fix(auth): set the session cookie's Secure flag behind a TLS-terminating proxy (M462 #4977)

The cookie set Secure from r.TLS, which is nil whenever TLS terminates at
Traefik or Cloudflare, so a public deployment's session cookie went out
without Secure. auth.IsHTTPS now decides it from X-Forwarded-Proto, believed
only through the trusted-hop count (rule 94) and read positionally like
X-Forwarded-For. Plain-HTTP and LAN logins are unchanged; nothing redirects.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/auth.go

@@ -140,7 +140,7 @@ func (h *handlers) handleLogin(w http.ResponseWriter, r *http.Request) {
 		Value:    token,
 		Path:     "/",
 		HttpOnly: true,
-		Secure:   r.TLS != nil, // dev over http stays functional; prod over https gets Secure
+		Secure:   auth.IsHTTPS(r, h.netSettings.Hops()), // plain HTTP and LAN stay functional (rule 94)
 		SameSite: http.SameSiteStrictMode,
 		MaxAge:   int(sessionCookieMaxAge.Seconds()),
 	})

internal/api/auth_register.go

@@ -195,7 +195,7 @@ func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
 		Value:    sessionToken,
 		Path:     "/",
 		HttpOnly: true,
-		Secure:   r.TLS != nil,
+		Secure:   auth.IsHTTPS(r, h.netSettings.Hops()), // plain HTTP and LAN stay functional (rule 94)
 		SameSite: http.SameSiteStrictMode,
 		MaxAge:   int(sessionCookieMaxAge.Seconds()),
 	})

internal/auth/clientip.go

@@ -65,6 +65,36 @@ func ClientIP(r *http.Request, trustedProxyHops int) string {
 	return remote
 }
 
+// IsHTTPS reports whether the client reached us over HTTPS, reading through
+// trustedProxyHops reverse proxies by the same rule as ClientIP.
+//
+// TLS terminates at the operator's proxy (rule 94), so r.TLS is nil in every
+// real deployment and only X-Forwarded-Proto can say what the client used.
+// That header is believed only when hops >= 1: with no trusted proxy it is
+// just something any client can send. Proxies that append rather than
+// overwrite produce a comma list, read positionally like X-Forwarded-For.
+//
+// This decides only HTTPS-only behaviour (the cookie Secure flag, HSTS). It
+// must never drive a redirect: plain-HTTP and LAN use keep working.
+func IsHTTPS(r *http.Request, trustedProxyHops int) bool {
+	if r.TLS != nil {
+		return true
+	}
+	if trustedProxyHops <= 0 {
+		return false
+	}
+	raw := r.Header.Get("X-Forwarded-Proto")
+	if strings.TrimSpace(raw) == "" {
+		return false
+	}
+	parts := strings.Split(raw, ",")
+	idx := len(parts) - trustedProxyHops
+	if idx < 0 {
+		idx = 0
+	}
+	return strings.EqualFold(strings.TrimSpace(parts[idx]), "https")
+}
+
 // forwardedChain returns the X-Forwarded-For entries in wire order, or the
 // single X-Real-IP value when XFF is absent.
 //

internal/auth/clientip_test.go

@@ -1,6 +1,7 @@
 package auth
 
 import (
+	"crypto/tls"
 	"net/http"
 	"testing"
 )
@@ -168,3 +169,45 @@ func TestClientIP(t *testing.T) {
 		})
 	}
 }
+
+func TestIsHTTPS(t *testing.T) {
+	tests := []struct {
+		name  string
+		hops  int
+		proto string
+		tls   bool
+		want  bool
+	}{
+		// The three cases rule 94 names.
+		{name: "hops 0 ignores a forwarded https", hops: 0, proto: "https", want: false},
+		{name: "hops 1 believes a forwarded https", hops: 1, proto: "https", want: true},
+		{name: "plain request is not https", hops: 1, want: false},
+
+		{name: "forwarded http is not https", hops: 1, proto: "http", want: false},
+		{name: "case-insensitive", hops: 1, proto: "HTTPS", want: true},
+		{name: "direct TLS is https whatever the hops", hops: 0, tls: true, want: true},
+		// An appending chain is read positionally: with one trusted hop the
+		// last entry is our proxy's word, and a client-supplied first entry
+		// cannot flip it.
+		{name: "appended chain reads our proxy's entry", hops: 1, proto: "https, http", want: false},
+		{name: "two hops read the CDN's entry", hops: 2, proto: "https, http", want: true},
+		{name: "over-counted hops clamp to the first entry", hops: 5, proto: "https", want: true},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			r, err := http.NewRequest(http.MethodGet, "/api/auth/login", nil)
+			if err != nil {
+				t.Fatalf("NewRequest: %v", err)
+			}
+			if tc.proto != "" {
+				r.Header.Set("X-Forwarded-Proto", tc.proto)
+			}
+			if tc.tls {
+				r.TLS = &tls.ConnectionState{}
+			}
+			if got := IsHTTPS(r, tc.hops); got != tc.want {
+				t.Errorf("IsHTTPS(hops=%d, proto=%q) = %v, want %v", tc.hops, tc.proto, got, tc.want)
+			}
+		})
+	}
+}