The three ways for the attacker to avoid raising alarms from intrusion detection systems are
Step-by-step explanation:
Session splicing and fragmentation - involve breaking, slicing, and splitting packets into multiple pieces such that no single packet causes the IDS to trigger an alert.
Invalid packets - Sending invalid TCP packets is another way of evading an IDS.
Polymorphic shellcodes -Most IDS systems have a standard default set of intrusion signatures. Attackers can modify the attack payload so that it doesn’t match the default IDS signature and gets through it.
Hope it helps, any confusion you may ask!