fix(main): bound HTTP response reads and use sync.Once for config init

- Limit all HTTP response body reads to 1 MB using io.LimitReader to prevent memory exhaustion
- Replace non-thread-safe configInitialized bool with sync.Once for safe concurrent initialization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: appleboy

main.go

@@ -28,21 +28,24 @@ import (
 )
 
 var (
-	serverURL         string
-	clientID          string
-	tokenFile         string
-	tokenStoreMode    string
-	flagServerURL     *string
-	flagClientID      *string
-	flagTokenFile     *string
-	flagTokenStore    *string
-	configInitialized bool
-	retryClient       *retry.Client
-	tokenStore        credstore.Store[credstore.Token]
+	serverURL      string
+	clientID       string
+	tokenFile      string
+	tokenStoreMode string
+	flagServerURL  *string
+	flagClientID   *string
+	flagTokenFile  *string
+	flagTokenStore *string
+	configOnce     sync.Once
+	retryClient    *retry.Client
+	tokenStore     credstore.Store[credstore.Token]
 )
 
 const defaultKeyringService = "authgate-device-cli"
 
+// maxResponseBodySize limits HTTP response body reads to prevent memory exhaustion (DoS).
+const maxResponseBodySize = 1 << 20 // 1 MB
+
 // Timeout configuration for different operations
 const (
 	deviceCodeRequestTimeout = 10 * time.Second
@@ -107,11 +110,12 @@ func init() {
 // initConfig parses flags and initializes configuration
 // Separated from init() to avoid conflicts with test flag parsing
 func initConfig() {
-	if configInitialized {
-		return
-	}
-	configInitialized = true
+	configOnce.Do(func() {
+		doInitConfig()
+	})
+}
 
+func doInitConfig() {
 	flag.Parse()
 
 	// Priority: flag > env > default
@@ -438,7 +442,7 @@ func requestDeviceCode(ctx context.Context) (*oauth2.DeviceAuthResponse, error)
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodySize))
 	if err != nil {
 		return nil, fmt.Errorf("failed to read response: %w", err)
 	}
@@ -638,7 +642,7 @@ func exchangeDeviceCode(
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodySize))
 	if err != nil {
 		return nil, fmt.Errorf("failed to read response: %w", err)
 	}
@@ -696,7 +700,7 @@ func verifyToken(ctx context.Context, accessToken string, d tui.Displayer) error
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodySize))
 	if err != nil {
 		return fmt.Errorf("failed to read response: %w", err)
 	}
@@ -746,7 +750,7 @@ func refreshAccessToken(
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodySize))
 	if err != nil {
 		return credstore.Token{}, fmt.Errorf("failed to read response: %w", err)
 	}
@@ -871,7 +875,7 @@ func makeAPICallWithAutoRefresh(
 		defer resp.Body.Close()
 	}
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxResponseBodySize))
 	if err != nil {
 		return fmt.Errorf("failed to read response: %w", err)
 	}