diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index 656631083177..3ac1af6f59fc 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -1068,6 +1068,13 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
 				ct->proto.tcp.last_flags |=
 					IP_CT_EXP_CHALLENGE_ACK;
 		}
+
+		/* possible challenge ack reply to syn */
+		if (old_state == TCP_CONNTRACK_SYN_SENT &&
+		    index == TCP_ACK_SET &&
+		    dir == IP_CT_DIR_REPLY)
+			ct->proto.tcp.last_ack = ntohl(th->ack_seq);
+
 		spin_unlock_bh(&ct->lock);
 		nf_ct_l4proto_log_invalid(skb, ct, state,
 					  "packet (index %d) in dir %d ignored, state %s",
@@ -1193,6 +1200,14 @@ int nf_conntrack_tcp_packet(struct nf_conn *ct,
 			 * segments we ignored. */
 			goto in_window;
 		}
+
+		/* Reset in response to a challenge-ack we let through earlier */
+		if (old_state == TCP_CONNTRACK_SYN_SENT &&
+		    ct->proto.tcp.last_index == TCP_ACK_SET &&
+		    ct->proto.tcp.last_dir == IP_CT_DIR_REPLY &&
+		    ntohl(th->seq) == ct->proto.tcp.last_ack)
+			goto in_window;
+
 		break;
 	default:
 		/* Keep compilers happy. */