Thank you for posting this question. Firewall threat messages could possibly be attacks, but not necessarily as they could also be caused by misconfigurations or peculiarities in the network. In AOS, threats have been categorized and been assigned a weight based on their possible severity. Threats with a higher severity have the potential to be more compromising to hosts behind the firewall than threats with a lower severity.
The threat you have displayed above is a minor threat, and virtually can be ignored. You can disable this message from appearing on the CLI, per session, by issuing the no events command. For future reference I have detailed this particular threat below, as described in our IPv4 Firewall Protection in AOS guide:
Short Definition: Reassembly timeout
Description: Indicates that the fragments necessary to complete reassembly
have not arrived in a timely manner. An attacker could have attempted to
consume all of the resources in the reassembly engine of a server or other
network device.
Action: The reassembly engine drops the offending fragments and frees the
resource.
I hope this makes sense, but please do not hesitate to reply to this discussion with further information or questions. I will be happy to assist you in any way I can.
Levi
