Tuesday, April 10, 2007

Defeating Evil Twin

The other day i was discussing with thrill about detecting and defeating Evil Twin and what are the best options beside using WEP or WPA. For folks who still don't know, both WEP and WPA can be broken and is not considered secure. Check my previous post and you will find the tools needed to break those keys. Apparently, it boils down to two options, but i will let you guys decide which one is more secure.

Thrill was suggesting about placing an Access Point in the DMZ area and make the IP a private one. A VPN server would also be needed to be setup in the DMZ area for listening to clients who wants to connect to the AP and use the wireless service. In that manner, an attacker who tries to setup an Evil Twin will not have access to the DMZ area because he would first need to use a VPN client to connect to the VPN server before any surfing can continue. This has several advantages and disadvantages and it all depends how you looked at it. The advantages being that all the traffic will be encrypted because it will be tunneled through a VPN server first before going out to the internet. Second being that SSID can be broadcast and no WEP/WPA security is needed. Of course if you are being paranoid, you can also include WPA key as an option. And third, it will defeat most Evil Twin. Below is a diagram which depicts the whole scenario.

According to thrill, its not necessary to implement a DMZ zone. It could be another extra network card on the VPN server that is going to have itself and the AP connected to it. Thrill quoted: "The trick is to not allow routing through this interface, and to set up a VPN server on that machine listening ONLY on that interface. And maybe a DHCP server on that interface as well. This is how this network becomes secure, and someone setting up an Evil Twin wouldn't be able to duplicate. And even if they did, the VPN client can be set up to authenticate a server side certificate easy enough." I wont say this is 100% secure, but it is the best solution he can think of and i do agree that it is good solution. The downside of this setup is every user needs to install OpenVPN client software on their machine and needs to be notified of the setup. That's a hassle.

On the other side, thrill also quote: "using 802.1x authentication along with using a Radius server for logging in the user. Some of you may have already heard of the technology, it's using the Odyssey client by Funk Software, along with their Steel Belted Radius. Using Cisco APs we were able to enable rotating WEP keys that were only given to the client if their Certificate could be authenticated, once they were connected to the wireless network, they then needed to authenticate their user/password via the radius which pointed to the LDAP portion of AD. The trick for rotating SSID/WEP keys is using a certificate to authenticate to the actual AP. The AP is set up to point to a radius server which has the certificate on it, then the client sends the AP the supplication requesting the SSID/Key, the AP forwards the request to the Radius server which authenticates the certificate and sends an OK to the AP, who in turn sends the client the SSID/Key to authenticate." Below depicts the scenarion:

Whichever is better, if something becomes too hard to use or requires too many steps, most people will be lazy and don't care about it. But then again, it all depends on the organization on how they want to implement their systems. Just my opinion.


Security4all said...

How is WPA-PSK flawed? From the FAQ of Aircrack-ng:

Actually, TKIP (WPA1) is not vulnerable: for each packet, the 48-bit IV is mixed with the 128-bit pairwise temporal key to create a 104-bit RC4 key, so there’s no statistical correlation at all. Furthermore, WPA provides counter-measures against active attacks (traffic reinjection), includes a stronger message integrity code (michael), and has a very robust authentication protocol (the 4-way handshake). The only vulnerability so far is a dictionnary attack, which fails if the passphrase is robust enough.

If your passphrase is secure enough, it should be considered safe. Only if your SSID isn't the default one because there are Rainbowtables computed with several default SSID's (which is used as salt as stated above).

Anyway, nice article! :)
PEAP+WPA is a better way to protect your network if you can implement 802.1x and a radius.
Generating a certificate with your own Root CA makes it more difficult for an attacker to gain access since he doesn't have the Root CA's public key. Drawback is that you need to install this private Root CA manually on each device that needs to connect to your network.
EAP-TLS is still the most secure way to go but on a wide scale alot of work since you need an full blown PKI infrastructure.

Some references:

* Cisco SAFE: Wireless LAN Security in Depth - version 2
* Microsoft Wireless Deployment Technology and Component Overview

Anonymous said...

security4all, first of all, i would like to thank you for taking the time to read and comment on my blog. I am not really a researcher into Wireless, but i know enough of how it operates and how it is vulnerable. If you continue to read on the faq, you will see this:

[ How can I crack a WPA-PSK network ?

