Your Smart Coffee Maker is Brewing Up Trouble

Tips & Advice

IOT devices are notoriously insecure and this claim can be backed up with a laundry list of examples. With more devices “needing” to connect to the internet, the possibility of your WiFi enabled toaster getting hacked and tweeting out your credit card number is, amazingly, no longer a joke.

With that in mind, I began to investigate the Mr. Coffee Coffee Maker with Wemo since we had previously bought one for our research lab (and we don’t have many coffee drinkers, so I didn’t feel bad about demolishing it!) My hope was to build on previous work done by my colleague Douglas McKee (@fulmetalpackets) and his Wemo Insight smart plug exploit. Finding the similar attack vector absent in this product, I explored a different avenue and was able to find another exploit. In this post I will explore my methodology and processes in detail.

All Wemo devices have two ways of communicating with the Wemo App, remotely via the internet or locally directly to the Wemo App. Remote connectivity is only present when the remote access setting is enabled, which it is by default. To allow the Wemo device to be controlled remotely, the Wemo checks Belkin’s servers periodically for updates. This way the Wemo doesn’t need to open any ports on your network. However, if you are trying to control your Wemo devices locally, or the remote access setting is disabled, the Wemo app connects directly to the Wemo. All my research is based on local device communication with the remote access setting turned off.

To gain insight on how the coffee maker communicates with its mobile application, I first set up a local network capture on my cellphone using an application called “SSL Capture.” SSL Capture allows the user to capture traffic from mobile applications. In this case, I selected the Wemo application. With the capture running, I went through the Wemo app and initiated several standard commands to generate network traffic. By doing this, I was able to view the communication between the coffee maker and the Wemo application. One of the unique characteristics about the app is that the user is able schedule the coffee maker to brew at a specified time. I made a few schedules and saved them.

I began analyzing the network traffic between the phone application and the Mr. Coffee machine. All transmissions between the two devices were issued in plaintext, meaning no encryption was used. I also noticed that the coffee maker and the mobile app were communicating over a protocol called UPNP (Universal Plug and Play), which has preset actions called “SOAP ACTIONS.” Digging deeper into the network capture from the device, I saw the SOAP action “SetRules.” This included XML content that pertained to the “brew schedule” I had set from the mobile application.

A Mr. Coffee “brew” being scheduled.

At this point I was able to see how the Wemo mobile application handled brewing schedules. Next, I wanted to see if the coffee maker performed any sort of validation of these schedules so I went back into the mobile application and disabled them all. I then copied the data and headers from the network capture and used the Linux Curl command to send the packet back to the coffee maker. I got the return header status of “200” which means “OK” in HTTP. This indicated there was no validation of the source of brewing schedules; I further verified with the mobile application and the newly scheduled brew appeared.

Curl command to send a “Brew” schedule to the Wemo Coffee maker.

Screenshot of the Curl command populating the Wemo app with a brew schedule

At this point I could change the coffee maker’s brew schedule without ever using the Wemo mobile application. To understand how the schedules were stored on the Wemo coffee maker, I decided to physically disassemble it and look at the electronics inside. Once disassembled, I saw there was a Wemo module connected to a larger PCB responsible for controlling the functions of the coffee maker. I then extracted the Wemo module from the coffee maker. This looked almost Identical to the Wemo module that was in the Wemo Insight device. I leveraged Doug’s blog on exploitation of the Wemo Insight to replicate the serial identification, firmware extraction, and root password change. After I obtained root access via the serial port on the Wemo device, I began to investigate the way in which the Wemo application is initiated from the underlying Linux Operating System. While looking through some of the most common Linux files and directories, I noticed something odd in the “crontab” file (used in Linux to execute and schedule commands).

It appeared the developers decided to take the easy route and used the Linux crontab file to schedule tasks instead of writing their own brew scheduling function. The crontab entry was the same as the scheduled brew I sent via the Wemo application (coffee-3) and executed as root. This was especially interesting; if I could add some sort of command to execute from the replayed UPNP packet, I could potentially execute my command as root over the network.

