Stop allowing cross-origin requests (closes #261)

The CORS middleware let every origin make credentialed requests, so a
page on another subdomain of the server's domain could read and act as
the signed-in user. Nothing needs cross-origin access, so the
middleware and the go-chi/cors dependency go. A test checks that a
request from another origin and a preflight get no
Access-Control-Allow-* headers.

Model: opus-5-5

README.md

@@ -168,8 +168,8 @@ either.
 - Request bodies for `POST /api/v1/session`, `POST /api/v1/login` and
   `POST /api/v1/messages` are limited to `MAX_MESSAGE_SIZE` bytes (default
   `4096`). A larger body gets `400` `invalid request body`.
-- The server allows cross-origin requests from any origin, with credentials. The
-  cookie is `SameSite=Strict`, so a browser sends it only from the same site.
+- The server allows no cross-origin requests. In a browser, only pages the
+  server itself serves, such as the web client, can use the API.
 
 ### The auth cookie
 
@@ -1892,14 +1892,13 @@ The IRC listener has no TLS.
   seconds. A request still running after 60 seconds is cancelled, and the
   connection closes without a response. Headers may be up to 1 MiB.
 - A long poll waits at most 30 seconds.
-- Every response to a well-formed request, except to a cross-origin preflight,
-  carries a `Content-Security-Policy` header:
+- Every response to a well-formed request carries a `Content-Security-Policy`
+  header:
   `default-src 'self'; script-src 'self'; style-src 'self'; connect-src 'self'; img-src 'self'; font-src 'self'; object-src 'none'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'`.
   The web client works within it.
-- Cross-origin requests are allowed from any origin, with credentials. The
-  allowed methods are `GET`, `POST`, `PUT`, `DELETE` and `OPTIONS`. The allowed
-  request headers are `Accept`, `Content-Type` and `X-CSRF-Token`. The cookie's
-  `SameSite=Strict` still keeps it to the same site.
+- The server allows no cross-origin requests. No response carries an
+  `Access-Control-Allow-*` header. A preflight request gets `405`, like any
+  other `OPTIONS` request.
 
 ### Logging
 
@@ -2002,8 +2001,8 @@ and their sessions to end. Then it closes the database.
   gets a message.
 - `internal/hashcash/`: stamp checking and minting.
 - `internal/ratelimit/`: the per-address login rate limiter.
-- `internal/middleware/`: finding the client's address, request logging,
-  cross-origin requests, the `Content-Security-Policy` header and metrics.
+- `internal/middleware/`: finding the client's address, request logging, the
+  `Content-Security-Policy` header and metrics.
 - `internal/healthcheck/` and `internal/stats/`: the health check and its
   counters.
 - `internal/logger/` and `internal/globals/`: logging setup and build

go.mod

@@ -7,7 +7,6 @@ require (
 	github.com/gdamore/tcell/v2 v2.13.8
 	github.com/getsentry/sentry-go v0.42.0
 	github.com/go-chi/chi/v5 v5.2.1
-	github.com/go-chi/cors v1.2.2
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
 	github.com/prometheus/client_golang v1.23.2

go.sum

@@ -20,8 +20,6 @@ github.com/getsentry/sentry-go v0.42.0 h1:eeFMACuZTbUQf90RE8dE4tXeSe4CZyfvR1MBL7
 github.com/getsentry/sentry-go v0.42.0/go.mod h1:eRXCoh3uvmjQLY6qu63BjUZnaBu5L5WhMV1RwYO8W5s=
 github.com/go-chi/chi/v5 v5.2.1 h1:KOIHODQj58PmL80G2Eak4WdvUzjSJSm0vG72crDCqb8=
 github.com/go-chi/chi/v5 v5.2.1/go.mod h1:L2yAIGWB3H+phAw1NxKwWM+7eUH/lU8pOMm5hHcoops=
-github.com/go-chi/cors v1.2.2 h1:Jmey33TE+b+rB7fT8MUy1u0I4L+NARQlK6LhzKPSyQE=
-github.com/go-chi/cors v1.2.2/go.mod h1:sSbTewc+6wYHBBCW7ytsFSn836hqM7JxpglAy2Vzc58=
 github.com/go-errors/errors v1.4.2 h1:J6MZopCL4uSllY1OfXM374weqZFFItUbrImctkmUxIA=
 github.com/go-errors/errors v1.4.2/go.mod h1:sIVyrIiJhuEF+Pj9Ebtd6P/rEYROXFi3BopGUQ5a5Og=
 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=

