(第1空)An advanced persistent threat (APT) is a covert ( ) attack on a computer networkwhere the attacker gains and maintains ( )access to the targeted network and remains undetected for a significant period. An APT is a sophisticated,long-term and ( ) attack,usually orchestrated by nation-state groups，or well-organized criminal enterprises. Duringthe time between infection and remediation the hacker will often monitor，intercept，and relayinformation and sensitive data. The intention of an APT is usually to ( ) or steal datarather than cause a network outage，denial of service or infect systems with malware.APTsoften use social engineering tactics or exploit security ( )in networks，applications orfiles to plant malware on target systems.A successful advanced persistent threat can beextremely effective and beneficial to the attacker.