A unique ICAO address is assigned to each Mode S transponder of an aircraft and serves as the unique identifier for each aircraft
However, as this report states military aircraft clearly often have a capability to modify the ICAO address.
It might be worth noting at this point that military aircraft can transmit non-assigned ICAO addresses. Potentially they could transmit anything else in the message, but I'm not sure that is important.
https://github.com/junzis/the-1090mhz-riddle/blob/90643baa6283f38d5432a89c7958b5dcae6b2803/content/ads-b/1-basics.tex#L56
In the linked section it indicates that
However, as this report states military aircraft clearly often have a capability to modify the ICAO address.
It might be worth noting at this point that military aircraft can transmit non-assigned ICAO addresses. Potentially they could transmit anything else in the message, but I'm not sure that is important.