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Attacking Automatic Wireless Network Selection
From: "Dino A. Dai Zovi" <ddz () matasano com>
Date: Tue, 17 Jan 2006 15:21:55 -0500


Simple Nomad recently discussed issues with Windows XP creating Ad- Hoc wireless networks at this year's ShmooCon. There are, however, many more similar and more serious problems with how Windows and MacOS X locate and automatically join wireless networks. These have been publicly discussed and demonstrated before, but do not seem to have been given the attention needed as many security professionals are still unaware of the risks they present.

Simple Nomad's work illustrated the problem when an ad-hoc network was present the Preferred Networks list used by Windows XP's Wireless Auto Configuration service (MacOS X has an similar list of "Trusted Wireless Networks"). In fact, Windows and MacOS X probe for *every* network in the preferred/trusted networks list upon boot up and waking from sleep. Under Windows, the entire list is probed for continually when the machine is not currently associated to a wireless network. In addition, any network joined is automatically added to the top of this list (MacOS X only adds the network to the trusted networks list if the user elects to do so when joining the network). Some wireless adapters, notably most 802.11b-only cards, will automatically probe for randomly generated network names. All of these behaviors can be taken advantage of by a nearby attacker.

To that effect, I would like to introduce KARMA [1], written by Shane "K2" Macaulay and myself. Our paper, "Attacking Automatic Wireless Network Selection" [2] describes serious vulnerabilities in how wireless networks are identified and automatically joined by Windows XP and MacOS X workstations. These vulnerabilities may cause nearby wireless clients to inadvertently and automatically join a rogue wireless network. KARMA is a proof-of-concept toolkit that demonstrates the risk of these vulnerabilities through a patch to the Linux MadWifi driver and client-side exploit toolkit.

Our driver responds to EVERY Probe Request as it operates in HostAP mode. The wireless network is "cloaked", so it does not send out any beacons, but when a client in range sends a Probe Request for a network ("tmobile", "linksys", "megacorp", etc), the driver will respond as if it were that network. In this way, it acts as a virtual AP for any network requested. This yields an extremely effective attack that is able to cause nearly all unassociated wireless clients within range to join the rogue network. KARMA also includes a tool for passively monitoring probe requests sent out by nearby wireless clients and a framework for exploiting client-side vulnerabilities once the client has joined the rogue network (no live exploits are included, though).

For example, we demonstrated this attack during our presentation at Microsoft's first BlueHat internal security conference. In a hall of 400-500 engineers, we hijacked upwards of 100 clients instantly, enough that our Linux laptop became unstable from all the wireless traffic passing through it. In practice, since nearly every roaming laptop has at least one unencrypted hotspot network in their preferred/trusted networks, almost all Windows XP and MacOS X laptops are susceptible to this kind of attack.

In addition, our driver uncovered vulnerabilities in drivers for 802.11b-only cards where they probe for randomly generated network names when the card is not associated to a network. When the KARMA driver responds to this probe, the card and host will join the network and DHCP an address, etc. I reported this to both Microsoft and Apple in the Spring last year. Apple has subsequently fixed the issue [3] and Microsoft said that a fix would be in the next service pack.

Again, this is not entirely new stuff. Max Moser released his HotSpotter [4] tool in April 2004 to create a HostAP based on sniffed Probe Requests. We first released our driver implementing the parallel attack in February 2005 at Immunity's Security Shindig in NYC. However, awareness of these issues appears to still be low.


Dino A. Dai Zovi
Matasano Security
ddz () matasano com


[1] - http://www.theta44.org/karma/
[2] - http://www.theta44.org/karma/aawns.pdf
[3] - http://docs.info.apple.com/article.html?artnum=301988
[4] - http://www.remote-exploit.org/index.php/Hotspotter_main

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