internal/middleware/middleware.go

@@ -12,7 +12,6 @@ import (
 
 	basicauth "github.com/99designs/basicauth-go"
 	chimw "github.com/go-chi/chi/v5/middleware"
-	"github.com/go-chi/cors"
 	metrics "github.com/slok/go-http-metrics/metrics/prometheus"
 	ghmm "github.com/slok/go-http-metrics/middleware"
 	"github.com/slok/go-http-metrics/middleware/std"
@@ -24,8 +23,6 @@ import (
 	"sneak.berlin/go/neoirc/internal/logger"
 )
 
-const corsMaxAge = 300
-
 // Params defines the dependencies for creating Middleware.
 type Params struct {
 	fx.In
@@ -200,28 +197,6 @@ func (mware *Middleware) Logging() func(http.Handler) http.Handler {
 	}
 }
 
-// CORS returns middleware that handles Cross-Origin Resource Sharing.
-// AllowCredentials is true so browsers include cookies in
-// cross-origin API requests.
-func (mware *Middleware) CORS() func(http.Handler) http.Handler {
-	return cors.Handler(cors.Options{ //nolint:exhaustruct // optional fields
-		AllowOriginFunc: func(
-			_ *http.Request, _ string,
-		) bool {
-			return true
-		},
-		AllowedMethods: []string{
-			"GET", "POST", "PUT", "DELETE", "OPTIONS",
-		},
-		AllowedHeaders: []string{
-			"Accept", "Content-Type", "X-CSRF-Token",
-		},
-		ExposedHeaders:   []string{"Link"},
-		AllowCredentials: true,
-		MaxAge:           corsMaxAge,
-	})
-}
-
 // Metrics returns middleware that records HTTP metrics.
 func (mware *Middleware) Metrics() func(http.Handler) http.Handler {
 	metricsMiddleware := ghmm.New(ghmm.Config{ //nolint:exhaustruct // optional fields

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))
 

internal/server/server_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"strconv"
 	"strings"
 	"testing"
@@ -188,6 +189,70 @@ func TestServeAfterStopDoesNotStopApp(t *testing.T) {
 	}
 }
 
+// TestNoCrossOriginAccess checks that the server allows no
+// cross-origin requests: neither a request from another origin
+// nor a preflight gets Access-Control-Allow-Origin or
+// Access-Control-Allow-Credentials, and a preflight gets 405
+// like any other OPTIONS request.
+func TestNoCrossOriginAccess(t *testing.T) {
+	var srv *server.Server
+
+	startApp(t, fx.Populate(&srv))
+
+	tests := []struct {
+		name          string
+		method        string
+		requestMethod string
+		status        int
+	}{
+		{
+			name:   "request",
+			method: http.MethodGet,
+			status: http.StatusOK,
+		},
+		{
+			name:          "preflight",
+			method:        http.MethodOptions,
+			requestMethod: http.MethodPost,
+			status:        http.StatusMethodNotAllowed,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			request := httptest.NewRequestWithContext(
+				t.Context(), test.method,
+				"http://chat.example.com/api/v1/server", 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)
+
+			if recorder.Code != test.status {
+				t.Errorf("status %d, want %d", recorder.Code, test.status)
+			}
+
+			for _, header := range []string{
+				"Access-Control-Allow-Origin",
+				"Access-Control-Allow-Credentials",
+			} {
+				value := recorder.Header().Get(header)
+				if value != "" {
+					t.Errorf("%s is %q, want none", header, value)
+				}
+			}
+		})
+	}
+}
+
 // startApp starts the app newApp builds, with the HTTP port
 // set to 0, so the system gives the server a free port.
 func startApp(