Index: pjproject/trunk/pjsip/src/pjsip/sip_transaction.c
===================================================================
--- pjproject/trunk/pjsip/src/pjsip/sip_transaction.c	(revision 2914)
+++ pjproject/trunk/pjsip/src/pjsip/sip_transaction.c	(revision 2915)
@@ -2841,4 +2841,13 @@
 	    /* Process incoming ACK request. */
 
+	    /* Verify that this is an INVITE transaction */
+	    if (tsx->method.id != PJSIP_INVITE_METHOD) {
+		PJ_LOG(2, (tsx->obj_name, 
+			   "Received illegal ACK for %.*s transaction",
+			   (int)tsx->method.name.slen,
+			   tsx->method.name.ptr));
+		return PJSIP_EINVALIDMETHOD;
+	    }
+
 	    /* Cease retransmission. */
 	    if (tsx->retransmit_timer.id != 0) {
Index: pjproject/trunk/pjsip/src/pjsip/sip_transport.c
===================================================================
--- pjproject/trunk/pjsip/src/pjsip/sip_transport.c	(revision 2914)
+++ pjproject/trunk/pjsip/src/pjsip/sip_transport.c	(revision 2915)
@@ -1440,4 +1440,12 @@
 		rdata->msg_info.via->rport_param = rdata->pkt_info.src_port;
 	    }
+	} else {
+	    /* Drop malformed responses */
+	    if (rdata->msg_info.msg->line.status.code < 100 ||
+		rdata->msg_info.msg->line.status.code >= 700)
+	    {
+		mgr->on_rx_msg(mgr->endpt, PJSIP_EINVALIDSTATUS, rdata);
+		goto finish_process_fragment;
+	    }
 	}
 
Index: pjproject/trunk/tests/pjsua/scripts-sipp/uac-bad-ack.xml
===================================================================
--- pjproject/trunk/tests/pjsua/scripts-sipp/uac-bad-ack.xml	(revision 2915)
+++ pjproject/trunk/tests/pjsua/scripts-sipp/uac-bad-ack.xml	(revision 2915)
@@ -0,0 +1,132 @@
+<?xml version="1.0" encoding="ISO-8859-1" ?>
+<!DOCTYPE scenario SYSTEM "sipp.dtd">
+
+<!-- This program is free software; you can redistribute it and/or      -->
+<!-- modify it under the terms of the GNU General Public License as     -->
+<!-- published by the Free Software Foundation; either version 2 of the -->
+<!-- License, or (at your option) any later version.                    -->
+<!--                                                                    -->
+<!-- This program is distributed in the hope that it will be useful,    -->
+<!-- but WITHOUT ANY WARRANTY; without even the implied warranty of     -->
+<!-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the      -->
+<!-- GNU General Public License for more details.                       -->
+<!--                                                                    -->
+<!-- You should have received a copy of the GNU General Public License  -->
+<!-- along with this program; if not, write to the                      -->
+<!-- Free Software Foundation, Inc.,                                    -->
+<!-- 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA             -->
+<!--                                                                    -->
+<!--                                                                    -->
+
+<scenario name="UAC with bad ACK">
+  <!-- UAC with bad ACK causes assertion with pjsip 1.4			-->
+  <send retrans="500">
+    <![CDATA[
+
+      INVITE sip:[service]@[remote_ip]:[remote_port] SIP/2.0
+      Via: SIP/2.0/[transport] [local_ip]:[local_port];branch=[branch]
+      From: sipp <sip:sipp@[local_ip]:[local_port]>;tag=[call_number]
+      To: sut <sip:[service]@[remote_ip]:[remote_port]>
+      Call-ID: [call_id]
+      CSeq: 1 INVITE
+      Contact: sip:sipp@[local_ip]:[local_port]
+      Max-Forwards: 70
+      Subject: Performance Test
+      Content-Type: application/sdp
+      Content-Length: [len]
+
+      v=0
+      o=user1 53655765 2353687637 IN IP[local_ip_type] [local_ip]
+      s=-
+      c=IN IP[media_ip_type] [media_ip]
+      t=0 0
+      m=audio [media_port] RTP/AVP 0
+      a=rtpmap:0 PCMU/8000
+
+    ]]>
+  </send>
+
+  <recv response="100"
+        optional="true">
+  </recv>
+
+  <recv response="180" optional="true">
+  </recv>
+
+  <!-- By adding rrs="true" (Record Route Sets), the route sets         -->
+  <!-- are saved and used for following messages sent. Useful to test   -->
+  <!-- against stateful SIP proxies/B2BUAs.                             -->
+  <recv response="200" rtd="true">
+  </recv>
+
+  <!-- Packet lost can be simulated in any send/recv message by         -->
+  <!-- by adding the 'lost = "10"'. Value can be [1-100] percent.       -->
+  <send>
+    <![CDATA[
+
+      ACK sip:[service]@[remote_ip]:[remote_port] SIP/2.0
+      Via: SIP/2.0/[transport] [local_ip]:[local_port];branch=[branch]
+      From: sipp <sip:sipp@[local_ip]:[local_port]>;tag=[call_number]
+      To: sut <sip:[service]@[remote_ip]:[remote_port]>[peer_tag_param]
+      Call-ID: [call_id]
+      CSeq: 1 ACK
+      Contact: sip:sipp@[local_ip]:[local_port]
+      Max-Forwards: 70
+      Subject: Performance Test
+      Content-Length: 0
+
+    ]]>
+  </send>
+
+  <!-- This delay can be customized by the -d command-line option       -->
+  <!-- or by adding a 'milliseconds = "value"' option here.             -->
+  <pause/>
+
+  <!-- The 'crlf' option inserts a blank line in the statistics report. -->
+  <send retrans="500">
+    <![CDATA[
+
+      BYE sip:[service]@[remote_ip]:[remote_port] SIP/2.0
+      Via: SIP/2.0/[transport] [local_ip]:[local_port];branch=z9hG4bK-1
+      From: sipp <sip:sipp@[local_ip]:[local_port]>;tag=[call_number]
+      To: sut <sip:[service]@[remote_ip]:[remote_port]>[peer_tag_param]
+      Call-ID: [call_id]
+      CSeq: 2 BYE
+      Contact: sip:sipp@[local_ip]:[local_port]
+      Max-Forwards: 70
+      Subject: Performance Test
+      Content-Length: 0
+
+    ]]>
+  </send>
+
+  <recv response="200" crlf="true">
+  </recv>
+
+  <pause milliseconds="2000"/>
+
+  <send>
+    <![CDATA[
+
+      ACK sip:[service]@[remote_ip]:[remote_port] SIP/2.0
+      Via: SIP/2.0/[transport] [local_ip]:[local_port];branch=z9hG4bK-1
+      From: sipp <sip:sipp@[local_ip]:[local_port]>;tag=[call_number]
+      To: sut <sip:[service]@[remote_ip]:[remote_port]>[peer_tag_param]
+      Call-ID: [call_id]
+      CSeq: 2 BYE
+      Contact: sip:sipp@[local_ip]:[local_port]
+      Max-Forwards: 70
+      Subject: Performance Test
+      Content-Length: 0
+
+    ]]>
+  </send>
+
+  <!-- definition of the response time repartition table (unit is ms)   -->
+  <ResponseTimeRepartition value="10, 20, 30, 40, 50, 100, 150, 200"/>
+
+  <!-- definition of the call length repartition table (unit is ms)     -->
+  <CallLengthRepartition value="10, 50, 100, 500, 1000, 5000, 10000"/>
+
+</scenario>
+
