I am not sure that there is any attractive answer for your situation. It looks like if you want to get code that fixes the problem (generally the preferred solution) that you would have to upgrade the hardware.
As I understand the description of the problem, to execute the attack the bad guy must complete the three way handshake with the router. So probably your best workaround is to control very tightly what is allowed to establish TCP connection to the router. I would start with the access list on the public facing interface (you do have an access list on that interface?). Make sure that connection to the router on TCP based services are denied or if they need to be allowed make sure that you restrict the addresses that are allowed to make the connection. After you have controlled TCP access from outside you might want to make a similar effort to control access from inside.
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...