The show the variety of aspects security covers in this sort of scenario:
Daniel posted the top answer, and it has nothing to do with IT, but instead focuses on the cause – if a user has installed an access point it is because they need something the existing network is not providing. This is always worth considering:
Discuss with the users what they are trying to accomplish. Perhaps create an official wifi network ( use all the security methods you wish – it will be ‘yours’ ). Or, better, two – Guest and Corporate WAPs.
Remember Immutable Law of Security #10: Technology is not a panacea. Whilst technology can do some amazing things, it can’t enforce user behaviour. You have a user that is bringing undue risk to the organisation, and that risk needs to be dealt with. The solution to your problem is _policy_, not technology. Set up a security policy that details explicitly disallowed behaviours, and have your users sign it. If they violate that policy, you can go to your superiors with evidence of the violation and a penalty can be enforced. As long as the users have physical access to the machines they use and their USB ports (that’s hard to avoid, unless you pour glue in all the USB ports…) and that the installed operating systems allow it (then again, hard to avoid if users are “administrators” on their systems, in particular in BYOD contexts), then the users can setup custom access points which gives access to, at least, their machine.
Rory McCune provided some information on the types of solutions which generally are used in large corporates, where they work well, including NAC and port lockdowns. Lie Ryan‘s comments tend to be appropriate on smaller networks.
k1DBLITZ also focuses on the use of technical solutions in addition to policy, and JasperWallace recommends looking for and blocking unapproved MAC addresses, and further answers discuss wireless scanning and scripted checks.
Overall, it would seem that a mixture of technical and management controls are required – the balance depending on your specific environment.
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