diff --git a/CMakeLists.txt b/CMakeLists.txt
index d9e4cb40f..9a97b7968 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -242,6 +242,7 @@ option(LWS_WITH_SPAWN "Spawn subprocesses with piped stdin/out/stderr" OFF)
 option(LWS_WITH_FSMOUNT "Overlayfs and fallback mounting apis" OFF)
 option(LWS_WITH_FANALYZER "Enable gcc -fanalyzer if compiler supports" OFF)
 option(LWS_HTTP_HEADERS_ALL "Override header reduction optimization and include all like older lws versions" OFF)
+option(LWS_WITH_SUL_DEBUGGING "Enable zombie lws_sul checking on object deletion" OFF)
 
 #
 # to use miniz, enable both LWS_WITH_ZLIB and LWS_WITH_MINIZ
diff --git a/include/libwebsockets/lws-timeout-timer.h b/include/libwebsockets/lws-timeout-timer.h
index 5eb975988..a02ba3485 100644
--- a/include/libwebsockets/lws-timeout-timer.h
+++ b/include/libwebsockets/lws-timeout-timer.h
@@ -302,6 +302,35 @@ lws_sul_earliest_wakeable_event(struct lws_context *ctx, lws_usec_t *pearliest);
 		lws_sul2_schedule(ctx, tsi, LWSSULLI_WAKE_IF_SUSPENDED, sul); \
 	}}
 
+#if defined(LWS_WITH_SUL_DEBUGGING)
+/**
+ * lws_sul_debug_zombies() - assert there are no scheduled sul in a given object
+ *
+ * \param ctx: lws_context
+ * \param po: pointer to the object that is about to be destroyed
+ * \param len: length of the object that is about to be destroyed
+ * \param destroy_description: string clue what any failure is related to
+ *
+ * This is an optional debugging helper that walks the sul scheduler lists
+ * confirming that there are no suls scheduled that live inside the object
+ * footprint described by po and len.  When internal objects are about to be
+ * destroyed, like wsi / user_data or secure stream handles, if
+ * LWS_WITH_SUL_DEBUGGING is enabled the scheduler is checked for anything
+ * in the object being destroyed.  If something found, an error is printed and
+ * an assert fired.
+ *
+ * Internal sul like timeouts should always be cleaned up correctly, but user
+ * suls in, eg, wsi user_data area, or in secure stream user allocation, may be
+ * the cause of difficult to find bugs if valgrind not available and the user
+ * code left a sul in the scheduler after destroying the object the sul was
+ * living in.
+ */
+LWS_VISIBLE LWS_EXTERN void
+lws_sul_debug_zombies(struct lws_context *ctx, void *po, size_t len,
+		      const char *destroy_description);
+#else
+#define lws_sul_debug_zombies(_a, _b, _c, _d)
+#endif
 
 /*
  * lws_validity_confirmed() - reset the validity timer for a network connection
diff --git a/lib/core-net/close.c b/lib/core-net/close.c
index a92f258fe..c078aa0f0 100644
--- a/lib/core-net/close.c
+++ b/lib/core-net/close.c
@@ -79,8 +79,12 @@ __lws_reset_wsi(struct lws *wsi)
 	 * or by specified the user. We should only free what we allocated.
 	 */
 	if (wsi->protocol && wsi->protocol->per_session_data_size &&
-	    wsi->user_space && !wsi->user_space_externally_allocated)
+	    wsi->user_space && !wsi->user_space_externally_allocated) {
+		/* confirm no sul left scheduled in user data itself */
+		lws_sul_debug_zombies(wsi->context, wsi->user_space,
+				wsi->protocol->per_session_data_size, __func__);
 		lws_free_set_NULL(wsi->user_space);
+	}
 
 	/*
 	 * Don't let buflist content or state from the wsi's previous life
@@ -91,7 +95,12 @@ __lws_reset_wsi(struct lws *wsi)
 	lws_dll2_remove(&wsi->dll_buflist);
 	lws_buflist_destroy_all_segments(&wsi->buflist_out);
 #if defined(LWS_WITH_UDP)
-	lws_free_set_NULL(wsi->udp);
+	if (wsi->udp) {
+		/* confirm no sul left scheduled in wsi->udp itself */
+		lws_sul_debug_zombies(wsi->context, wsi->udp,
+				      sizeof(*wsi->udp), "close udp wsi");
+		lws_free_set_NULL(wsi->udp);
+	}
 #endif
 	wsi->retry = 0;
 
