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.
DocumentationPrerequisite and code download linksGoalLimitations/RestrictionsTopologyHow OpenDNS worksStep by Step ConfigurationUpgrade the router image to Polaris (16.3) or higher imageUpgrade rommomImport CA certificate to the trust poolGet the t...
DocumentationPrerequisite and code download linksGoalLimitations/RestrictionsTopologyHow OpenDNS worksStep by Step ConfigurationUpgrade the router image to Polaris (16.3) or higher imageUpgrade rommomImport CA certificate to the trust poolGet the token to...
Show Name: ASA/FTD Troubleshooting Enhancements and Cisco Live US 2018
Contributors: Kevin Klous, Jay Johnston, and Magnus Mortensen
Posting Date: June 2018
Description: The team discusses the recently released troubleshooting...