Mitigating the KRACK in WPA2 with WIPS

On Monday, security researcher Mathy Vanhoef disclosed a new vulnerability in the WPA/WPA2 four-way handshake, which has been branded KRACK. The attack is targeted and sophisticated, and it results in decrypting a TKIP or CCMP/AES encrypted session without knowledge of the PTK. WPA/WPA2-Personal and WPA/WPA2-Enterprise networks are vulnerable.

The attack takes advantage of client side implementations of the WPA/WPA2 protocol, which in some cases allows clients to reinstall the PTK and reuse cryptographic information in a way that allows the the attacker to decrypt the session. The PSK or 802.1X credentials are not compromised by this attack. I know that description is vague so if you want more, my favorite resource on this is this serious of videos from Hemant Chaska of Mojo Networks. Do yourself a favor and watch them all.

The ultimate solution to the vulnerability is to patch clients to prevent them from reusing the same cryptographic information when EAPOL keys are retransmitted. That will take some time, and there are a lot of clients, like IoT clients, which are unlikely to ever be patched. Windows and iOS clients with the latest security patches are already protected.

Fortunately, the attack relies on the attacker deploying an easy to detect and mitigate rogue AP. Today, without patching clients or the WLAN infrastructure, KRACK can be totally mitigated on a WLAN by configuring WIPS to auto-contain rogue AP’s that broadcast one of your own SSID’s. You need to tread lightly and understand the legal consequences before enabling auto-containment of rogue AP’s (Configure it for alerting-only first!). It’s best to get management and your InfoSec teams involved before taking this step so that the benefits and risks of auto-containment are understood by the organization.

Another solution is to disable retransmission of EAPOL frame M3 on the WLAN, but sometimes M3 needs to be retransmitted. If there was a collision or the frame arrived to quickly for the client to process, it should be retransmitted to complete the four-way handshake and prevent the client from going through a full reassociation. This is especially true for latency-sensitive voice clients which roam frequently, resulting in many four-way handshakes. These clients may be short on CPU cycles and free memory to quickly process EAPOL frames, and may require an occassional EAPOL frame retransmission.

Therefore, I prefer to mitigate KRACK by using WIPS to contain rogues that use the organization’s own SSID’s. As you can see from the test below, a Cisco monitor-mode AP will deauth a new client on a rogue AP before any data frames are transmitted.

In my testing, every client that associated to the rogue AP was deauthed before any data frames could be transmitted by the client.

Even simple probing on the channel resulted in a flood of deauth/disassociate frames from the monitor-mode AP to the client:


This attack works by setting up the rogue AP on a different channel from the target AP, so make sure you are scanning all channels for rogues. It’s also a good idea to setup notifications from your NMS in the event that a rogue is contained so that you are aware of potential attacks as well as false positives that require correction.

The one exception to WIPS protection appears to be CVE-2017-13082, which will require an infrastructure-side patch. This only affects SSID’s that use 802.11r.

So patch your clients, tune your WIPS, and relax! The sky is not falling.

macOS Wi-Fi Roaming

One of the nice things about Intel wireless chipsets is that the drivers expose a lot of controls to help tune the chipset’s operation. One of my favorite of these controls is “Prefered Band,” which I usually adjust to instruct the chipset to prefer the 5 GHz band over the 2.4 GHz band. There are some other useful controls like “Roaming Aggressiveness” and you can also enable Fat Channel Intolerance if a neighbor is rudely using 40 MHz of spectrum in 2.4 GHz.


Although macOS has many advantages over Windows when it comes to Wi-Fi, such as the ability to natively do packet captures with the internal chipset, macOS doesn’t have the same level of customization as a Windows machine with an Intel chipset. And my experience has been that Mac clients don’t roam particularly well. Too often they are “sticky clients” and you need to disable/enable Wi-Fi on them to get them to associate with a better BSS.

Here’s a screenshot for a MacBook Air which wouldn’t roam away from a BSS whose RSSI has fallen to -80 dBm, while the laptop was only able to transmit at MCS 0, 7 Mbps. However there was another BSS in the -60’s which would have allowed for much better Wi-Fi performance.

Why is the native macOS Wi-Fi menu showing a full signal with -80 dBm RSSI and MCS 0? Wi-Fi Signal tells the real story.

In 2016 Apple published a webpage that explains how macOS makes roaming decisions and what roaming features it supports. This is very helpful and I wish other manufacturers would do the same. The algorithms that control client roaming are usually a black box, so Wi-Fi engineers have make a lot of assumptions about them when designing WLAN’s for clients that require efficient roaming. That said, while Apple says Macs should usually roam at -75 dBm, that doesn’t match my experience. Sometimes Macs are just sticky.

