diff --git a/internal/core/auth/totp/totp.go b/internal/core/auth/totp/totp.go
index 19a10ba..48ebb6d 100644
--- a/internal/core/auth/totp/totp.go
+++ b/internal/core/auth/totp/totp.go
@@ -10,17 +10,24 @@ package totp
 
 import (
 	"context"
+	"crypto/subtle"
 	"errors"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/pquerna/otp"
 	"github.com/pquerna/otp/totp"
 )
 
-// ErrInvalidCode is returned when a code does not match the stored secret.
+// ErrInvalidCode is returned when a code does not match the stored secret — or
+// when it does but has already been accepted (a replay, L5s). The two cases are
+// deliberately indistinguishable: never tell an attacker which.
 var ErrInvalidCode = errors.New("totp: invalid code")
 
+// errNoStore is returned when the service was built without a Store.
+var errNoStore = errors.New("totp: no store configured")
+
 // Store persists an identity's TOTP secret. It is implemented by the auth
 // service, which encrypts at rest through the AEAD sealer (A9) — so this
 // package never touches the ciphertext or the key.
@@ -51,6 +58,13 @@ type Generator struct {
 	store  Store
 	now    func() time.Time
 	opts   totp.ValidateOpts
+
+	// mu guards last, the highest counter accepted per identity (L5s). Replay
+	// defence is in-process: a restart forgets the counters, but the skew
+	// window is only ±90s, so the exposure is bounded and the alternative (a
+	// DB round-trip per verification) is not worth it at M0.
+	mu   sync.Mutex
+	last map[int64]uint64
 }
 
 var _ Service = (*Generator)(nil)
@@ -71,6 +85,7 @@ func New(issuer string, store Store) *Generator {
 			Digits:    otp.DigitsSix,
 			Algorithm: otp.AlgorithmSHA1,
 		},
+		last: make(map[int64]uint64),
 	}
 }
 
@@ -82,10 +97,11 @@ func (g *Generator) SetClock(fn func() time.Time) {
 }
 
 // Begin issues a fresh secret, stores it encrypted (A9) and returns the otpauth
-// URL for the authenticator app.
+// URL for the authenticator app. A fresh secret resets the identity's replay
+// counter (L5s): the new secret is a new sequence.
 func (g *Generator) Begin(ctx context.Context, identityID int64, account string) (Issued, error) {
 	if g.store == nil {
-		return Issued{}, errors.New("totp: no store configured")
+		return Issued{}, errNoStore
 	}
 	key, err := totp.Generate(totp.GenerateOpts{Issuer: g.issuer, AccountName: account})
 	if err != nil {
@@ -95,29 +111,82 @@ func (g *Generator) Begin(ctx context.Context, identityID int64, account string)
 	if err := g.store.PutTOTPSecret(ctx, identityID, []byte(key.Secret())); err != nil {
 		return Issued{}, err
 	}
+	g.forget(identityID)
 	return Issued{URL: key.URL()}, nil
 }
 
 // Verify validates code against the stored secret, then records the
 // verification. The code and the secret never reach an error string or a log.
+//
+// A code that matches but whose counter has already been accepted is rejected
+// as a replay (L5s), so a code observed (shoulder-surfed, logged, phished) in
+// its 90-second validity cannot be used twice.
 func (g *Generator) Verify(ctx context.Context, identityID int64, code string) error {
 	if code == "" {
 		return ErrInvalidCode
 	}
+	if g.store == nil {
+		return errNoStore
+	}
 	secret, err := g.store.TOTPSecret(ctx, identityID)
 	if err != nil {
 		return err
 	}
-	ok, err := totp.ValidateCustom(code, string(secret), g.now().UTC(), g.opts)
-	if err != nil {
-		// A malformed code is an invalid code, not a server fault.
-		return ErrInvalidCode
-	}
+	counter, ok := g.matchCounter(secret, code)
 	if !ok {
 		return ErrInvalidCode
 	}
+	if !g.acceptCounter(identityID, counter) {
+		return ErrInvalidCode // a replay — indistinguishable from a bad code
+	}
 	if err := g.store.MarkTOTPVerified(ctx, identityID); err != nil {
 		return err
 	}
 	return nil
 }
+
+// matchCounter finds the time step whose code equals code, searching the skew
+// window newest-first (L5s: newest-first means the returned counter is the
+// highest matching step, so an earlier step is never preferred over a later
+// one). The comparison is constant-time.
+func (g *Generator) matchCounter(secret []byte, code string) (uint64, bool) {
+	period := int64(g.opts.Period)
+	if period <= 0 {
+		period = 30
+	}
+	skew := int64(g.opts.Skew)
+	base := g.now().UTC().Unix() / period
+	for d := skew; d >= -skew; d-- {
+		step := base + d
+		if step < 0 {
+			continue
+		}
+		want, err := totp.GenerateCodeCustom(string(secret), time.Unix(step*period, 0).UTC(), g.opts)
+		if err != nil {
+			continue
+		}
+		if subtle.ConstantTimeCompare([]byte(want), []byte(code)) == 1 {
+			return uint64(step), true
+		}
+	}
+	return 0, false
+}
+
+// acceptCounter records counter as the highest accepted for identityID and
+// reports whether it is strictly newer than the last one (L5s).
+func (g *Generator) acceptCounter(identityID int64, counter uint64) bool {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	if prev, ok := g.last[identityID]; ok && counter <= prev {
+		return false
+	}
+	g.last[identityID] = counter
+	return true
+}
+
+// forget drops an identity's replay counter (a new secret, L5s).
+func (g *Generator) forget(identityID int64) {
+	g.mu.Lock()
+	defer g.mu.Unlock()
+	delete(g.last, identityID)
+}
diff --git a/internal/core/auth/totp/totp_test.go b/internal/core/auth/totp/totp_test.go
index 00973a7..fc264a3 100644
--- a/internal/core/auth/totp/totp_test.go
+++ b/internal/core/auth/totp/totp_test.go
@@ -80,7 +80,7 @@ func TestVerifyAcceptsAValidCodeAndMarksVerified(t *testing.T) {
 	}
 }
 
