You can read more about it on the links below, there are some suggestions. Please be aware that I have not yet tested it myself on any ASA that is in production so I don't know how well the suggestions actually work.
"Mitigation Different kinds of mitigations can be implemented to minimise the impact of the attack. On firewalls and other kinds of equipment a list of trusted sources for which ICMP is allowed could be configured. Disabling ICMP Type 3 Code 3 on the WAN interface can mitigate the attackquite easily. This is the best mitigation weknow of so far."
This seems to be affecting all ASAs, we ran those tests yesterday and a 5545 increased to 42% CPU and a 5585 ssp20 took a 9% cpu-hit from just one computer with the same test. The fewer cores your ASA has the worse the impact seems to be.
If you are under attack I think your best bet is to filter it out further out in the network. I.e. configure a PACL on a switch between your ISP and your ASA that blocks icmp unreachable before it hits your ASA, that is until the original issue has been solved properly.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...