fix(panel): accept 2FA codes from adjacent TOTP windows (#6546)

* 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

* fix(panel): share TOTP skew tolerance with VerifyTwoFactorCode

Move the +/-1 window helper to internal/util/totp so both 2FA
acceptance points use it: login (CheckUser) and disable/rebind plus
username/password changes (VerifyTwoFactorCode). Also shrink comments
to the 2-line house rule and anchor the unit test mid-window to avoid
a step-boundary flake.

Addresses review on #6546 (MEDIUM + 2 LOWs).

---------

Co-authored-by: sdhfsl <sdhfsl@users.noreply.github.com>

internal/util/totp/totp.go

@@ -0,0 +1,22 @@
+package totp
+
+import (
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+// SkewWindows is how many 30s steps around now VerifyWithSkew accepts.
+// Standard TOTP clock-drift tolerance, see MHSanaei/3x-ui#6535.
+const SkewWindows = 1
+
+// VerifyWithSkew accepts the code for the current step plus/minus SkewWindows.
+func VerifyWithSkew(secret, code string, now time.Time) bool {
+	totp := gotp.NewDefaultTOTP(secret)
+	for i := -SkewWindows; i <= SkewWindows; i++ {
+		if totp.AtTime(now.Add(time.Duration(i*30)*time.Second)) == code {
+			return true
+		}
+	}
+	return false
+}

internal/util/totp/totp_test.go

@@ -0,0 +1,34 @@
+package totp
+
+import (
+	"testing"
+	"time"
+
+	"github.com/xlzd/gotp"
+)
+
+func TestVerifyWithSkew(t *testing.T) {
+	secret := "JBSWY3DPEHPK3PXP"
+	totp := gotp.NewDefaultTOTP(secret)
+	// Anchor mid-window so a step boundary can't fall between sampling and verify.
+	now := time.Unix((time.Now().Unix()/30)*30+15, 0).UTC()
+
+	if !VerifyWithSkew(secret, totp.AtTime(now), now) {
+		t.Fatal("current window code should verify")
+	}
+	if !VerifyWithSkew(secret, totp.AtTime(now.Add(-30*time.Second)), now) {
+		t.Fatal("previous window code should verify (clock skew)")
+	}
+	if !VerifyWithSkew(secret, totp.AtTime(now.Add(30*time.Second)), now) {
+		t.Fatal("next window code should verify (clock skew)")
+	}
+	if VerifyWithSkew(secret, totp.AtTime(now.Add(-60*time.Second)), now) {
+		t.Fatal("code two windows old should not verify")
+	}
+	if VerifyWithSkew(secret, totp.AtTime(now.Add(60*time.Second)), now) {
+		t.Fatal("code two windows ahead should not verify")
+	}
+	if VerifyWithSkew(secret, "000000", now) {
+		t.Fatal("wrong code should not verify")
+	}
+}

internal/web/service/panel/user.go

@@ -2,8 +2,8 @@ package panel
 
 import (
 	"errors"
+	"time"
 
-	"github.com/xlzd/gotp"
 	"gorm.io/gorm"
 
 	"github.com/mhsanaei/3x-ui/v3/internal/database"
@@ -11,6 +11,7 @@ import (
 	"github.com/mhsanaei/3x-ui/v3/internal/logger"
 	"github.com/mhsanaei/3x-ui/v3/internal/util/crypto"
 	ldaputil "github.com/mhsanaei/3x-ui/v3/internal/util/ldap"
+	"github.com/mhsanaei/3x-ui/v3/internal/util/totp"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
 )
 
@@ -97,7 +98,7 @@ func (s *UserService) CheckUser(username string, password string, twoFactorCode
 			return nil, err
 		}
 
-		if gotp.NewDefaultTOTP(twoFactorToken).Now() != twoFactorCode {
+		if !totp.VerifyWithSkew(twoFactorToken, twoFactorCode, time.Now()) {
 			return nil, errors.New("invalid 2fa code")
 		}
 	}

internal/web/service/setting.go

@@ -15,7 +15,6 @@ import (
 	"time"
 
 	"github.com/google/uuid"
-	"github.com/xlzd/gotp"
 	"gorm.io/gorm"
 
 	"github.com/mhsanaei/3x-ui/v3/internal/config"
@@ -26,6 +25,7 @@ import (
 	"github.com/mhsanaei/3x-ui/v3/internal/util/netproxy"
 	"github.com/mhsanaei/3x-ui/v3/internal/util/random"
 	"github.com/mhsanaei/3x-ui/v3/internal/util/reflect_util"
+	"github.com/mhsanaei/3x-ui/v3/internal/util/totp"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/entity"
 	"github.com/mhsanaei/3x-ui/v3/internal/xray"
 	"github.com/mhsanaei/3x-ui/v3/internal/xray/dnsconf"
@@ -655,7 +655,7 @@ func (s *SettingService) VerifyTwoFactorCode(code string) error {
 	if err != nil {
 		return err
 	}
-	if strings.TrimSpace(token) == "" || !gotp.NewDefaultTOTP(token).Verify(strings.TrimSpace(code), time.Now().Unix()) {
+	if strings.TrimSpace(token) == "" || !totp.VerifyWithSkew(token, strings.TrimSpace(code), time.Now()) {
 		return common.NewError("invalid two factor code")
 	}
 	return nil

internal/web/service/setting_security_test.go

@@ -4,6 +4,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"testing"
+	"time"
 
 	"github.com/xlzd/gotp"
 
@@ -230,6 +231,9 @@ func TestVerifyTwoFactorCode(t *testing.T) {
 	if err := s.VerifyTwoFactorCode(gotp.NewDefaultTOTP(token).Now()); err != nil {
 		t.Fatalf("valid code rejected: %v", err)
 	}
+	if err := s.VerifyTwoFactorCode(gotp.NewDefaultTOTP(token).AtTime(time.Now().Add(-30 * time.Second))); err != nil {
+		t.Fatalf("previous window code rejected: %v", err)
+	}
 	if err := s.VerifyTwoFactorCode("000000"); err == nil {
 		t.Fatal("invalid code accepted")
 	}