Penetration testing offers strong advantages in the discovery of hidden vulnerabilities in a network and assessing network security. However, it can be carried out by only security analysts, which costs considerable time and money. The natural way to deal with the above problem is automated penetration testing, the essential part of which is automated attack planning. Although previous studies have explored various ways to discover attack paths, all of them require perfect network information beforehand, which is contradictory to realistic penetration testing scenarios. To vividly mimic intruders to find all possible attack paths hidden in a network from the perspective of hackers, we propose a network information gain based automated attack planning (NIG-AP) algorithm to achieve autonomous attack path discovery. The algorithm formalizes penetration testing as a Markov decision process and uses network information to obtain the reward, which guides an agent to choose the best response actions to discover hidden attack paths from the intruder’s perspective. Experimental results reveal that the proposed algorithm demonstrates substantial improvement in training time and effectiveness when mining attack paths.