diff --git a/ChangeLog.txt b/ChangeLog.txt
index c7146ff549..215ca2041f 100644
--- a/ChangeLog.txt
+++ b/ChangeLog.txt
@@ -11,6 +11,12 @@ Security:
   unaffected. Users who have only used the mosquitto_passwd utility to create
   and modify their password files are unaffected by this vulnerability.
   Affects version 1.0 to 1.5.5 inclusive.
+- CVE-2018-xxxxx: If an ACL file is empty, or has only blank lines or
+  comments, then mosquitto treats the ACL file as not being defined, which
+  means that no topic access is denied. Although denying access to all topics
+  is not a useful configuration, this behaviour is unexpected and could lead
+  to access being incorrectly granted in some circumstances. This is now
+  fixed. Affects versions 1.0 to 1.5.5 inclusive.
 
 Broker:
 - Fixed comment handling for config options that have optional arguments.
diff --git a/src/security_default.c b/src/security_default.c
index 743020c1a9..5a886a53df 100644
--- a/src/security_default.c
+++ b/src/security_default.c
@@ -317,7 +317,7 @@ int mosquitto_acl_check_default(struct mosquitto_db *db, struct mosquitto *conte
 	}else{
 		security_opts = &db->config->security_options;
 	}
-	if(!security_opts->acl_list && !security_opts->acl_patterns){
+	if(!security_opts->acl_file && !security_opts->acl_list && !security_opts->acl_patterns){
 			return MOSQ_ERR_PLUGIN_DEFER;
 	}
 
@@ -535,6 +535,10 @@ static int aclfile__parse(struct mosquitto_db *db, struct mosquitto__security_op
 					fclose(aclfptr);
 					return 1;
 				}
+			}else{
+				log__printf(NULL, MOSQ_LOG_ERR, "Error: Invalid line in acl_file \"%s\": %s.", security_opts->acl_file, buf);
+				fclose(aclfptr);
+				return 1;
 			}
 		}
 	}
diff --git a/test/broker/09-acl-empty-file.py b/test/broker/09-acl-empty-file.py
new file mode 100755
index 0000000000..a6d71cf60b
--- /dev/null
+++ b/test/broker/09-acl-empty-file.py
@@ -0,0 +1,71 @@
+#!/usr/bin/env python
+
+# Test for CVE-2018-xxxxx
+
+import inspect, os, sys
+# From http://stackoverflow.com/questions/279237/python-import-a-module-from-a-folder
+cmd_subfolder = os.path.realpath(os.path.abspath(os.path.join(os.path.split(inspect.getfile( inspect.currentframe() ))[0],"..")))
+if cmd_subfolder not in sys.path:
+    sys.path.insert(0, cmd_subfolder)
+
+import mosq_test
+import signal
+
+def write_config(filename, port, per_listener):
+    with open(filename, 'w') as f:
+        f.write("per_listener_settings %s\n" % (per_listener))
+        f.write("port %d\n" % (port))
+        f.write("acl_file %s\n" % (filename.replace('.conf', '.acl')))
+
+def write_acl(filename):
+    with open(filename, 'w') as f:
+        f.write('#comment\n')
+        f.write('\n')
+
+
+def do_test(port, per_listener):
+    conf_file = os.path.basename(__file__).replace('.py', '.conf')
+    write_config(conf_file, port, per_listener)
+
+    acl_file = os.path.basename(__file__).replace('.py', '.acl')
+    write_acl(acl_file)
+
+    rc = 1
+    keepalive = 60
+    connect_packet = mosq_test.gen_connect("acl-check", keepalive=keepalive)
+    connack_packet = mosq_test.gen_connack(rc=0)
+
+    mid = 1
+    publish_packet = mosq_test.gen_publish("test/topic", qos=0, payload="message")
+    subscribe_packet = mosq_test.gen_subscribe(mid, "test/topic", 0)
+    suback_packet = mosq_test.gen_suback(mid, 0)
+
+    pingreq_packet = mosq_test.gen_pingreq()
+    pingresp_packet = mosq_test.gen_pingresp()
+
+    broker = mosq_test.start_broker(filename=os.path.basename(__file__), use_conf=True, port=port)
+
+    try:
+        sock = mosq_test.do_client_connect(connect_packet, connack_packet, port=port)
+        mosq_test.do_send_receive(sock, subscribe_packet, suback_packet, "suback")
+
+        sock.send(publish_packet)
+
+        # If we receive the message, this will fail.
+        mosq_test.do_send_receive(sock, pingreq_packet, pingresp_packet, "pingresp")
+        rc = 0
+
+        sock.close()
+    finally:
+        os.remove(conf_file)
+        os.remove(acl_file)
+        broker.terminate()
+        broker.wait()
+        (stdo, stde) = broker.communicate()
+        if rc:
+            print(stde)
+            exit(rc)
+
+port = mosq_test.get_port()
+do_test(port, "false")
+do_test(port, "true")
diff --git a/test/broker/Makefile b/test/broker/Makefile
index 3299d7dbe1..d3b3e43889 100644
--- a/test/broker/Makefile
+++ b/test/broker/Makefile
@@ -118,6 +118,7 @@ endif
 endif
 
 09 :
+	./09-acl-empty-file.py
 	./09-plugin-auth-unpwd-success.py
 	./09-plugin-auth-unpwd-fail.py
 	./09-plugin-auth-acl-sub.py
diff --git a/test/broker/ptest.py b/test/broker/ptest.py
index eb10895da5..28f14f97e2 100755
--- a/test/broker/ptest.py
+++ b/test/broker/ptest.py
@@ -90,6 +90,7 @@
     (2, './08-tls-psk-pub.py'),
     (3, './08-tls-psk-bridge.py'),
 
+    (1, './09-acl-empty-file.py'),
     (1, './09-plugin-auth-unpwd-success.py'),
     (1, './09-plugin-auth-unpwd-fail.py'),
     (1, './09-plugin-auth-acl-sub.py'),