@@ -161,6 +170,9 @@ __lws_free_wsi(struct lws *wsi)
 			wsi->context->count_wsi_allocated,
 			wsi->context->pt[(int)wsi->tsi].fds_count);
 
+	/* confirm no sul left scheduled in wsi itself */
+	lws_sul_debug_zombies(wsi->context, wsi, sizeof(wsi), __func__);
+
 	lws_free(wsi);
 }
 
@@ -434,8 +446,13 @@ just_kill_connection:
 		lws_buflist_destroy_all_segments(&wsi->http.buflist_post_body);
 #endif
 #if defined(LWS_WITH_UDP)
-	if (wsi->udp)
+	if (wsi->udp) {
+		/* confirm no sul left scheduled in wsi->udp itself */
+		lws_sul_debug_zombies(wsi->context, wsi->udp,
+					sizeof(*wsi->udp), "close udp wsi");
+
 		lws_free_set_NULL(wsi->udp);
+	}
 #endif
 
 	if (wsi->role_ops->close_kill_connection)
diff --git a/lib/core-net/sorted-usec-list.c b/lib/core-net/sorted-usec-list.c
index 895865c56..e00de7b0a 100644
--- a/lib/core-net/sorted-usec-list.c
+++ b/lib/core-net/sorted-usec-list.c
@@ -259,3 +259,63 @@ lws_sul_earliest_wakeable_event(struct lws_context *ctx, lws_usec_t *pearliest)
 
 	return 0;
 }
+
+#if defined(LWS_WITH_SUL_DEBUGGING)
+
+/*
+ * Sanity checker for any sul left scheduled when its containing object is
+ * freed... code scheduling suls must take care to cancel them when destroying
+ * their object.  This optional debugging helper checks that when an object is
+ * being destroyed, there is no live sul scheduled from inside the object.
+ */
+
+void
+lws_sul_debug_zombies(struct lws_context *ctx, void *po, size_t len,
+		      const char *destroy_description)
+{
+	struct lws_context_per_thread *pt;
+	int n, m;
+
+	for (n = 0; n < ctx->count_threads; n++) {
+		pt = &ctx->pt[n];
+
+		lws_pt_lock(pt, __func__);
+
+		for (m = 0; m < LWS_COUNT_PT_SUL_OWNERS; m++) {
+
+			lws_start_foreach_dll(struct lws_dll2 *, p,
+				      lws_dll2_get_head(&pt->pt_sul_owner[m])) {
+				lws_sorted_usec_list_t *sul =
+					lws_container_of(p,
+						lws_sorted_usec_list_t, list);
+
+				/*
+				 * Is the sul resident inside the object that is
+				 * indicated as being deleted?
+				 */
+
+				if (sul >= po && lws_ptr_diff(sul, po) < len) {
+					lwsl_err("%s: ERROR: Zombie Sul "
+						 "(on list %d) %s\n", __func__,
+						 m, destroy_description);
+					/*
+					 * This assert fires if you have left
+					 * a sul scheduled to fire later, but
+					 * are about to destroy the object the
+					 * sul lives in.  You must take care to
+					 * do lws_sul_cancel(&sul) on any suls
+					 * that may be scheduled before
+					 * destroying the object the sul lives
+					 * inside.
+					 */
+					assert(0);
+				}
+
+			} lws_end_foreach_dll(p);
+		}
+
+		lws_pt_unlock(pt);
+	}
+}
+
+#endif
diff --git a/lib/secure-streams/secure-streams.c b/lib/secure-streams/secure-streams.c
index d0097e98f..4373023d2 100644
--- a/lib/secure-streams/secure-streams.c
+++ b/lib/secure-streams/secure-streams.c
@@ -742,6 +742,10 @@ lws_ss_destroy(lws_ss_handle_t **ppss)
 
 	lws_sul_cancel(&h->sul);
 
+	/* confirm no sul left scheduled in handle or user allocation object */
+	lws_sul_debug_zombies(h->context, h, sizeof(*h) + h->info.user_alloc,
+			      __func__);
+
 	lws_free_set_NULL(h);
 }