Allow CORS origins from CORS_ALLOWED_ORIGINS

Echo compares allowed origins literally, so "https://*" and "http://*" never
matched a real origin and no CORS headers were sent. The allowed origins now
come from CORS_ALLOWED_ORIGINS (comma-separated, checked at startup). When it
is empty, only same-origin requests work, which is enough for the bundled
frontend.

Credentials are no longer allowed since the API uses Bearer tokens. Browsers
may now send Range and X-Queue-Id and read X-Queue-Id and Content-Range.

internal/config/config.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"net/url"
 	"os"
 	"strconv"
 	"strings"
@@ -38,6 +39,10 @@ type Config struct {
 	// TrustedProxies are CIDR ranges whose X-Forwarded-For header is trusted when resolving client IPs.
 	TrustedProxies []*net.IPNet
 
+	// CORSAllowedOrigins are the browser origins, like https://music.example.com, that may call the API
+	// from another site. Empty means only same-origin requests, which is enough for the bundled frontend.
+	CORSAllowedOrigins []string
+
 	ReleasesURL string
 }
 
@@ -91,6 +96,11 @@ func load(getenv func(string) string) (Config, error) {
 		errs = append(errs, fmt.Errorf("TRUSTED_PROXIES: %w", err))
 	}
 
+	cfg.CORSAllowedOrigins, err = parseOrigins(getenv("CORS_ALLOWED_ORIGINS"))
+	if err != nil {
+		errs = append(errs, fmt.Errorf("CORS_ALLOWED_ORIGINS: %w", err))
+	}
+
 	return cfg, errors.Join(errs...)
 }
 
@@ -110,6 +120,27 @@ func parseCIDRs(value string) ([]*net.IPNet, error) {
 	return nets, nil
 }
 
+// parseOrigins parses a comma-separated list of origins (scheme://host[:port], nothing else).
+func parseOrigins(value string) ([]string, error) {
+	var origins []string
+	for part := range strings.SplitSeq(value, ",") {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+		u, err := url.Parse(part)
+		if err != nil {
+			return nil, err
+		}
+		if (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" || u.Path != "" || u.RawQuery != "" ||
+			u.Fragment != "" || u.User != nil || strings.Contains(u.Host, "*") {
+			return nil, fmt.Errorf("%q is not an origin like https://music.example.com", part)
+		}
+		origins = append(origins, u.Scheme+"://"+u.Host)
+	}
+	return origins, nil
+}
+
 func withDefault(value, fallback string) string {
 	if value == "" {
 		return fallback

internal/config/config_test.go

@@ -31,6 +31,7 @@ func TestLoadDefaults(t *testing.T) {
 	assert.Equal(t, "songs", cfg.SoundCheckPath)
 	assert.Equal(t, DefaultReleasesURL, cfg.ReleasesURL)
 	assert.Empty(t, cfg.TrustedProxies)
+	assert.Empty(t, cfg.CORSAllowedOrigins)
 }
 
 func TestLoadMissingRequired(t *testing.T) {
@@ -56,6 +57,21 @@ func TestLoadTrustedProxies(t *testing.T) {
 	assert.ErrorContains(t, err, "TRUSTED_PROXIES")
 }
 
+func TestLoadCORSAllowedOrigins(t *testing.T) {
+	values := validEnv()
+	values["CORS_ALLOWED_ORIGINS"] = "https://music.example.com, http://localhost:5173,"
+	cfg, err := load(env(values))
+	require.NoError(t, err)
+	assert.Equal(t, []string{"https://music.example.com", "http://localhost:5173"}, cfg.CORSAllowedOrigins)
+
+	for _, invalid := range []string{"music.example.com", "ftp://example.com", "https://example.com/app",
+		"https://*.example.com", "https://*", "*", "https://example.com?x=1", "https://user@example.com"} {
+		values["CORS_ALLOWED_ORIGINS"] = invalid
+		_, err = load(env(values))
+		assert.ErrorContains(t, err, "CORS_ALLOWED_ORIGINS", invalid)
+	}
+}
+
 func TestLoadInvalidPort(t *testing.T) {
 	values := validEnv()
 	values["PORT"] = "abc"