With the firmware dumped, I decided to look at the “rtng_run_rule” executable that was called in the crontab. The rtng_run_rule is a Lua script. As Lua is a scripting language, it was written in plaintext and not compiled like all the other Wemo executables. I followed the flow of execution until I noticed the rule passing parameters to a template for execution. At this point, I knew it would be useless trying to inject commands directly into the rule and instead looked at modifying the template performing the execution.

I went back to the Wemo mobile application network captures and started to dig around again. I found the application also sends the templates to the Wemo coffee maker. If I could figure out how to modify the template and still have the Wemo think it is valid, I could get arbitrary code execution.

Template with the correct syntax to pass Wemo’s verification

There were 3 templates sent over, “do,” “do_if,” and “do_unless.” Each of the templates were Lua scripts and encoded with base64. Based on this, I knew it would be trivial to insert my own code; the only remaining challenge would be the MD5 hash included at the top of the template. As it turned out, that was hardly an obstacle.

I created an MD5 hash of the base-64 decoded Lua script and the base64 encoded script separately, simply to see if one or the other matched the hash that was being sent; however, neither matched the MD5 being sent in the template. I began to think the developers used some sort of HMAC or clever way to hash the template, which would have made it much harder to upload a malicious template. Instead, I was astounded to find out that it was simply the base64 code prepended by the string “begin-base64 644 <template name>” and appended with the string “====.”

At last I had the ability to upload any template of my choice and have it pass all the Wemo’s verification steps necessary to be used by a scheduled rule.

I appended a new template called “hack” and added a block of code within the template to download and execute a shell script.

Within that shell command, I instructed the Mr. Coffee Coffee Maker with Wemo to download a cross-complied version of Netcat so I can get a reverse shell, and also added an entry to “rc.local.” This was done so that if the coffee maker was power cycled, I would have persistent access to the device after reboot, via the Netcat reverse shell.

The final aspect of this exploit was to use what I learned earlier to schedule a brew with my new “hack” template executing my shell script. I took the schedule I was able to replay earlier and modified it to have the “hack” template execute 5 minutes from the time of sending. I did have to convert the time value required into the epoch time format.

Converting time to Epoch time.

Now, I sat back and waited as the coffee maker (at my specified time delay) connected to my computer, downloaded my shell script, and ran it. I verified that I had a reverse shell and that it ran as intended, perfectly.

This vulnerability does require network access to the same network the coffee maker is on. Depending on the complexity of the user’s password, WiFi cracking can be a relatively simple task to accomplish with today’s computing power. For example, we demonstrate a quick and easy brute force dictionary attack to crack a semi-complex WPA2 password (10 characters alpha-numeric) in the demo for the Wemo Insight smart plug.  However, even a slightly more complex password, employing special characters, would exponentially increase the difficulty of a brute force attack. We contacted Belkin (who owns Wemo) on November 16th, 2018 and disclosed this issue to them. While the vendor did not respond to this report, we were pleasantly surprised to see that the latest firmware update has patched the issue. Despite a general lack of communication, we’re delighted to see the results of our research further securing home automation devices.

This vulnerability shows that not all exploits are overly complicated or require an exceptional amount of effort to pull off, if you know what to look for. This vulnerability exists solely because a few poor coding decisions were made in conjunction with a lack of input sanitation and validation. Even though this target does not contain sensitive data and is limited to your local network, it doesn’t mean malicious hackers are not targeting IOT devices like this. These devices may serve as a sought-after target as they are often overlooked from a security standpoint and can provide a simple and unmonitored foothold into your home or business network. It is very important for any consumer, when purchasing new IOT gadgets, to ask themself: “Does this really need to be connected to the internet?” In the case of a coffee maker, I’ll let you be the judge.

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