By Richard Bejtlich, Principal Security Strategist, Corelight

Welcome to the first in a regular series of blog posts on network security monitoring (NSM).

In 2002 Bamm Visscher and I defined NSM as “the collection, analysis, and escalation of indications and warnings to detect and respond to intrusions.” We were inspired by our work in the late 1990s and early 2000s at the Air Force Computer Emergency Response Team (AFCERT), and the operations built on the NSM software written by Todd Heberlein. Although NSM methodology applies to any sort of evidence or environment, these posts will largely describe NSM for network traffic in the enterprise.

As might be appropriate for the first post in a series on NSM, I will explain why I believe NSM is the first step one should take when implementing a security program. This may sound like a bold claim. Shouldn’t one collect logs from all your devices first, or perhaps roll out a shiny new endpoint detection and response (EDR) agent? While those steps may indeed benefit your security posture, they are not the first steps you should take.

In 2001 Bruce Schneier neatly summarized a shared vision for security: “monitor first”. I concur with this strategy, because I advocate basing security decisions on evidence, not faith. In other words, before making changes to one’s security posture, it is more efficient and effective to determine what is happening, and address the resulting discoveries first. My 2005 post Soccer Goal Security expands on this concept.

If one accepts the need to gather evidence, and identify what is happening in one’s environment as a necessary precursor to making changes, then we must determine how best to gather that evidence. Elsewhere I have advocated for four rough categories of intelligence, which I repeat here. They are ordered by increasing difficulty of implementation, but also likely increasing granularity of information.

The first way to identify what is happening in your environment is to rely on third party notification. As Mandiant’s M-Trends reports have been documenting for years, as of 2018, 38% of the firm’s incident response workload began with the victim learning of an intrusion via a third party. This is a cheap way to get insights into your security posture, as law enforcement, or worse, reporter Brian Krebs, is acting as your free threat intelligence provider. However, you are already days or weeks behind the intruder, and you must soon hire a consultancy to instrument and protect your network. It is important to maintain good relations with law enforcement and the media, but you should not rely on them for network intelligence

The second method, and the focus of this blog series, is network security monitoring. Begin by deploying a NSM sensor collecting, at a minimum, Zeek data at the gateway connecting your environment to the public Internet. This will see so-called “north-south” traffic (visibility for “east-west” traffic will be covered in a later post). By collecting NSM data, one has not interrupted daily IT operations or users, other than perhaps a brief outage to install a network tap. If administrators decide to (temporarily) use a switch SPAN port to see network traffic, users will suffer no interruption of service whatsoever. With a simple deployment, security teams gather a wealth of data about their environment and threat activity. I will address the specific benefits in future posts.

The third method is to collect logs from systems, servers, architecture, and other devices throughout the network. This step requires deploying not only a log management platform to collect, store, and present the data, but also reconfiguring each device to send its logs to the log management platform. Unlike the NSM deployment, installing and configuring a log management system is a demanding project. While the benefits are ultimately worthwhile, the project is much more involved, hence its status as the third step one should take.

The fourth way to learn about threat activity in the enterprise is to instrument the endpoints with an EDR agent. This is even a bigger project than the log collection effort, as the EDR agent could interfere with business operations while trying to observe and possibly interdict malicious activity. As with log management, I am not arguing against EDR. EDR is a tool that yields wonderful benefits for visibility and control. EDR is especially attractive the more mobile and distributed one’s workforce is, and the greater the amount of encrypted network traffic one encounters. However, the level of effort and return associated with NSM means I prefer network-centric visibility strategies prior to installing log management or EDR.

At this point you may ask “isn’t third party visibility the first step when trying to learn about threat activity? You listed NSM as second!” That is true, but I don’t consider third parties as a reliable method, or an especially proactive one. When called by the FBI, one should be able to reply “yes, thank you for calling, but I already detected the activity and we are handling it now.”

Some of you may also ask “how can NSM be first, when I already have a security program?” In that case, I suggest you make “NSM next!” In other words, augment your existing environment with NSM, and let the data help guide future security decisions.

Finally, you might ask if this is a workable solution. Has anyone ever done this? I’ve used or recommended the methodology in this blog series to dozens of organizations, from small start-ups of less than 100 people, to the largest corporate entities of half a million identities under management with global presence.

In future posts I will expand upon all things NSM. I look forward to you joining me on this journey.