Fix segfault on client sending malformed CONNACk.

CVE-xxxx-xxxx: If an authenticated client connected with MQTT v5 sent a
malformed CONNACK message to the broker a NULL pointer dereference occurred,
most likely resulting in a segfault. This will be updated with the CVE
number when it is assigned.
Affects versions 2.0.0 to 2.0.9 inclusive.

Closes #2163. Thanks to Bryan Pearson.

ChangeLog.txt

@@ -1,9 +1,17 @@
-2.0.10 - 2021-xx-xx
+2.0.10 - 2021-04-03
 ==================
 
+Security:
+- CVE-xxxx-xxxx: If an authenticated client connected with MQTT v5 sent a
+  malformed CONNACK message to the broker a NULL pointer dereference occurred,
+  most likely resulting in a segfault. This will be updated with the CVE
+  number when it is assigned.
+  Affects versions 2.0.0 to 2.0.9 inclusive.
+
 Broker:
 - Don't over write new receive-maximum if a v5 client connects and takes over
   an old session. Closes #2134.
+- Fix CVE-xxxx-xxxx. Closes #2163.
 
 Clients:
 - Set `receive-maximum` to not exceed the `-C` message count in mosquitto_sub

src/handle_connack.c

@@ -39,7 +39,7 @@ int handle__connack(struct mosquitto *context)
 	uint16_t server_keepalive;
 	uint8_t max_qos = 255;
 
-	if(!context){
+	if(context == NULL || context->bridge == NULL){
 		return MOSQ_ERR_INVAL;
 	}
 	log__printf(NULL, MOSQ_LOG_DEBUG, "Received CONNACK on connection %s.", context->id);

test/broker/01-connect-connack-2163.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+
+# Test https://github.com/eclipse/mosquitto/issues/2163
+# Does the broker cope with a malformed CONNACK sent to it after a valid CONNECT?
+
+from mosq_test_helper import *
+
+def do_test(proto_ver):
+    rc = 1
+    keepalive = 10
+    connect_packet = mosq_test.gen_connect("connect-connack-2163", keepalive=keepalive, proto_ver=proto_ver)
+    connack_packet = mosq_test.gen_connack(rc=0, proto_ver=proto_ver)
+    connack_malformed = struct.pack("BBBBB", 0x02, 0x00, 0x01, 0xE0, 0x00)
+    connack_malformed = struct.pack("BBBB", 0x29, 0x02, 0x00, 0x01)
+    pingreq_packet = mosq_test.gen_pingreq()
+
+    port = mosq_test.get_port()
+    broker = mosq_test.start_broker(filename=os.path.basename(__file__), port=port)
+
+    try:
+        sock = mosq_test.do_client_connect(connect_packet, connack_packet, port=port)
+        sock.send(connack_malformed)
+        try:
+            mosq_test.do_send_receive(sock, pingreq_packet, b"", "pingreq")
+        except ConnectionResetError:
+            pass
+        sock.close()
+
+        # Does the broker still exist?
+        sock = mosq_test.do_client_connect(connect_packet, connack_packet, port=port)
+        mosq_test.do_ping(sock)
+        sock.close()
+
+        rc = 0
+    except mosq_test.TestError:
+        pass
+    finally:
+        broker.terminate()
+        broker.wait()
+        (stdo, stde) = broker.communicate()
+        if rc:
+            print(stde.decode('utf-8'))
+            print("proto_ver=%d" % (proto_ver))
+            exit(rc)
+
+
+do_test(proto_ver=3)
+do_test(proto_ver=4)
+do_test(proto_ver=5)
+exit(0)

test/broker/Makefile

@@ -22,6 +22,7 @@ test : test-compile 01 02 03 04 05 06 07 08 09 10 11 12 13 14
 01 :
 	./01-connect-allow-anonymous.py
 	./01-connect-bad-packet.py
+	./01-connect-connack-2163.py
 	./01-connect-disconnect-v5.py
 	./01-connect-duplicate.py
 	./01-connect-invalid-id-0.py

test/broker/test.py

@@ -7,6 +7,7 @@
     #(ports required, 'path'),
     (1, './01-connect-allow-anonymous.py'),
     (1, './01-connect-bad-packet.py'),
+    (1, './01-connect-connack-2163.py'),
     (1, './01-connect-disconnect-v5.py'),
     (1, './01-connect-duplicate.py'),
     (1, './01-connect-invalid-id-0.py'),