-func TestVerifyRejectsBadOrEmptyCode(t *testing.T) {
+func TestVerifyRejectsABadOrEmptyCode(t *testing.T) {
 	st := newMapStore()
 	svc := totp.New("MyDay", st)
 	if _, err := svc.Begin(context.Background(), 1, "guid"); err != nil {
@@ -96,3 +96,55 @@ func TestVerifyRejectsBadOrEmptyCode(t *testing.T) {
 		t.Error("a bad code marked the secret verified")
 	}
 }
+
+// TestVerifyRejectsAReplay covers L5s: a code that already worked must not work
+// again within the skew window.
+func TestVerifyRejectsAReplay(t *testing.T) {
+	st := newMapStore()
+	svc := totp.New("MyDay", st)
+	ctx := context.Background()
+	if _, err := svc.Begin(ctx, 1, "guid"); err != nil {
+		t.Fatalf("Begin: %v", err)
+	}
+	now := time.Now()
+	svc.SetClock(func() time.Time { return now })
+	secret := string(st.secrets[1])
+
+	code, err := pqotp.GenerateCode(secret, now)
+	if err != nil {
+		t.Fatalf("GenerateCode: %v", err)
+	}
+	if err := svc.Verify(ctx, 1, code); err != nil {
+		t.Fatalf("first Verify = %v, want nil", err)
+	}
+	// The same code, still inside its window, is a replay (L5s).
+	if err := svc.Verify(ctx, 1, code); !errors.Is(err, totp.ErrInvalidCode) {
+		t.Errorf("replayed Verify = %v, want ErrInvalidCode (L5s)", err)
+	}
+
+	// A code from an EARLIER step is refused once a newer step was accepted.
+	newer, err := pqotp.GenerateCode(secret, now.Add(30*time.Second))
+	if err != nil {
+		t.Fatalf("GenerateCode: %v", err)
+	}
+	svc.SetClock(func() time.Time { return now.Add(30 * time.Second) })
+	if err := svc.Verify(ctx, 1, newer); err != nil {
+		t.Fatalf("Verify(newer) = %v, want nil", err)
+	}
+	svc.SetClock(func() time.Time { return now }) // the skew window still covers step N
+	if err := svc.Verify(ctx, 1, code); !errors.Is(err, totp.ErrInvalidCode) {
+		t.Errorf("an older step after a newer one = %v, want ErrInvalidCode (L5s)", err)
+	}
+
+	// A fresh secret resets the counter (re-enrollment, L5s).
+	if _, err := svc.Begin(ctx, 1, "guid"); err != nil {
+		t.Fatalf("Begin (re-enroll): %v", err)
+	}
+	fresh, err := pqotp.GenerateCode(string(st.secrets[1]), now)
+	if err != nil {
+		t.Fatalf("GenerateCode: %v", err)
+	}
+	if err := svc.Verify(ctx, 1, fresh); err != nil {
+		t.Errorf("Verify after re-enrollment = %v, want nil", err)
+	}
+}