Your PIX is only vulnerable if you are running 6.3 code or earlier and have IPsec enabled (which is not enabled by default). The vulnerability is due to CSCef57566. Also, I would recommend taking a look at the follow link, which has all the details of this vulnerability:
As a workaround, you can disable IPSec, though this is probably not a viable solution for you. Therefore, to be protected against this vulnerability, you should upgrade to the latest 6.3.5.x interim release, or move to the 7.x or 8.x trains where this vulnerability does not exist.
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...