diff --git a/changes/ticket40870 b/changes/ticket40870
new file mode 100644
index 0000000000..c33c83e1a6
--- /dev/null
+++ b/changes/ticket40870
@@ -0,0 +1,4 @@
+  o Minor bugfixes (conflux, client):
+    - Avoid a non fatal assert caused by data coming in on a conflux set that is
+      being freed during shutdown. Fixes bug 40870; bugfix on 0.4.8.1-alpha.
+
diff --git a/src/app/main/shutdown.c b/src/app/main/shutdown.c
index b3f1c6d058..7f0d112c90 100644
--- a/src/app/main/shutdown.c
+++ b/src/app/main/shutdown.c
@@ -120,6 +120,7 @@ tor_free_all(int postfork)
   dirserv_free_all();
   rep_hist_free_all();
   bwhist_free_all();
+  conflux_notify_shutdown();
   circuit_free_all();
   conflux_pool_free_all();
   circpad_machines_free();
diff --git a/src/core/or/conflux_pool.c b/src/core/or/conflux_pool.c
index 4a7e941372..ec2938aefc 100644
--- a/src/core/or/conflux_pool.c
+++ b/src/core/or/conflux_pool.c
@@ -1607,7 +1607,22 @@ linked_circuit_free(circuit_t *circ, bool is_client)
 
   /* Circuit can be freed without being closed and so we try to delete this leg
    * so we can learn if this circuit is the last leg or not. */
-  cfx_del_leg(circ->conflux, circ);
+  if (cfx_del_leg(circ->conflux, circ)) {
+    /* Check for instances of bug #40870, which we suspect happen
+     * during exit. If any happen outside of exit, BUG and warn. */
+    if (!circ->conflux->in_full_teardown) {
+      /* We should bug and warn if we're not in a shutdown process; that
+       * means we got here somehow without a close. */
+      if (BUG(!shutting_down)) {
+        log_warn(LD_BUG,
+                 "Conflux circuit %p being freed without being marked for "
+                 "full teardown via close, with shutdown state %d. "
+                 "Please report this.", circ, shutting_down);
+        conflux_log_set(LOG_WARN, circ->conflux, is_client);
+      }
+      circ->conflux->in_full_teardown = true;
+    }
+  }
 
   if (CONFLUX_NUM_LEGS(circ->conflux) > 0) {
     /* The last leg will free the streams but until then, we nullify to avoid
@@ -2126,14 +2141,36 @@ conflux_log_set(int loglevel, const conflux_t *cfx, bool is_client)
   }
 }
 
+/**
+ * Conflux needs a notification when tor_shutdown() begins, so that
+ * when circuits are freed, new legs are not launched.
+ *
+ * This needs a separate notification from conflux_pool_free_all(),
+ * because circuits must be freed before that function.
+ */
+void
+conflux_notify_shutdown(void)
+{
+  shutting_down = true;
+}
+
+#ifdef TOR_UNIT_TESTS
+/**
+ * For unit tests: Clear the shutting down state so we resume building legs.
+ */
+void
+conflux_clear_shutdown(void)
+{
+  shutting_down = false;
+}
+#endif
+
 /** Free and clean up the conflux pool subsystem. This is called by the subsys
  * manager AFTER all circuits have been freed which implies that all objects in
  * the pools aren't referenced anymore. */
 void
 conflux_pool_free_all(void)
 {
-  shutting_down = true;
-
   digest256map_free(client_linked_pool, free_conflux_void_);
   digest256map_free(server_linked_pool, free_conflux_void_);
   digest256map_free(client_unlinked_pool, free_unlinked_void_);
diff --git a/src/core/or/conflux_pool.h b/src/core/or/conflux_pool.h
index afa4d9d058..eba726b03a 100644
--- a/src/core/or/conflux_pool.h
+++ b/src/core/or/conflux_pool.h
@@ -12,6 +12,7 @@
 #include "core/or/or.h"
 
 void conflux_pool_init(void);
+void conflux_notify_shutdown(void);
 void conflux_pool_free_all(void);
 
 origin_circuit_t *conflux_get_circ_for_conn(const entry_connection_t *conn,
@@ -41,6 +42,7 @@ void conflux_log_set(int loglevel, const conflux_t *cfx, bool is_client);
 
 #ifdef TOR_UNIT_TESTS
 bool launch_new_set(int num_legs);
+void conflux_clear_shutdown(void);
 digest256map_t *get_linked_pool(bool is_client);
 digest256map_t *get_unlinked_pool(bool is_client);
 extern uint8_t DEFAULT_CLIENT_UX;
diff --git a/src/test/test_conflux_pool.c b/src/test/test_conflux_pool.c
index fc30677377..05dc7b14ff 100644
--- a/src/test/test_conflux_pool.c
+++ b/src/test/test_conflux_pool.c
@@ -396,6 +396,7 @@ test_setup(void)
 static void
 test_clear_circs(void)
 {
+  conflux_notify_shutdown();
   SMARTLIST_FOREACH(circ_pairs, circ_pair_t *, circ_pair, {
     tor_free(circ_pair);
   });
@@ -430,6 +431,9 @@ test_clear_circs(void)
   tor_assert(smartlist_len(mock_cell_delivery) == 0);
 
   (void)free_fake_origin_circuit;
+
+  /* Clear shutdown flag so we can resume testing again. */
+  conflux_clear_shutdown();
 }
 
 static void