diff --git a/internal/runtime/unshare.go b/internal/runtime/unshare.go
index 987f8fe..134e147 100644
--- a/internal/runtime/unshare.go
+++ b/internal/runtime/unshare.go
@@ -17,6 +17,14 @@ import (
 // pause process exists. The environment podman unshare would set is set the
 // same way, so helpers that check for it behave identically.
 func inPodmanNS(argv ...string) *exec.Cmd {
+	// Already inside it -- `box __fc` and the other helpers run there --
+	// the namespace cannot be entered again: setns into the user namespace
+	// one is in fails with EINVAL. Run argv directly. Without this, a
+	// Firecracker VM torn down from inside never got its crun kill, and every
+	// run and snapshot left its VM running.
+	if os.Getenv("_CONTAINERS_USERNS_CONFIGURED") != "" {
+		return exec.Command(argv[0], argv[1:]...)
+	}
 	if pid := pausePID(); pid > 0 {
 		args := []string{"--user", "--mount", "--preserve-credentials", "--target", strconv.Itoa(pid), "--"}
 		cmd := exec.Command("nsenter", append(args, argv...)...)
diff --git a/security/escape-test.sh b/security/escape-test.sh
index 5f671be..690a46b 100755
--- a/security/escape-test.sh
+++ b/security/escape-test.sh
@@ -261,6 +261,18 @@ t0=$(date +%s%N); ls / >/dev/null; t1=$(date +%s%N)
 [ $(( (t1 - t0) / 1000000 )) -lt 1000 ] && pass "host responsive during a guest fork bomb" \
   || fail "host slowed by a guest fork bomb"
 
+echo
+echo "nothing left behind"
+# A run's VM is gone when the run is; only the sandbox that is up stays.
+"$BB" run "$NAME" -- true >/dev/null 2>&1
+sleep 1
+case "$BACKEND" in
+  firecracker) left=$(podman unshare crun --root "${XDG_RUNTIME_DIR:-/tmp}/box/fc" list 2>/dev/null | awk -v up="box-up-$NAME" 'NR>1 && $1 ~ /^box-/ && $1 != up' | wc -l) ;;
+  krun) left=$(podman unshare krun --root "${XDG_RUNTIME_DIR:-/tmp}/box/krun" list 2>/dev/null | awk -v up="box-up-$NAME" 'NR>1 && $1 ~ /^box-/ && $1 != up' | wc -l) ;;
+  *) left=$(podman ps -a --format '{{.Names}}' 2>/dev/null | grep "^box-$NAME-" | wc -l) ;;
+esac
+[ "$left" -eq 0 ] && pass "no VM outlives its run" || fail "$left VM(s) still running after their run ended"
+
 echo
 if [ "$FAILS" -eq 0 ]; then
   echo "no escapes."