diff --git a/backend/internal/ads/service.go b/backend/internal/ads/service.go
index ff6a7e0..939771d 100644
--- a/backend/internal/ads/service.go
+++ b/backend/internal/ads/service.go
@@ -139,14 +139,23 @@ func (s *Service) AddMessage(ctx context.Context, campaignID uuid.UUID, bodyEn,
 	return s.store.AddMessage(ctx, Message{CampaignID: campaignID, Position: len(c.Messages), BodyEn: en, BodyRu: ru})
 }
 
-// EditMessage validates and updates a message's bilingual bodies (its position is
-// unchanged).
-func (s *Service) EditMessage(ctx context.Context, id uuid.UUID, bodyEn, bodyRu string) error {
+// EditMessage validates and updates the bilingual bodies of a message that
+// belongs to campaignID (its position is unchanged). It returns ErrNotFound when
+// the message is not part of that campaign, so a mismatched campaign/message pair
+// never edits an unrelated campaign's message.
+func (s *Service) EditMessage(ctx context.Context, campaignID, messageID uuid.UUID, bodyEn, bodyRu string) error {
 	en, ru, err := validBodies(bodyEn, bodyRu)
 	if err != nil {
 		return err
 	}
-	return s.store.UpdateMessageBodies(ctx, id, en, ru)
+	c, err := s.store.Campaign(ctx, campaignID)
+	if err != nil {
+		return err
+	}
+	if !campaignOwnsMessage(c, messageID) {
+		return ErrNotFound
+	}
+	return s.store.UpdateMessageBodies(ctx, messageID, en, ru)
 }
 
 // MoveMessage reorders a message within its campaign by swapping its position
@@ -178,19 +187,35 @@ func (s *Service) MoveMessage(ctx context.Context, campaignID, messageID uuid.UU
 	return s.store.SetMessagePosition(ctx, b.ID, idx)
 }
 
-// DeleteMessage removes a message, refusing to remove the default campaign's last
-// remaining message (the default must always have a creative to show).
+// DeleteMessage removes a message that belongs to campaignID, refusing to remove
+// the default campaign's last remaining message (the default must always have a
+// creative to show). It returns ErrNotFound when the message is not part of that
+// campaign — so a mismatched campaign/message pair can neither delete an
+// unrelated campaign's message nor bypass the default's last-message guard.
 func (s *Service) DeleteMessage(ctx context.Context, campaignID, messageID uuid.UUID) error {
 	c, err := s.store.Campaign(ctx, campaignID)
 	if err != nil {
 		return err
 	}
+	if !campaignOwnsMessage(c, messageID) {
+		return ErrNotFound
+	}
 	if c.IsDefault && len(c.Messages) <= 1 {
 		return fmt.Errorf("%w: the default campaign must keep at least one message", ErrValidation)
 	}
 	return s.store.DeleteMessage(ctx, messageID)
 }
 
+// campaignOwnsMessage reports whether messageID is one of the campaign's messages.
+func campaignOwnsMessage(c Campaign, messageID uuid.UUID) bool {
+	for _, m := range c.Messages {
+		if m.ID == messageID {
+			return true
+		}
+	}
+	return false
+}
+
 // Settings returns the global display timings.
 func (s *Service) Settings(ctx context.Context) (Timings, error) {
 	return s.store.Settings(ctx)
diff --git a/backend/internal/inttest/banner_console_test.go b/backend/internal/inttest/banner_console_test.go
index 1cf0095..67ee9fa 100644
--- a/backend/internal/inttest/banner_console_test.go
+++ b/backend/internal/inttest/banner_console_test.go
@@ -144,6 +144,49 @@ func TestBannerConsoleCRUD(t *testing.T) {
 	}
 }
 
+// TestBannerMessageOwnership guards the campaign/message pairing: a message may
+// only be edited or deleted through the URL of the campaign it belongs to. This
+// closes a default-protection bypass — deleting the default's last message via an
+// unrelated campaign's URL must be refused.
+func TestBannerMessageOwnership(t *testing.T) {
+	ctx := context.Background()
+	srv, adsSvc := bannerServer(t)
+	h := srv.Handler()
+	base := "http://admin.test/_gm"
+	const origin = "http://admin.test"
+
+	def := defaultCampaign(t, adsSvc)
+	if len(def.Messages) == 0 {
+		t.Fatal("default campaign has no seeded message")
+	}
+	defMsg := def.Messages[0].ID
+
+	// A separate timed campaign whose URL we will (mis)use.
+	otherID, err := adsSvc.CreateCampaign(ctx, ads.Campaign{Name: "Own-" + uuid.NewString()[:8], Weight: 10, Enabled: true})
+	if err != nil {
+		t.Fatalf("create campaign: %v", err)
+	}
+	t.Cleanup(func() { _ = adsSvc.DeleteCampaign(ctx, otherID) })
+
+	// Deleting the default's message through the other campaign's URL is refused,
+	// and the default keeps its message (the bypass is closed).
+	if code, body := consoleDo(h, http.MethodPost, base+"/banners/"+otherID.String()+"/messages/"+defMsg.String()+"/delete", "", origin); code != http.StatusOK || !strings.Contains(body, "Not found") {
+		t.Fatalf("cross-campaign delete = %d, has 'Not found'=%v", code, strings.Contains(body, "Not found"))
+	}
+	// Editing it through the wrong campaign is refused too.
+	if code, body := consoleDo(h, http.MethodPost, base+"/banners/"+otherID.String()+"/messages/"+defMsg.String(), "body_en=Hijacked&body_ru=Взлом", origin); code != http.StatusOK || !strings.Contains(body, "Not found") {
+		t.Fatalf("cross-campaign edit = %d, has 'Not found'=%v", code, strings.Contains(body, "Not found"))
+	}
+	// The default's message is intact.
+	again := defaultCampaign(t, adsSvc)
+	if len(again.Messages) != len(def.Messages) {
+		t.Fatalf("default messages = %d, want %d (unchanged)", len(again.Messages), len(def.Messages))
+	}
+	if again.Messages[0].BodyEn != def.Messages[0].BodyEn {
+		t.Fatalf("default message body changed: %q -> %q", def.Messages[0].BodyEn, again.Messages[0].BodyEn)
+	}
+}
+
 // profileBanner is the banner block of the profile.get JSON response.
 type profileBanner struct {
 	Banner *struct {
diff --git a/backend/internal/server/handlers_admin_banners.go b/backend/internal/server/handlers_admin_banners.go
index 86b89a5..0a8ecc6 100644
--- a/backend/internal/server/handlers_admin_banners.go
+++ b/backend/internal/server/handlers_admin_banners.go
@@ -159,7 +159,7 @@ func (s *Server) consoleEditBannerMessage(c *gin.Context) {
 		return
 	}
 	back := "/_gm/banners/" + cid.String()
-	if err := s.ads.EditMessage(c.Request.Context(), mid, trimForm(c, "body_en"), trimForm(c, "body_ru")); err != nil {
+	if err := s.ads.EditMessage(c.Request.Context(), cid, mid, trimForm(c, "body_en"), trimForm(c, "body_ru")); err != nil {
 		s.bannerError(c, err, back)
 		return
 	}