document pf issue.

UPDATING

@@ -1,5 +1,17 @@
 Updating Information for MidnightBSD users.
 
+20231205:
+	pf security issue: 
+	As part of its stateful TCP connection tracking implementation, pf
+	performs sequence number validation on inbound packets.  This makes it
+	difficult for a would-be attacker to spoof the sender and inject packets
+	into a TCP stream, since crafted packets must contain sequence numbers
+	which match the current connection state to avoid being rejected by the
+	firewall. A bug in the implementation of sequence number validation means that the
+	sequence number is not in fact validated, allowing an attacker who is
+	able to impersonate the remote host and guess the connection's port
+	numbers to inject packets into the TCP stream.
+
 20231129:
 	telnetd(8) removed. Use mports/net/freebsd-telnetd if you need it