internal/server/routes.go

@@ -40,14 +40,17 @@ func (s *Server) routes() (*echo.Echo, huma.API) {
 
 	e.Use(logging.RequestLogger())
 	e.Use(echoMiddleware.Recover())
-	e.Use(echoMiddleware.CORSWithConfig(echoMiddleware.CORSConfig{
-		AllowOrigins:     []string{"https://*", "http://*"},
-		AllowMethods:     []string{"GET", "POST", "PUT", "DELETE", "OPTIONS", "PATCH"},
-		AllowHeaders:     []string{"Accept", "Authorization", "Content-Type", "X-CSRF-Token", middleware.QueueHeader},
-		ExposeHeaders:    []string{middleware.QueueHeader},
-		AllowCredentials: true,
-		MaxAge:           300,
-	}))
+	// Without allowed origins no CORS headers are sent, so only same-origin browser requests work.
+	// The API uses Bearer tokens, not cookies, so credentials are not allowed.
+	if len(s.cfg.CORSAllowedOrigins) > 0 {
+		e.Use(echoMiddleware.CORSWithConfig(echoMiddleware.CORSConfig{
+			AllowOrigins:  s.cfg.CORSAllowedOrigins,
+			AllowMethods:  []string{"GET", "POST", "PUT", "DELETE", "OPTIONS", "PATCH"},
+			AllowHeaders:  []string{"Accept", "Authorization", "Content-Type", "Range", middleware.QueueHeader},
+			ExposeHeaders: []string{middleware.QueueHeader, "Content-Range", "Accept-Ranges"},
+			MaxAge:        300,
+		}))
+	}
 
 	// The API: /api/v1 is documented at /docs and /openapi.json.
 	a := newAPI(e)

internal/server/routes_test.go

@@ -196,3 +196,30 @@ func TestTrustedProxies(t *testing.T) {
 	assert.Equal(t, http.StatusTooManyRequests, e.doBody(http.MethodPost, "/api/v1/token", `{}`, "X-Forwarded-For", "10.0.0.1").Code)
 	assert.Equal(t, http.StatusOK, e.doBody(http.MethodPost, "/api/v1/token", `{}`, "X-Forwarded-For", "10.0.0.2").Code)
 }
+
+func TestCORSAllowedOrigins(t *testing.T) {
+	e := newTestEnv(t, func(c *config.Config) { c.CORSAllowedOrigins = []string{"https://music.example.com"} })
+
+	rec := e.do(http.MethodGet, "/api/v1/music/list", "Origin", "https://music.example.com")
+	assert.Equal(t, http.StatusOK, rec.Code)
+	assert.Equal(t, "https://music.example.com", rec.Header().Get("Access-Control-Allow-Origin"))
+	assert.Contains(t, rec.Header().Get("Access-Control-Expose-Headers"), "X-Queue-Id")
+	assert.Empty(t, rec.Header().Get("Access-Control-Allow-Credentials"))
+
+	preflight := e.do(http.MethodOptions, "/api/v1/music/list", "Origin", "https://music.example.com",
+		"Access-Control-Request-Method", "GET", "Access-Control-Request-Headers", "authorization,x-queue-id")
+	assert.Equal(t, http.StatusNoContent, preflight.Code)
+	assert.Equal(t, "https://music.example.com", preflight.Header().Get("Access-Control-Allow-Origin"))
+	assert.Contains(t, preflight.Header().Get("Access-Control-Allow-Headers"), "X-Queue-Id")
+	assert.Contains(t, preflight.Header().Get("Access-Control-Allow-Headers"), "Authorization")
+
+	other := e.do(http.MethodGet, "/api/v1/music/list", "Origin", "https://evil.example.com")
+	assert.Empty(t, other.Header().Get("Access-Control-Allow-Origin"), "other origins get no CORS headers")
+}
+
+func TestCORSWithoutAllowedOrigins(t *testing.T) {
+	e := newTestEnv(t)
+	rec := e.do(http.MethodGet, "/api/v1/version", "Origin", "https://music.example.com")
+	assert.Equal(t, http.StatusOK, rec.Code, "the request itself works; the browser blocks reading it")
+	assert.Empty(t, rec.Header().Get("Access-Control-Allow-Origin"))
+}