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.

CORS() now uses the CONVENTIONS.md template's options (AllowedOrigins "*", AllowCredentials false) and covers only the two public, read-only routes, GET /api/v1/server and GET /.well-known/healthcheck.json. Every other route sends no Access-Control-Allow-* header. A preflight to the public routes gets 405 like any other path.

Deviation: CORS() is not in the global middleware chain CONVENTIONS.md lists, because REPO_POLICIES.md forbids * on authenticated routes.

Model: opus-5-5

internal/middleware/middleware.go

@@ -200,24 +200,19 @@ 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.
+// CORS returns middleware that lets a page on any origin read the
+// response. Use it only on public, read-only routes.
 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
-		},
+		AllowedOrigins: []string{"*"},
 		AllowedMethods: []string{
 			"GET", "POST", "PUT", "DELETE", "OPTIONS",
 		},
 		AllowedHeaders: []string{
-			"Accept", "Content-Type", "X-CSRF-Token",
+			"Accept", "Authorization", "Content-Type", "X-CSRF-Token",
 		},
 		ExposedHeaders:   []string{"Link"},
-		AllowCredentials: true,
+		AllowCredentials: false,
 		MaxAge:           corsMaxAge,
 	})
 }