diff --git a/ChangeLog.txt b/ChangeLog.txt
index 47723dbe..3ad5ec14 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -2,6 +2,9 @@
 ===================
 
 Security:
+- An MQTT v5 client connecting with a large number of user-property properties
+  could cause excessive CPU usage, leading to a loss of performance and
+  possible denial of service. This has been fixed.
 - Fix `max_keepalive` not applying to MQTT v3.1.1 and v3.1 connections.
   These clients are now rejected if their keepalive value exceeds
   max_keepalive. This option allows CVE-2020-13849, which is for the MQTT
diff --git a/lib/property_mosq.c b/lib/property_mosq.c
index b4dc73dc..f8d15408 100644
--- a/lib/property_mosq.c
+++ b/lib/property_mosq.c
@@ -962,14 +962,14 @@ int mosquitto_property_check_all(int command, const mosquitto_property *properti
 		if(rc) return rc;
 
 		/* Check for duplicates */
-		tail = p->next;
-		while(tail){
-			if(p->identifier == tail->identifier
-					&& p->identifier != MQTT_PROP_USER_PROPERTY){
-
-				return MOSQ_ERR_DUPLICATE_PROPERTY;
+		if(p->identifier != MQTT_PROP_USER_PROPERTY){
+			tail = p->next;
+			while(tail){
+				if(p->identifier == tail->identifier){
+					return MOSQ_ERR_DUPLICATE_PROPERTY;
+				}
+				tail = tail->next;
 			}
-			tail = tail->next;
 		}
 
 		p = p->next;
diff --git a/test/broker/01-connect-575314.py b/test/broker/01-connect-575314.py
new file mode 100755
index 00000000..4a8f314d
--- /dev/null
+++ b/test/broker/01-connect-575314.py
@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+
+# Check for performance of processing user-property on CONNECT
+
+from mosq_test_helper import *
+
+def do_test():
+    rc = 1
+    props = mqtt5_props.gen_string_pair_prop(mqtt5_props.PROP_USER_PROPERTY, "key", "value")
+    for i in range(0, 20000):
+        props += mqtt5_props.gen_string_pair_prop(mqtt5_props.PROP_USER_PROPERTY, "key", "value")
+    connect_packet_slow = mosq_test.gen_connect("connect-user-property", proto_ver=5, properties=props)
+    connect_packet_fast = mosq_test.gen_connect("a"*65000, proto_ver=5)
+    connack_packet = mosq_test.gen_connack(rc=0, proto_ver=5)
+
+    port = mosq_test.get_port()
+    broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
+
+    try:
+        t_start = time.monotonic()
+        sock = mosq_test.do_client_connect(connect_packet_slow, connack_packet, port=port)
+        t_stop = time.monotonic()
+        sock.close()
+
+        t_diff_slow = t_stop - t_start
+
+        t_start = time.monotonic()
+        sock = mosq_test.do_client_connect(connect_packet_fast, connack_packet, port=port)
+        t_stop = time.monotonic()
+        sock.close()
+
+        t_diff_fast = t_stop - t_start
+        # 20 is chosen as a factor that works in plain mode and running under
+        # valgrind. The slow performance manifests as a factor of >100. Fast is <10.
+        if t_diff_slow / t_diff_fast < 20:
+            rc = 0
+    except mosq_test.TestError:
+        pass
+    finally:
+        broker.terminate()
+        broker.wait()
+        (stdo, stde) = broker.communicate()
+        if rc:
+            print(stde.decode('utf-8'))
+            exit(rc)
+
+
+do_test()
+exit(0)
diff --git a/test/broker/Makefile b/test/broker/Makefile
index 51cb64b7..2f93ee55 100644
--- a/test/broker/Makefile
+++ b/test/broker/Makefile
@@ -23,6 +23,7 @@ msg_sequence_test:
 	./msg_sequence_test.py
 
 01 :
+	./01-connect-575314.py
 	./01-connect-allow-anonymous.py
 	./01-connect-disconnect-v5.py
 	./01-connect-max-connections.py
diff --git a/test/broker/test.py b/test/broker/test.py
index 08dd2f8c..c022f887 100755
--- a/test/broker/test.py
+++ b/test/broker/test.py
@@ -5,6 +5,7 @@ import ptest
 
 tests = [
     #(ports required, 'path'),
+    (1, './01-connect-575314.py'),
     (1, './01-connect-allow-anonymous.py'),
     (1, './01-connect-disconnect-v5.py'),
     (1, './01-connect-max-connections.py'),