You must sniff until a handshake takes place between a wireless client and the access point. To force the client to reauthenticate, you can start a deauth attack with aireplay-ng. Also, a good dictionary is required. FYI, it’s not possible to pre-compute large tables of Pairwise Master Keys like rainbowcrack does, since the passphrase is salted with the ESSID. ]

You must actually force the user to reauthenticate and deauth the user again to sniff the first handshake.

Actually, i do like the idea of PEAP-WPA with 802.1x and a radius. As for the private CA, is it possible that once the user has authenticated, direct him to your internal server where the root ca resides and make the private root ca installs automatically into his machine?

As for EAP-TLS, i am not so sure about it, care to elaborate more?


Anonymous said...

Well, i researched further and i now how EAP-TLS works now. It is indeed the most secure. Below is an excerpt:

"EAP-Transport Layer Security or EAP-TLS, defined in RFC 2716, is an IETF open standard, and is well-supported among wireless vendors. It offers a good deal of security, since TLS is considered the successor of the SSL standard. It uses PKI to secure communication to the RADIUS authentication server, and this fact may make it seem like a daunting task to set up. So even though EAP-TLS provides excellent security, the overhead of client-side certificates may be its Achilles' heel.

EAP-TLS is the original standard wireless LAN EAP authentication protocol. Although it is rarely deployed, it is still considered one of the most secure EAP standards available and is universally supported by all manufacturers of wireless LAN hardware and software including Microsoft. The requirement for a client-side certificate, however unpopular it may be, is what gives EAP-TLS its authentication strength and illustrates the classic convenience vs. security trade-off. A compromised password is not enough to break into EAP-TLS enabled systems because the hacker still needs to have the client-side certificate. When the client-side certificates are housed in smartcards, this offers the most security available because there is no way to steal a certificate's private key from a smartcard without stealing the smartcard itself. It is significantly more likely that physical theft of a smartcard would be immediately noticed and the smartcard revoked and a new card issued than that password theft would be noticed and the password changed or account disabled"


thrill said...


Do a search on google for coWPAtty.

Now, it's been quite a long time, but from what I remember, EPA-TLS is the prefered form of authentication between the client and the AP while it is requesting authentication of the certificate.

Your suggestion on the Root CA was actually a very good one, and one I forgot to mention in my original thread with hackathology. I did set up my own CA at my place of work, and not only for the security portion of it, but also because we were rolling out dozens of Radius servers around the world which would have required a $500 certificate from another CA. But as you mentioned, this is a very good method for securing your systems and ensuring that no one else can acquire your identity, ala microsoft's stolen certificate.


Anonymous said...

hey thrill, glad that you are here. Yep, i will do some research on coWPAtty. Thanks for the suggestions. Is your forum launched?


thrill said...

Not really.. I gotta have you guys take a closer look at the software I'm running.. :)

I'll probably be asking for some help tomorrow on sla.ckers


Anonymous said...

sure thrill, once it launched, do let me know. I will start creating account there.


Security4all said...

So PEAP with a private CA or EAP-TLS are still the two secure enterprise setups. Glad that it's confirmed! :-)

I had a look at coWPAtty... it's an offline dictionary attack. if you choose your SSID randomly enough so that there is no precomputed (rainbow)table for it, and a strong password. It should not be possible to crack the WPA-PSK.

From Wireless Defence:
genpmk is used to precompute the hash files in a similar way to Rainbow tables is used to pre-hash passwords in Windows LANMan attacks. There is a slight difference however in WPA in that the SSID of the network is used as well as the WPA-PSK to "salt" the hash. This means that we need a different set of hashes for each and every unique SSID i.e. a set for "linksys" a set for "tsunami" etc.. The Church of Wifi have produced some lookup tables for 1000 SSID's computed against a 170,000 word password file. The resultant table are approximately 7 Gigabytes in size.

At least not for now.... but faster bruteforcing (or generating tables) is the greatest danger. Check it out: Faster PwninG Assured or the video

Anonymous said...

well, in my opinion, setting a random and hardtoguess with alphanumeric SSID should do ther trick.


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