Stop allowing credentialed cross-origin requests (closes #261)

CORS() allowed every origin with credentials, so a page on another
origin of the same site could read what the signed-in user sees. It
now uses the options of the CONVENTIONS.md template: every origin as
*, no credentials. It applies only to GET /api/v1/server and
GET /.well-known/healthcheck.json, the public read-only routes, since
REPO_POLICIES.md forbids * on authenticated routes. Every other route
sends no Access-Control-Allow-* header.

Model: opus-5-5

internal/server/routes.go

@@ -30,7 +30,6 @@ func (srv *Server) SetupRoutes() {
 		srv.router.Use(srv.mw.Metrics())
 	}
 
-	srv.router.Use(srv.mw.CORS())
 	srv.router.Use(srv.mw.CSP())
 	srv.router.Use(middleware.Timeout(routeTimeout))
 
@@ -45,7 +44,7 @@ func (srv *Server) SetupRoutes() {
 	}
 
 	// Health check.
-	srv.router.Get(
+	srv.router.With(srv.mw.CORS()).Get(
 		"/.well-known/healthcheck.json",
 		srv.handlers.HandleHealthCheck(),
 	)
@@ -69,7 +68,7 @@ func (srv *Server) SetupRoutes() {
 }
 
 func (srv *Server) setupAPIv1(router chi.Router) {
-	router.Get(
+	router.With(srv.mw.CORS()).Get(
 		"/server",
 		srv.handlers.HandleServerInfo(),
 	)

internal/server/server_test.go

@@ -6,8 +6,10 @@ package server_test
 
 import (
 	"fmt"
+	"maps"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"strconv"
 	"strings"
 	"testing"
@@ -188,6 +190,88 @@ func TestServeAfterStopDoesNotStopApp(t *testing.T) {
 	}
 }
 
+// TestCORSHeaders checks the Access-Control-Allow-* headers on the
+// answers to requests from another origin. The public, read-only
+// routes answer with Access-Control-Allow-Origin: * alone. Routes
+// that need the cookie, and a preflight to one, answer with none.
+func TestCORSHeaders(t *testing.T) {
+	var srv *server.Server
+
+	startApp(t, fx.Populate(&srv))
+
+	tests := []struct {
+		name          string
+		method        string
+		path          string
+		requestMethod string
+		wantHeaders   map[string]string
+	}{
+		{
+			name:        "server information",
+			method:      http.MethodGet,
+			path:        "/api/v1/server",
+			wantHeaders: map[string]string{"Access-Control-Allow-Origin": "*"},
+		},
+		{
+			name:        "health check",
+			method:      http.MethodGet,
+			path:        "/.well-known/healthcheck.json",
+			wantHeaders: map[string]string{"Access-Control-Allow-Origin": "*"},
+		},
+		{
+			name:   "send",
+			method: http.MethodPost,
+			path:   "/api/v1/messages",
+		},
+		{
+			name:   "state",
+			method: http.MethodGet,
+			path:   "/api/v1/state",
+		},
+		{
+			name:          "preflight for send",
+			method:        http.MethodOptions,
+			path:          "/api/v1/messages",
+			requestMethod: http.MethodPost,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			request := httptest.NewRequestWithContext(
+				t.Context(), test.method,
+				"http://chat.example.com"+test.path, nil,
+			)
+			request.Header.Set("Origin", "http://blog.example.com")
+
+			if test.requestMethod != "" {
+				request.Header.Set(
+					"Access-Control-Request-Method", test.requestMethod,
+				)
+			}
+
+			recorder := httptest.NewRecorder()
+
+			srv.ServeHTTP(recorder, request)
+
+			gotHeaders := map[string]string{}
+
+			for name := range recorder.Header() {
+				if strings.HasPrefix(name, "Access-Control-Allow-") {
+					gotHeaders[name] = recorder.Header().Get(name)
+				}
+			}
+
+			if !maps.Equal(gotHeaders, test.wantHeaders) {
+				t.Errorf(
+					"Access-Control-Allow-* headers are %v, want %v",
+					gotHeaders, test.wantHeaders,
+				)
+			}
+		})
+	}
+}
+
 // startApp starts the app newApp builds, with the HTTP port
 // set to 0, so the system gives the server a free port.
 func startApp(