QotW #24: Why do people tell me not to use VLANs for security?

This week’s question of the week was asked by user jtnire who was asking a question very near and dear to those security professionals who came out of networking or systems backgrounds. He was doing some network design and came across a classic statement that, “VLANs are Not a Security Tool”.

As of this writing, jtnire had not accepted any answers, however user Rory McCune was leading the pack of answers. Rory focused primarily on the classic human problem of misconfiguration, particularly easy when we’re talking about typing Gi/0/4 when you meant Gi/1/4, as opposed to plugging a cable into the wrong port. He also specifically called out VLAN Hopping, which can abuse a misconfiguration to allow a malicious user access to a non-authorized VLAN.

User and moderator Rory Alsop, speaking from an audit perspective, expanded on what the other Rory mentioned and focused more generally on what would make him double-take. He pointed out that VLANs are generally used for cheap network segmentation and that if you’re using them for as a security tool, then you probably want to do it right and use a physically isolated network instead.

Jakob Borg came in with a completely different approach. He explained that, as an ISP, VLANs are a crucial component of their environment and when done right can be a very powerful tool from both a security and service prospective. User jliendo largely agreed with Jakob that configuration is king, and when configured properly is an excellent tool in your security arsenal. He also went into more technical detail about some of the possible attacks against VLANs and how they can be mitigated.

In this author’s opinion this is a fantastic question as VLANs are becoming an extremely common mechanism for network isolation. The answers also did a great job of coming at the problem from all manner of angles, from external auditors to in the trenches technicians.

Liked this question of the week? Interested in reading it or adding an answer? See the question in full. Have questions of a security nature of your own? Security expert and want to help others? Come and join us at security.stackexchange.com.