Multiscale Stepping-Stone Detection: Detecting Pairs of Jittered Interactive Streams by Exploiting Maximum Tolerable Delay

Computer attackers frequently relay their attacks through a compromised host at an innocent site, thereby obscuring the true origin of the attack. There is a growing literature on ways to detect that an interactive connection into a site and another outbound from the site give evidence of such a “stepping stone.” This has been done based on monitoring the access link connecting the site to the Internet (Eg. [7, 11, 8]). The earliest work was based on connection content comparisons but more recent work has relied on timing information in order to compare encrypted connections. Past work on this problem has not yet attempted to cope with the ways in which intruders might attempt to modify their traffic to defeat stepping stone detection. In this paper we give the first consideration to constraining such intruder evasion. We present some unexpected results that show there are theoretical limits on the ability of attackers to disguise their traffic in this way for sufficiently long connections. We consider evasions that consist of local jittering of packet arrival times (without addition and subtraction of packets), and also the addition of superfluous packets which will be removed later in the connection chain (chaff). To counter such evasion, we assume that the intruder has a “maximum delay tolerance.” By using wavelets and similar multiscale methods, we show that we can separate the shortterm behavior of the streams – where the jittering or chaff indeed masks the correlation – from the long-term behavior of the streams – where the correlation remains. It therefore appears, at least in principle, that there is an effective countermeasure to this particular evasion tactic, at least for sufficiently long-lived interactive connections.