Index: sys/netinet/tcp_syncache.c
===================================================================
--- sys/netinet/tcp_syncache.c
+++ sys/netinet/tcp_syncache.c
@@ -1514,20 +1514,22 @@
 #endif
 
 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
-	/*
-	 * If listening socket requested TCP digests, check that received
-	 * SYN has signature and it is correct. If signature doesn't match
-	 * or TCP_SIGNATURE support isn't enabled, drop the packet.
-	 */
-	if (ltflags & TF_SIGNATURE) {
-		if ((to->to_flags & TOF_SIGNATURE) == 0) {
-			TCPSTAT_INC(tcps_sig_err_nosigopt);
-			goto done;
-		}
+	/* RFC 2385: upon receiving a signed packet, validate it */
+	if (to->to_flags & TOF_SIGNATURE) {
 		if (!TCPMD5_ENABLED() ||
 		    TCPMD5_INPUT(m, th, to->to_signature) != 0)
 			goto done;
 	}
+	else if (ltflags & TF_SIGNATURE) {
+		/*
+		 * The listening socket requested TCP digests, but no
+		 * signature was provided. If there is no security
+		 * association setup, let the packet in.
+		 */
+		if (!TCPMD5_ENABLED() ||
+		    TCPMD5_INPUT(m, NULL, NULL) != ENOENT)
+			goto done;
+	}
 #endif	/* TCP_SIGNATURE */
 	/*
 	 * See if we already have an entry for this connection.
@@ -1725,11 +1727,11 @@
 	}
 #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
 	/*
-	 * If listening socket requested TCP digests, flag this in the
+	 * If the incoming packet has an MD5 signature, flag this in the
 	 * syncache so that syncache_respond() will do the right thing
 	 * with the SYN+ACK.
 	 */
-	if (ltflags & TF_SIGNATURE)
+	if (to->to_flags & TOF_SIGNATURE)
 		sc->sc_flags |= SCF_SIGNATURE;
 #endif	/* TCP_SIGNATURE */
 	if (to->to_flags & TOF_SACKPERM)
Index: sys/netipsec/xform_tcp.c
===================================================================
--- sys/netipsec/xform_tcp.c
+++ sys/netipsec/xform_tcp.c
@@ -269,6 +269,11 @@
 		KMOD_TCPSTAT_INC(tcps_sig_err_buildsig);
 		return (ENOENT);
 	}
+	if (buf == NULL) {
+		key_freesav(&sav);
+		KMOD_TCPSTAT_INC(tcps_sig_err_nosigopt);
+		return (EPERM);
+	}
 	/*
 	 * tcp_input() operates with TCP header fields in host
 	 * byte order. We expect them in network byte order.