One reason for this is that once the roaming threshold is crossed, a Mac will only roam to a BSS that is 12 dBm louder than the current BSS, which would require a roaming candidate BSS to have an RSSI of -63 dBm or better before roaming will occur at -75 dBm. There doesn’t appear to be any way to modify this value.

Enabling 802.11k or 802.11v won’t help because macOS does not yet support those features, although they don’t prevent Macs from using an SSID that has them enabled. 802.11k and .11v are supported in Windows 10, however, if the wireless adapter supports those features.

There is an old plist that once controlled “opportunistic” roaming behavior, which I suspect meant roaming above -75 dBm RSSI.


…which has these defaults in macOS 10.12 Sierra:

    deltaRSSI = 10;
    disabled = 0;
    useBonjour = 0;
    useBroadcastBSSID = 1;

That looks promising, however, this plist hasn’t been used by macOS since macOS 10.10 Yosemite. It’s ignored by the OS now, and when it was utilized, it wasn’t intended to be user-editable, so changes were likely to be overwritten by the OS.

So if you are an enterprise with a fleet of Macs to manage and you run into sticky client issues, consider infrastructure features like Cisco’s Optimized Roaming or Aruba ClientMatch to force better roaming behavior among these clients.

To observe roaming behavior on a Mac, I recommend WiFi Signal from Adrian Granados. It can be setup to generate macOS notifications when roaming events occur or the RSSI of the AP drops below a certain threshold.


Splunking Wi-Fi DFS Events


One aspect of wireless networking that I’ve always struggled with is visibility into DFS events. Usually I catch them by chance by noticing two nearby AP’s on a site map using the same non-DFS channel, or maybe by casually looking through logs, but I’ve never felt like I had the reporting and alerting that should be in place for DFS events, because they can be very disruptive. An AP will abruptly change the channel it is operating on, and if it switches back, it may observe a “quiet period” of 60 seconds in which is does not transmit any data. Not good.

Enter Splunk.

Splunk is a powerful log analysis tool that you can think of as “Google for the data center.” It takes log data from almost any source and makes it as searchable as Google has made the web. For wireless network engineers, you can quickly and easily search syslog and SNMP data, build reports, and create alerts. Splunk Light is free and will process up to 5 GB of data a day, which should be plenty for most WLAN’s. It also runs easily on macOS if you just want to demo it locally.

Using Splunk I very quickly created this dashboard of real DFS data from SNMP traps coming from a Cisco WLC. It’s a little rough around the edges still (I need to figure out how to clean-up those AP names and channels), but it still shows me a lot of the valuable data.

Yes, DFS is a problem at this site.

I can easily create email alerts too, so that if a DFS event occurs an email is triggered, or if say 10 DFS events occur within 30 minutes an email is triggered.

How To

I installed Splunk on a Mac then setup the built-in snmptrapd to listen for incoming traps and log them to a file. For snmptrapd to interpret the SNMP traps from a Cisco WLC, download the Cisco MIB’s and copy them to /usr/share/snmp/mibs/. Then you can start snmptrapd.

Here’s the CLI one-liner to do that:

sudo snmptrapd -Lf /var/log/snmp-traps --disableAuthorization=yes -m +ALL

Next configure the WLC to send SNMP traps to the Splunk box by adding its IP address under Management -> SNMP -> Trap Receivers. While you’re there go to Trap Controls and turn everything on you want to analyze.


Even though DFS events only generate SNMP traps, it’s still a good idea to send syslog messages to Splunk too, so do that under Management -> Logs -> Config. Set the Syslog Level to “Informational” to get a lot of good data. “Debugging” is probably way too much. The Syslog Facility isn’t important.


Monitor the file snmptrapd is writing traps to to make sure it is working. Run this command on the Mac and you should see traps streaming in. If not you have some troubleshooting to do.

tail -f /var/log/snmp-traps

Now add the file to Splunk under Data -> Data inputs -> Files & directories, and you should be able to see the traps in searches.

Have a look at Splunk’s documentation on SNMP data for more setup help. Setting up syslog is easier. Under Data -> Data inputs -> UDP add UDP port 514 with the Source type “syslog.”

Once the data is coming into Splunk you can start searching it and creating fields. Search “RADIO_RC_DFS” (with quotes) to see all the DFS traps. From that search click “Extract new fields” and select the tab delimiter to parse the data. Give the AP name field a label, and then you can create visualizations of DFS events by AP name. Any search can also be used to trigger an alert, such as an email.

Cisco has published a WLC SNMP Trap Guide as well as a WLC syslog Guide that is helpful when working with this data. Find the messages you are looking for in those guides, then search for them in Splunk.

From there it’s all up to your own creativity. DFS events is just scratching the surface of Splunk’s potential. You can look at authentication events, monitor RRM, and there might be some interesting roaming analysis that can be done with this data as well. I’m sure there are some bright engineers out there that have taken this a lot farther. Please share your work!