Affected: github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).
Summary
The v1.6.6 fix for CVE-2025-48865 sweeps the client Connection header against a hardcoded allowlist protectHeaders (proxy/http_headers.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.
Details
The three unprotected trust headers and where fabio sets them:
- ClientIPHeader (
proxy.header.clientip) - http_headers.go:73 r.Header.Set(cfg.ClientIPHeader, remoteIP)
- TLSHeader (
proxy.header.tls) - http_headers.go:153 r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue) (TLS connections)
- RequestID (
proxy.header.requestid) - http_proxy.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.
PoC
Configure proxy.header.clientip=X-Client-IP, proxy.header.requestid=X-Request-ID. Raw request:
GET / HTTP/1.1
Host: foo.com
Connection: close, X-Client-IP, X-Request-ID
Backend sees X-Client-IP: <empty> and X-Request-ID: <empty>. Baseline (Connection: close) => backend sees the real client IP and a request id. The same works for proxy.header.tls over TLS (Connection: close, X-Secure strips the TLS assertion). Reproduced by the two PASS-ing tests in proxy/poc_cve48865_incomplete_test.go on master HEAD (Go 1.26.4). Control: X-Forwarded-For (in protectHeaders) survives, confirming the gap is specific to the configured headers.
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.
References
thientd Reporter
Affected: github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).
Summary
The v1.6.6 fix for CVE-2025-48865 sweeps the client
Connectionheader against a hardcoded allowlistprotectHeaders(proxy/http_headers.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.Details
The three unprotected trust headers and where fabio sets them:
proxy.header.clientip) - http_headers.go:73r.Header.Set(cfg.ClientIPHeader, remoteIP)proxy.header.tls) - http_headers.go:153r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue)(TLS connections)proxy.header.requestid) - http_proxy.go:90r.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, whoseremoveHopByHopHeaders(Go stdlib net/http/httputil/reverseproxy.go) iterates the inboundConnectionheader andh.Dels every listed header. The three trust headers fabio just set are therefore deleted before the request reaches the backend.PoC
Configure
proxy.header.clientip=X-Client-IP,proxy.header.requestid=X-Request-ID. Raw request:Backend sees
X-Client-IP: <empty>andX-Request-ID: <empty>. Baseline (Connection: close) => backend sees the real client IP and a request id. The same works forproxy.header.tlsover TLS (Connection: close, X-Securestrips the TLS assertion). Reproduced by the two PASS-ing tests in proxy/poc_cve48865_incomplete_test.go on master HEAD (Go 1.26.4). Control:X-Forwarded-For(in protectHeaders) survives, confirming the gap is specific to the configured headers.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.
References