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Improvements in Rogue AP attacks – MANA 1/2

Reading time ~9 min

At Defcon 22 we presented several improvements in wifi rogue access point attacks. We entitled the talk “Manna from heaven” and released the MANA toolkit. I’ll be doing two blog entries. The first will describe the improvements made at a wifi layer, and the second will cover the network credential interception stuff. If you just want the goodies, you can get them at the end of this entry for the price of scrolling down.

Introduction

This work is about rogue access points, by which we mean a wireless access point that mimics real ones in an attempt to get users to connect to it. The initial work on this was done in 2004 by Dino dai Zovi and Shaun Macaulay. They realised that the way wifi devices probe for wireless networks that they’ve “remembered” happens without authentication, and that if a malicious access point merely responds to these directed probes, it can trick wireless clients into connecting to it. They called this a KARMA attack.

Additionally, Josh Wright and Brad Antoniewicz in 2008 worked out that if you man in the middle the EAP authentication on secured networks, you could crack that hash and gain access to the network yourself. They implemented this in freeradius-wpe (wireless pwnage edition).

However, KARMA attacks no longer work well, and we wanted to know why. Also, the WPE stuff seemed ripe for use in rogue access points rather than just for gaining access to the original network. This is what we implemented.

Changes in Probing

After a significant amount of time poring over radio captures of the ways in which various devices probed, and informed by our previous work on Snoopy, we realised two things. The first is that modern devices, particularly mobile ones, won’t listen to directed probe responses for open, non-hidden networks if that AP didn’t also/first respond to a broadcast probe. What this means is that our rogue access point needs to implement the same. However, the challenge is, what do we respond to the broadcast probe *with*?

To overcome that, we took the existing KARMA functionality built by Digininja, ported it to the latest version of hostapd and extended it to store a view of the “remembered networks” (aka the Preferred Network List (PNL)) for each device it sees. Then when hostapd-mana sees a broadcast probe from that device, it will respond with a directed probe response for each network hostapd-mana knows to be in that device’s PNL. This is based on our finding, that wifi clients don’t have a problem with a single BSSID (i.e. AP MAC address) to have several ESSIDs (aka SSID aka network name).

Practically, suppose there are two devices, one probing for a network foo, and the other probing for two networks bar and baz. When device1 sends a broadcast probe, hostapd-mana will respond with a directed probe response for foo to device1. Likewise when device2 sends a broadcast probe, hostapd-mana will respond with two directed probe responses to device 2, one for bar and one for baz. In addition, the “normal” KARMA functionality of responding to directed probes will also occur. Practically, we found this significantly improved the effectiveness of our rogue AP.

iOS and hidden networks

iOS presented an interesting challenge to us when it came to hidden networks. A hidden network is one configured not to broadcast its ESSID in either its beacons or broadcast probe response. Practically, the only way a client device can know if the hidden network it has remembered is nearby is by constantly sending out directed probes for the network. This is why hidden networks aren’t a very good design, as their clients need to spew their names out all over the place. However, when observing iOS devices, while they could join a hidden network just fine, they seemed to not probe for it most of the time. This had us constructing faraday cages, checking other factors like BSSID and geolocation to no avail. Until we realised that iOS will not probe for any hidden networks in its PNL, unless there is at least one hidden network nearby. So, if you’d like to maximise your rogue’ing, make sure you have a hidden network nearby. It doesn’t even need to be a real network; use a mifi, use airbase-ng or just create another hostapd network.

Limits in probing

Modern mobile devices probe for networks on their PNL *significantly* less than laptops or older devices do. In an ideal world, manufacturers would change the implementation to never probe for open network, and only wait for a response to a broadcast, effectively limiting these attacks to requiring pre-knowledge, common networks or being performed in the vicinity of the actual network. Actually, a patch was pushed to wpa_supplicant to limit the stupid probing behaviour Android does in low power mode a few months ago, this will make it into Android proper sometime soon. Also, iOS has significantly reduced how often it probes.

There are two ways to work around this. The first is manual; go for a common network. The rise of city-wide wifi projects makes this somewhat easy. Or if you’re going for a corporate network, just do some recon and name one of your access points after that. But, we wanted to make things work better than that. The default behaviour of hostapd-mana is to build up a view of each devices PNL and only respond to broadcasts with networks specific to that device. However, we can remove that limitation and build a global PNL, and respond to each broadcast with every network every device has probed for. We call this loud mode, and it’s configurable in the hostapd-mana config. This relies on the fact that many devices, particularly laptops and older mobile devices still probe for networks a lot. It also relies on the fact that many devices have networks in common (have they been in the same city, same airport, same conference, same company, same pocket etc.). This works *very* well in less crowded areas, and you’ll get a much higher number of devices connecting.

However, in busy areas, or if your antenna is large enough, you’ll quickly exceed the capacity for your average wifi device to respond fast enough to all of the devices, and as the number of response probes grows exponentially with each new device, even in quiet areas over time, this problem crops up (but didn’t on stage at Defcon miraculously). So, it’s *good enough* for now, but needs an in-kernel or in-firmware implementation with some network ageing to scale a bit better (one of the many opportunities for extending this work if you’re up for some open source contribution).

Auto Crack ‘n Add

freeradius-wpe is great, it provides a nice way to grab EAP hashes for clients that don’t validate certificates presented via EAP’s that implement SSL (PEAP, EAP-GTC, PEAL-TTLS). However, the patches are for freeradius v1 and, much like the KARMA patches for hostapd, have aged. But, hostapd contains a radius server, and so we could port the freeradius-wpe work to that, something we based off some initial but incomplete patches by Brad Antoniewicz. So hostapd-mana will also let you grab EAP hashes without needing another tool.

However, the KARMA attacks only work against open wifi networks. EAP networks are increasingly common (especially corporate ones) and we wanted to be able to have a go at getting devices probing for those to connect to our rogue AP. To do this, we modified hostapd-mana to always accept any EAP hash, but send it off for cracking. It simply writes these to a file, from which the simple python tool crackapd (included) will grab it and send it off to another process for cracking. Currently, we use asleap (also by Josh) and the rockyou password list, but these can all be easily modified. For example, to use CloudCracker and its incredibly optimised MS-CHAPv2 cracking setup.

The net result is pretty great for simple EAP hashes. The device will try and connect, and fail as we don’t know enough to do the challenge response right. But after the hash is cracked, when it retries to connect (something a device will keep doing) it will succeed (and you’ll have your first creds). For simple hashes, this is transparent to the user. Of course, very complex hashes will only work if you crack them in time. Worst case scenario, you leave with hashes.

Conclusion

So that’s what we built into hostapd-mana. You get improved KARMA attacks, a modern hostapd version, an integrated hash stealer, and the possibility of rogue’ing some EAP networks. You can get the full toolkit at MANA toolkit on GitHub or our hostapd-mana at hostapd-mana on GitHub.

The next blog entry will cover what we did once we got a device to connect.

The Goodies

The Defcon talk:

The supporting slide deck with more information:

The final toolkit: MANA toolkit on GitHub You can also get this on Kali with “apt-get install mana-toolkit”

The modified hostapd (for hackers or people who want to build their own setup): hostapd-mana on GitHub