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 short-term 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.
Keywords Network intrusion detection - Evasion - Stepping Stone - Interactive Session - Multiscale Methods - Wavelets - Universal Keystroke - Interarrival Distribution