Thursday, March 10, 2011

Optimal Methods for Spam and DDoS Offender Discovery

As botnet threats go, Spam and DDoS are probably the most widely known and discussed tactics employed by criminal operators. Despite being some of the last things that career botnet operators employ their compromised victims for, and despite offering the lowest monetization rates for the criminals, DDoS and Spam volume have continued to rise annually.

A question was asked to me recently as to which techniques worked best for dealing with DDoS and Spam participation from within large enterprises or residential DSL/Cable networks – Network Anomaly Detection Systems (NADS) or botnet command-and-control (CnC) enumeration techniques (such as those employed by Damballa)?

It’s not the kind of question that can be answered succinctly. Both approaches are designed to scale to very large networks – and as such are components of a robust protection strategy. In fact the technologies are rather complementary – although I do think that the CnC enumeration approach is more elegant and efficient in the grand scheme of things.

The NADS approach to Spam and DDoS participation detection is simple enough – you monitor netflow (a compact summary of network packet flow – usually to/from IP address, port, protocol, date/time and packet size information), determine a baseline for traffic levels, set alert thresholds for potential anomalies, and define responses when a threshold alert is received. In the context of a simple DDoS threat, you set up a threshold for the volume of HTTP traffic directed at a single destination by a single IP host and label that host as initiating a DDoS attack. If multiple hosts within the network being monitored also reach the HTTP threshold(s) against the same target IP address, you label them all as being part of a DDoS botnet. The same basic principles apply to Spam botnet detection.

An alternative and generally complementary approach to the problem is to automatically identify hosts within the monitored network that are already infected with malware and/or engaged in conversations with botnet CnC servers. This can be achieved in a variety of ways, but one of the simplest ways is to merely observe the DNS requests made by the hosts and the responses from the resolving DNS servers. Having identified suspicious DNS request profiles along with DNS responses that have high probabilities of association with criminal hosting infrastructure, it’s possible to quickly match victims with particular botnets – and label the new (or previously known) CnC fully qualified domain name. Any other hosts exhibiting similar DNS resolution characteristics are members of the same botnet. The beauty of this approach is that this method of detection and botnet enumeration (and labeling) can be done before the botnet victims actually participate in any subsequent Spam or DDoS campaigns.

When it comes to mitigating the threat, the historical way is to effectively block the attack traffic by either firewalling off specific ports or destination IP addresses, or walled gardening the malignant hosts. So, while the botnet host is spewing spam or DDoS traffic, it’s not being routed to its final (target) destination.

That approach may have been OK in the past if you were only dealing with IP-based threat responses and could stomach the voluminous traffic internally, but with more advanced CnC and botnet enumeration technologies you’re able to bring to bear some additional (and more versatile) mitigation techniques. Since you’re constantly identifying and tracking botnet membership and you know which CnC’s these victims are being controlled by, you could perform one or more of the following actions:

  1. As botnet members begin to participate in the DDoS attack or Spam campaign, traffic to and from the CnC server could be blocked. By doing so, no new commands are sent to the botnet victims and they typically cease their attacks. In addition, any other botnet members within the network who have not yet been tasked to participate in the attack will similarly not be able to receive instructions.
  2. Walled Gardens can be selectively initiated around the infected botnet population – blocking just the ports and protocols being used (or likely to be used) in the attack against remote targets – without applying the same blocking to all hosts or subscribers within the network. For example, a botnet may be tasked with DDoSing a popular financial services web portal using a HTTP-based payload. It would therefore be important to only block the attack traffic and allow legitimate traffic through. A walled garden approach could be used in this scenario without having to utilize Deep Packet Inspection (DPI) to differentiate between the attack and legitimate traffic.
  3. The ability to differentiate CnC server activity at the domain name level is important for botnets that utilize fast flux infrastructure to distribute command over large numbers of IP addresses. If recursive DNS services are provided by the organization to their enterprise hosts or subscribers, an alternative DNS response could be sent to the botnet victims – e.g. making resolve to localhost (
  4. If DPI or PCAP capabilities exist within the organization, they could be selectively deployed to catalog the criminal communications between the botnet members and the CnC server. This detailed evidence of the attack (including the commands being sent by the CnC) can be used for takedown or prosecution purposes.
  5. If the botnet malware agent is relatively unsophisticated or if the CnC server itself is vulnerable to third-party takeover (e.g. a hacked server that the legitimate owner regains control and can now issue commands to the botnet, or if the Botnet CnC portal code contains remotely exploitable vulnerabilities), it may be possible to issue commands “on behalf” of the criminal operator instructing all the botnet members to stop their attack and to automatically uninstall the malware agent.

There are of course many other imaginative ways to use the knowledge of the botnet CnC and its members in preemptive protection strategies too.

I think that NADS-based botnet detection (or more precisely botnet attack traffic detection) is useful for identifying triggers for remediation action – but I think that botnet CnC enumeration techniques can provide greater flexibility in long-term threat management approaches.

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