Intrusion Prevention vs Active Response

In today's varied world of computer security products, techniques, and solutions, the term intrusion prevention has received widespread attention. Much of this attention probably stems from the perhaps overly powerful implications of the term, but this is not to say that the concept of proactively preventing security compromises is without merit. Intrusion protection techniques range from host level stack-hardening mechanisms (see the PaX project at to inline network devices with software that can prevent malicious packets from ever reaching their intended targets, while simultaneously allowing all other traffic through unimpeded.

In contrast, active response refers to the set of mechanisms that can be employed against an attacker (once an attack is detected) that do not necessarily thwart the attack. The fact that active response isn't always able to prevent the initial attack is an important distinction, and it solidly delineates the difference between intrusion prevention and active response. One of the best ways to see this is with a motivating example.

The Witty worm of 2004 ( exploited a vulnerability in the PAM ICQ module in several products developed by Internet Security Systems (, now part of IBM), including BlackICE and RealSecure. The worm was transmitted from system to system via a single UDP packet with a source port of 4000 and an arbitrary destination port. When a vulnerable system monitored such a packet, the contents of the packet payload would be executed, instead ofjust inspected. In the specific case of the Witty worm, the packet payload contained code that would write 65K of data (derived from the same DLL that contained the vulnerability) to random points within the local disk drive, thus slowly causing filesystem corruption. While this would not immediately destroy a system upon initial infection (say, by completely formatting the disk), it would certainly break a system in subtle ways over time.

For anyone still running a vulnerable version of BlackICE or RealSecure, the first priority would be to download and install a patch from http://www Another option is to configure a local packet filter to not forward any UDP packets with a source port of 4000 into the internal network; however, this would be at the expense of potentially breaking ICQ services that span the firewall. Obviously, this is not an optimal solution, so what is really needed is the ability to detect packets that are specifically associated with the Witty worm, and then stop them from entering the local network. The detection requirement is easily met (Snort rules were quickly written after the initial discovery of the Witty worm), but any active response mechanism (such as sending ICMP Port Unreachable messages or dynamically reconfiguring a firewall ruleset) is completely ineffectual against the worm. Because the entire attack is encapsulated within a single packet, the attacker is able to take advantage of two important facts:

• Sending an ICMP Port Unreachable message back to the source IP address is worthless because the attack has already made it through to the target. The source IP address does not have to care whether or not the targeted UDP service appears to be unreachable.

• The attack packet can be spoofed. From the perspective of the target, the attack might appear to originate from Yahoo!, an external DNS server, or an upstream router. Sending any kind of response packet or instantiating a firewall-blocking rule could therefore interfere with basic network connectivity.

The only way to really stop the Witty worm is with an inline device that can make fine-grained decisions about the contents of packets that should or should not be forwarded. Both Snort running in inline mode and iptables running a translated Snort rule can provide this functionality. Because it is useless to respond to a single packet attack after such an attack is forwarded to a target system, this class of attacks highlights the differences between active response and intrusion prevention mechanisms.

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