.

The record
Technical detail
CVSS v4.0
9.0 · CRITICAL
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
EPSS
0.00560 · 45.8th percentile
Weakness
CWE-644 · Improper Neutralization of HTTP Headers for Scripting Syntax
Published
2026-04-15T10:13Z
EPSS history
Timeline
  • 15 APR 10:13Z
    @fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers
    cvelistv5