To defend against a multi-step network intrusion, its progress needs to be monitored and predicted in real-time. For this purpose, isolated alerts must be correlated into attack scenarios as soon as the alerts arrive. Such efficient correlation of alerts demands an in-memory index to be built on received alerts. However, the finite memory implies that only a limited number of alerts inside a sliding window can be considered for correlation. Knowing this fact, an attacker can prevent two attack steps from both falling into the sliding window by either passively delaying the second step or actively invoking bogus alerts between the two steps. In either case, the correlation effort is defeated. In this paper, we first address the above issue with a novel queue graph (QG) approach. Instead of explicitly correlating a new alert to all the old ones that prepare for it, the approach only correlates the new alert to the latest copy of each type of alerts. The correlation with other alerts is kept implicit using the temporal order between alerts. Consequently, the approach has a quadratic (in the number of alert types) memory requirement, and it can correlate two alerts that are arbitrarily far away (namely, an infinitely large sliding window with a quadratic memory requirement). Our second contribution is a unified method based on the QG approach that can correlate received alerts, hypothesize missing alerts, and predict future alerts all at the same time. Empirical results show that our method can fulfill those tasks faster than an IDS can report alerts. The method is thus a promising solution for administrators to monitor and predict the progress of an intrusion, and thus to take appropriate countermeasures in a timely manner.
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