diff --git a/internal/localapi/escrow_ceremony.go b/internal/localapi/escrow_ceremony.go
index 47926ad..87ef68c 100644
--- a/internal/localapi/escrow_ceremony.go
+++ b/internal/localapi/escrow_ceremony.go
@@ -226,6 +226,9 @@ func (s *Server) handleEscrowCeremonyStart(w http.ResponseWriter, r *http.Reques
 	if base == nil {
 		base = context.Background()
 	}
+	// The reply's fields are read HERE, before the job goroutine may write job.Phase (a data race the race
+	// detector caught, 2026-10-13 — the reply could show the finished phase or a torn value).
+	jobID, phase := job.JobID, job.Phase
 	done := make(chan struct{})
 	go func() {
 		defer close(done)
@@ -274,7 +277,7 @@ func (s *Server) handleEscrowCeremonyStart(w http.ResponseWriter, r *http.Reques
 			"claim_ttl_s", int(escrowClaimTTL.Seconds()), "duration_ms", time.Since(start).Milliseconds())
 	}()
 	s.escrowDone = done // tests wait on it; production polls the status endpoint
-	writeStatus(w, http.StatusAccepted, true, map[string]any{"vmid": vmid, "job_id": job.JobID, "phase": job.Phase}, "")
+	writeStatus(w, http.StatusAccepted, true, map[string]any{"vmid": vmid, "job_id": jobID, "phase": phase}, "")
 }
 
 // handleEscrowCeremonyStatus is GET /escrow/ceremony/status: phase + the NON-SECRET summary.
diff --git a/internal/localapi/hostpoweroff.go b/internal/localapi/hostpoweroff.go
index 30bde6f..d2be5e8 100644
--- a/internal/localapi/hostpoweroff.go
+++ b/internal/localapi/hostpoweroff.go
@@ -209,18 +209,8 @@ func (s *Server) powerOffBlocker(ctx context.Context, vmid int) (string, string)
 	if s.inFlight == nil || s.staleLock == nil {
 		return PowerOffUnknown, "the heavy-operation gate or the Proxmox task reader is not wired"
 	}
-	running, err := c.Running()
-	if err != nil {
-		return PowerOffUnknown, "cannot list the running guests: " + err.Error()
-	}
-	var others []string
-	for _, v := range running {
-		if v != vmid {
-			others = append(others, strconv.Itoa(v))
-		}
-	}
-	if len(others) > 0 {
-		return PowerOffOtherGuests, "other guests or VMs running: " + strings.Join(others, ", ")
+	if code, detail := s.otherGuestsBlocker(vmid); code != "" {
+		return code, detail
 	}
 	if busy := s.inFlight.Busy(); busy != "" {
 		switch {
@@ -324,6 +314,24 @@ func runningGuestsFromProc(procDir string) ([]int, error) {
 	return out, nil
 }
 
+// otherGuestsBlocker refuses while any guest or VM other than the caller's runs (or the list cannot be read).
+func (s *Server) otherGuestsBlocker(vmid int) (string, string) {
+	running, err := s.powerOff.Running()
+	if err != nil {
+		return PowerOffUnknown, "cannot list the running guests: " + err.Error()
+	}
+	var others []string
+	for _, v := range running {
+		if v != vmid {
+			others = append(others, strconv.Itoa(v))
+		}
+	}
+	if len(others) > 0 {
+		return PowerOffOtherGuests, "other guests or VMs running: " + strings.Join(others, ", ")
+	}
+	return "", ""
+}
+
 // waitJobsClear waits until jobBlocker finds nothing, at most JobWait. false = still busy (or unreadable) at the end:
 // the power-off is CANCELLED — the box stays on, the gate is released, the log says why. Better a box that did not
 // switch off than a backup, update or format cut in the middle.
@@ -332,6 +340,12 @@ func (s *Server) waitJobsClear(base context.Context, vmid int, when string) bool
 	ctx, cancel := context.WithTimeout(base, c.JobWait)
 	defer cancel()
 	for {
+		// Another guest or VM started after acceptance (security review 2026-10-13): no waiting — cancel at once.
+		if code, detail := s.otherGuestsBlocker(vmid); code != "" {
+			s.logger.Error("host-poweroff: CANCELLED — another guest is running now; the box stays on", "when", when,
+				"reason", code, "detail", detail)
+			return false
+		}
 		code, detail := s.jobBlocker(ctx, vmid)
 		if code == "" {
 			return true
diff --git a/internal/localapi/hostpoweroff_test.go b/internal/localapi/hostpoweroff_test.go
index cae5b99..316a25c 100644
--- a/internal/localapi/hostpoweroff_test.go
+++ b/internal/localapi/hostpoweroff_test.go
@@ -328,3 +328,26 @@ func TestHostPowerOff_JobStartedAfterAcceptance(t *testing.T) {
 		t.Fatalf("the heavy gate is still held after a cancel: %q", busy)
 	}
 }
+
+// A guest or VM started AFTER acceptance cancels the power-off at once (security review 2026-10-13).
+func TestHostPowerOff_OtherGuestStartedAfterAcceptance(t *testing.T) {
+	r := newPowerRig(t, time.Hour)
+	var started atomic.Bool
+	r.srv.powerOff.Running = func() ([]int, error) {
+		if started.Load() {
+			return []int{8200, 341}, nil
+		}
+		return []int{8200}, nil
+	}
+	r.srv.powerOff.ReportNow = func(context.Context) bool { started.Store(true); return true } // a VM starts now
+	if w := do(t, r.h, "POST", "/host/poweroff", "A", ""); w.Code != http.StatusAccepted {
+		t.Fatalf("accept: %d %s", w.Code, w.Body.String())
+	}
+	time.Sleep(50 * time.Millisecond)
+	if s := r.power.got(); len(s) != 0 {
+		t.Fatalf("power-off ran although a VM started: %v", s)
+	}
+	if busy := r.srv.inFlight.Busy(); busy != "" {
+		t.Fatalf("the heavy gate is still held after a cancel: %q", busy)
+	}
+}