GHSA-fq95-v8xc-jm3v Fabio - Incomplete fix for CVE-2025-48865: operator-configured trust headers (clientip/tls/requestid) still strippable via the Connection header

• Go github.com/fabiolb/fabio: >= 1.6.6, < 1.7.2 → fixed in 1.7.2

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.Del`s 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: ` and `X-Request-ID: `. 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.