Security in Context: The BeyondTrust Blog

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Bringing you news and commentary on solutions and strategies for protecting critical IT infrastructure in the context of your business.

Where Passive Scanning Falls Short

Posted July 11, 2014    Morey Haber

In many sports, as in business, teams will promote a strategy to gain an edge – even if the concept is possibly flawed. Consider an American football hurry-up offense: will it cause the defense to stumble, or will it just exhaust the offense? The play has potential pros and cons, and many strategic technologies are no different. However, passive scanning technology is one of those ideas that is clearly bad and borderline awful. Here’s why …

First, passive scanning technology is based on sniffer technology. The technology must be put on a span port or tap to inspect network traffic and perform a vulnerability analysis. This requires you to identify key locations throughout a WAN to create a span port and locate passive scanners for monitoring traffic. This approach fails in a myriad of scenarios:

  • Passive scanners can’t identify network traffic routed outside of the span port, such as all local subnet traffic.
  • Passive scanning can’t handle encrypted traffic; only packets sent in clear text.
  • Devices on the network, but not communicating (e.g., in listen-only mode), are never discovered.
  • There is a high false positive rate for patched applications that do not modify headers or banners when communicating on the network. This is especially true for Linux variations with clear text protocols such as FTP and Telnet.
  • Large quantities of passive scanners are required to sufficiently cover highly routed and geographically disperse networks.
  • Environments that’s have invested in 10GB networks or faster can’t use span ports effectively for this technology.

The only scenario we have seen that successfully utilizes passive scanning technology is one with a flat, small and non-routable network. This limits passive scanning’s viability to small businesses with simple networks that do not require scalability.

BeyondTrust offers a better solution: continuous scheduled discovery throttled to meet the needs of your network. Our BeyondInsight technology, leveraged by Retina CS Enterprise Vulnerability Management and several of our PowerBroker Privileged Account Management solutions, offers the capability to schedule multiple discovery scans all day (with scan windows), process new assets, and send alerts when new devices are detected. BeyondInsight can therefore avoid span ports and discover devices on any subnet at any time, AND perform a deep vulnerability scan whenever a new device is detected. Below is a screen shot of a built in Smart Rule that triggers alerts when new assets are discovered:


This philosophy and superior method has the following advantages:

  • No need for span ports or special network changes
  • Throttle Scans, Retina Scan Pooling, and Scan Windows can quickly discover large address spaces.
  • New devices or services are identified immediately and alerted on or grouped for assessments.
  • Vulnerability assessments can be targeted reliably on devices using Smart Rules based on device age, open ports, or other techniques that would be associated with clear text protocols (i.e., Telnet or FTP detected).
  • Devices communicating on the same subnet, or not communicating at all, are identified and classified.
  • False positives are minimized because a true vulnerability assessment is performed, instead of an inference model that relies on network traffic to identify vulnerabilities.
  • Discovery scans can be scheduled multiple times per day for continuous monitoring initiatives to isolate and identify devices requiring assessment.

It’s fun watching teams and companies boast, but some just execute the wrong plays on the field. In the case of passive scanning, the play fails to advance the team down the field in too many situations. Fortunately, BeyondTrust customers have a strategy that is flexible and scalable enough to get them to the end zone.

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