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wireless lan security

14 Jun 2009

one issue with securing a wireless lan most ppl forget is that the access point is running as a bridge. using a bridge is a good thing in general but it has it’s drawbacks concerning security

client1 < ~ ~wlan link~ ~~> ap bridge <------wire-----> client2

this is the common setup on the average home wlan you can find. assume the hacker got a valid wpa2 key from somewhere or your wep key was broken (which can be done in less than 10minutes - common knowledge).

since most access points are used as internet routers as well you can now redirect connections from client2 over clien1 with some arp-spoofing (also common knowledge, just search in google). this would of course not work if the router wasn’t configured as bridge! for example one could create one subnet for wired lan and for the wireless lan. this would cure this critical security issue.

but having the stream redirected what could an attacker do with it? you can first of all intercept all traffic. you can alter the traffic and upload trojans/viruses using a http proxy and then you could insert it into a download.

another security issue which is common with autoupdaters is weather they check the updates for authenticity. I did not check any but i would assume there could be some interesting way getting things done.

just remember that next time you have to secure a network. consider using no bridge.

i would like to debunk a security myth about wireless security guides. many tell you to turn on mac filters. mac filters don’t do any good since anyone can spoof your mac address (if you use that ap with a wireless client from time to time) and one can use two clients with equal mac addresses but different ip addresses (this might be dependent on the vendor) but i tested this for an access point using a madwifi card and it worked well. the only thing which is not working is communicating between the two clients sharing the same mac.

this works since wireless lan is a broadcast medium and both clients see the data incoming (and both decode them). but since they use a different mac address the data won’t be handled on the wrong destination. some protocols might break tough

enabling mac filters on the other hand degrades usability greatly. so if in doubt don’t use it.