fix(panel): accept 2FA codes from adjacent TOTP windows

CheckUser compared only gotp.Now(), so a code submitted at the end of
its 30s window (or with slight client/server clock drift) failed with
'invalid 2fa code', while the immediate retry in the next window
succeeded. Accept current +/-1 window, the standard TOTP skew
tolerance.

Fixes MHSanaei/3x-ui#6535

internal/web/service/panel/user.go

@@ -2,6 +2,7 @@ package panel
 
 import (
 	"errors"
+	"time"
 
 	"github.com/xlzd/gotp"
 	"gorm.io/gorm"
@@ -97,7 +98,7 @@ func (s *UserService) CheckUser(username string, password string, twoFactorCode
 			return nil, err
 		}
 
-		if gotp.NewDefaultTOTP(twoFactorToken).Now() != twoFactorCode {
+		if !verifyTOTPWithSkew(twoFactorToken, twoFactorCode) {
 			return nil, errors.New("invalid 2fa code")
 		}
 	}
@@ -105,6 +106,26 @@ func (s *UserService) CheckUser(username string, password string, twoFactorCode
 	return user, nil
 }
 
+// totpSkewWindows is how many 30s steps around now are accepted. Client and
+// server clocks are rarely perfectly in sync, and a code submitted at the end
+// of its window may arrive after the server has rolled over — without skew
+// the first attempt fails and the immediate retry (in the next window)
+// succeeds, see #6535.
+const totpSkewWindows = 1
+
+// verifyTOTPWithSkew accepts the code for the current step plus/minus
+// totpSkewWindows steps, the standard tolerance for TOTP clock drift.
+func verifyTOTPWithSkew(secret, code string) bool {
+	totp := gotp.NewDefaultTOTP(secret)
+	now := time.Now()
+	for i := -totpSkewWindows; i <= totpSkewWindows; i++ {
+		if totp.AtTime(now.Add(time.Duration(i*30)*time.Second)) == code {
+			return true
+		}
+	}
+	return false
+}
+
 func (s *UserService) BumpLoginEpoch() error {
 	db := database.GetDB()
 	return db.Model(model.User{}).

internal/web/service/panel/user_totp_test.go

@@ -0,0 +1,33 @@
+package panel
+
+import (
+	"testing"
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+func TestVerifyTOTPWithSkew(t *testing.T) {
+	secret := "JBSWY3DPEHPK3PXP"
+	totp := gotp.NewDefaultTOTP(secret)
+	now := time.Now()
+
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now)) {
+		t.Fatal("current window code should verify")
+	}
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now.Add(-30*time.Second))) {
+		t.Fatal("previous window code should verify (clock skew)")
+	}
+	if !verifyTOTPWithSkew(secret, totp.AtTime(now.Add(30*time.Second))) {
+		t.Fatal("next window code should verify (clock skew)")
+	}
+	if verifyTOTPWithSkew(secret, totp.AtTime(now.Add(-60*time.Second))) {
+		t.Fatal("code two windows old should not verify")
+	}
+	if verifyTOTPWithSkew(secret, totp.AtTime(now.Add(60*time.Second))) {
+		t.Fatal("code two windows ahead should not verify")
+	}
+	if verifyTOTPWithSkew(secret, "000000") {
+		t.Fatal("wrong code should not verify")
+	}
+}