diff --git a/callback.go b/callback.go
index a03c348..b965548 100644
--- a/callback.go
+++ b/callback.go
@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"crypto/subtle"
 	"fmt"
 	"html"
 	"net"
@@ -71,9 +72,10 @@ func startCallbackServer(
 			return
 		}
 
-		// Validate state (CSRF protection).
+		// Validate state (CSRF protection) using constant-time comparison.
 		state := q.Get("state")
-		if state != expectedState {
+		if len(state) != len(expectedState) ||
+			subtle.ConstantTimeCompare([]byte(state), []byte(expectedState)) != 1 {
 			writeCallbackPage(w, false, "state_mismatch",
 				"State parameter does not match. Possible CSRF attack.")
 			sendResult(callbackResult{
diff --git a/main.go b/main.go
index 3585854..718348a 100644
--- a/main.go
+++ b/main.go
@@ -36,7 +36,6 @@ var (
 	callbackPort   int
 	scope          string
 	tokenFile      string
-	tokenStoreMode string
 	tokenStore     credstore.Store[credstore.Token]
 	configOnce     sync.Once
 	retryClient    *retry.Client
@@ -170,16 +169,16 @@ func doInitConfig() {
 	var err error
 	retryClient, err = retry.NewBackgroundClient(retry.WithHTTPClient(baseHTTPClient))
 	if err != nil {
-		panic(fmt.Sprintf("failed to create retry client: %v", err))
+		fmt.Fprintf(os.Stderr, "Error: failed to create retry client: %v\n", err)
+		os.Exit(1)
 	}
 
 	const defaultKeyringService = "authgate-oauth-cli"
-	tokenStoreMode = getConfig(*flagTokenStore, "TOKEN_STORE", "auto")
+	tokenStoreMode := getConfig(*flagTokenStore, "TOKEN_STORE", "auto")
 	var warnings []string
-	var err2 error
-	tokenStore, warnings, err2 = initTokenStore(tokenStoreMode, tokenFile, defaultKeyringService)
-	if err2 != nil {
-		fmt.Fprintf(os.Stderr, "Error: %v\n", err2)
+	tokenStore, warnings, err = initTokenStore(tokenStoreMode, tokenFile, defaultKeyringService)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 		os.Exit(1)
 	}
 	configWarnings = append(configWarnings, warnings...)
@@ -298,6 +297,16 @@ func parseOAuthError(statusCode int, body []byte, action string) error {
 	return fmt.Errorf("%s failed with status %d: %s", action, statusCode, string(body))
 }
 
+// isRefreshTokenError checks whether the response body indicates an expired
+// or invalid refresh token (invalid_grant / invalid_token).
+func isRefreshTokenError(body []byte) bool {
+	var errResp ErrorResponse
+	if err := json.Unmarshal(body, &errResp); err == nil {
+		return errResp.Error == "invalid_grant" || errResp.Error == "invalid_token"
+	}
+	return false
+}
+
 // validateTokenResponse performs basic sanity checks on a token response.
 func validateTokenResponse(accessToken, tokenType string, expiresIn int) error {
 	if accessToken == "" {
@@ -438,12 +447,8 @@ func refreshAccessToken(ctx context.Context, refreshToken string) (*tui.TokenSto
 
 	if resp.StatusCode != http.StatusOK {
 		// Check for expired/invalid refresh token before general error handling.
-		var errResp ErrorResponse
-		if jsonErr := json.Unmarshal(body, &errResp); jsonErr == nil && errResp.Error != "" {
-			if errResp.Error == "invalid_grant" || errResp.Error == "invalid_token" {
-				return nil, tui.ErrRefreshTokenExpired
-			}
-			return nil, fmt.Errorf("%s: %s", errResp.Error, errResp.ErrorDescription)
+		if isRefreshTokenError(body) {
+			return nil, tui.ErrRefreshTokenExpired
 		}
 		return nil, parseOAuthError(resp.StatusCode, body, "refresh")
 	}
@@ -522,10 +527,9 @@ func makeAPICallWithAutoRefresh(ctx context.Context, storage *tui.TokenStorage)
 	if err != nil {
 		return fmt.Errorf("API request failed: %w", err)
 	}
-	defer resp.Body.Close()
 
 	if resp.StatusCode == http.StatusUnauthorized {
-		// Drain and close body immediately so the HTTP transport can reuse the connection.
+		// Drain and close body so the HTTP transport can reuse the connection.
 		_, _ = io.Copy(io.Discard, resp.Body)
 		resp.Body.Close()
 
@@ -554,8 +558,8 @@ func makeAPICallWithAutoRefresh(ctx context.Context, storage *tui.TokenStorage)
 		if err != nil {
 			return fmt.Errorf("retry failed: %w", err)
 		}
-		defer resp.Body.Close()
 	}
+	defer resp.Body.Close()
 
 	body, err := readResponseBody(resp.Body)
 	if err != nil {
diff --git a/main_test.go b/main_test.go
index 5479c3d..76215a4 100644
--- a/main_test.go
+++ b/main_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -179,7 +180,7 @@ func TestBuildAuthURL_ContainsRequiredParams(t *testing.T) {
 		"code_challenge=test-challenge",
 		"code_challenge_method=S256",
 	} {
-		if !containsSubstring(u, want) {
+		if !strings.Contains(u, want) {
 			t.Errorf("auth URL missing %q\nURL: %s", want, u)
 		}
 	}
@@ -273,7 +274,7 @@ func TestInitTokenStore_Invalid(t *testing.T) {
 	if store != nil {
 		t.Errorf("expected nil store on error, got %T", store)
 	}
-	if !containsSubstring(err.Error(), "invalid token-store value") {
+	if !strings.Contains(err.Error(), "invalid token-store value") {
 		t.Errorf("unexpected error message: %v", err)
 	}
 }
@@ -309,17 +310,3 @@ func TestReadResponseBody(t *testing.T) {
 		}
 	})
 }
-
-// containsSubstring is a helper to avoid importing strings in tests.
-func containsSubstring(s, sub string) bool {
-	return len(s) >= len(sub) && findSubstring(s, sub)
-}
-
-func findSubstring(s, sub string) bool {
-	for i := 0; i <= len(s)-len(sub); i++ {
-		if s[i:i+len(sub)] == sub {
-			return true
-		}
-	}
-	return false
-}