This paper presents the attack tree modeling technique of quantifying cyber-\nattacks on a hypothetical school network system. Attack trees are constructed\nby decomposing the path in the network system where attacks are\nplausible. Considered for the network system are two possible network attack\npaths. One network path represents an attack through the Internet, and the\nother represents an attack through the Wireless Access Points (WAPs) in the\nschool network. The probabilities of success of the events, that is, 1) the attack\npayoff, and 2) the commitment of the attacker to infiltrate the network\nare estimated for the leaf nodes. These are used to calculate the Returns on\nAttacks (ROAs) at the Root Nodes. For Phase I, the â??As Isâ? network, the ROA\nvalues for both attack paths, are higher than 7 (8.00 and 9.35 respectively),\nwhich are high values and unacceptable operationally. In Phase II, countermeasures\nare implemented, and the two attack trees reevaluated. The probabilities\nof success of the events, the attack payoff and the commitment of the\nattacker are then re-estimated. Also, the Returns on Attacks (ROAs) for the\nRoot Nodes are re-assessed after executing the countermeasures. For one attack\ntree, the ROA value of the Root Node was reduced to 4.83 from 8.0,\nwhile, for the other attack tree, the ROA value of the Root Node changed to\n3.30 from 9.35. ROA values of 4.83 and 3.30 are acceptable as they fall within\nthe medium value range. The efficacy of this method whereby, attack trees are\ndeployed to mitigate computer network risks, as well as using it to assess the\nvulnerability of computer networks is quantitatively substantiated.
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