I've never heard of an attack or anything else using this. As Nessus says, the "Risk factor: Low".
If it can only be used for port scans, and to be honest I can't see how it would help in that either, then there is little risk, since the PIX should only be configured to allow through specific ports anyway. Make sure you have your internal servers up to date with all software patches, etc, and only allow through those specific ports that need to be let through.
If you want to see if someone is port scanning your network, which can be a prelude to a more serious attack, then you can get an IDS system.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...