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

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