Fabio - Incomplete fix for CVE-2025-48865: operator-configured trust headers (clientip/tls/requestid) still strippable via the Connection header

QFinch assessment

The v1.6.6 fix for CVE-2025-48865 sweeps the client Connection header against a hardcoded allowlist protectHeaders (proxy/httpheaders.go:28-36) containing only the 7 X-Forwarded family headers. Fabio also injects three operator-configured server-side trust headers that are NOT in that allowlist, so the original hop-by-hop stripping attack still works against them.

Potential impact

Where a backend trusts these fabio-set headers, an external unauthenticated client can strip/downgrade the trust signal: bypass/poison IP-based ACL or audit that reads the clientip header, downgrade a TLS-terminated request to appear non-TLS to a backend keying off the TLS header, or drop request-id correlation. Scope: deployments that enable the relevant proxy.header. option (all empty by default). Fix: add the configured ClientIPHeader/TLSHeader/RequestID names to the protectHeaders set (or strip their token from the inbound Connection header) inside addHeaders. Parent: CVE-2025-48865 / GHSA-q7p4-7xjv-j3wf (fixed in v1.6.6). This is an incomplete-fix sibling - the same hop-by-hop stripping primitive applies to the operator-configured trust headers the allowlist does not cover.

Technical context

The three unprotected trust headers and where fabio sets them: - ClientIPHeader (proxy.header.clientip) - httpheaders.go:73 r.Header.Set(cfg.ClientIPHeader, remoteIP) - TLSHeader (proxy.header.tls) - httpheaders.go:153 r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue) (TLS connections) - RequestID (proxy.header.requestid) - httpproxy.go:90 r.Header.Set(p.Config.RequestID, id()) Order of operations in HTTPProxy.ServeHTTP: (1) line 90 sets RequestID; (2) line 175 calls addHeaders, which sweeps the Connection header (keeping the three configured names because they are absent from protectHeaders) and then sets ClientIPHeader/TLSHeader; (3) line 223 runs the Go httputil.ReverseProxy, whose removeHopByHopHeaders (Go stdlib net/http/httputil/reverseproxy.go) iterates the inbound Connection header and h.Dels every listed header. The three trust headers fabio just set are therefore deleted before the request reaches the backend.