In Fall 2016 I was invited to come to Miami as part of a team that independently validated some alleged flaws in implantable cardiac devices manufactured by St. Jude Medical (now part of Abbott Labs). These flaws were discovered by a company called MedSec. The story got a lot of traction in the press at the time, primarily due to the fact that a hedge fund called Muddy Waters took a large short position on SJM stock as a result of these findings. SJM subsequently sued both parties for defamation. The FDA later issued a recall for many of the devices.

Due in part to the legal dispute (still ongoing!), I never had the opportunity to write about what happened down in Miami, and I thought that was a shame: because it’s really interesting. So I’m belatedly putting up this post, which talks a bit MedSec’s findings, and implantable device security in general.

By the way: “we” in this case refers to a team of subject matter experts hired by Bishop Fox, and retained by legal counsel for Muddy Waters investments. I won’t name the other team members here because some might not want to be troubled by this now, but they did most of the work — and their names can be found in this public expert report (as can all the technical findings in this post.)

Quick disclaimers: this post is my own, and any mistakes or inaccuracies in it are mine and mine alone. I’m not a doctor so holy cow this isn’t medical advice. Many of the flaws in this post have since been patched by SJM/Abbot. I was paid for my time and travel by Bishop Fox for a few days in 2016, but I haven’t worked for them since. I didn’t ask anyone for permission to post this, because it’s all public information.

A quick primer on implantable cardiac devices 

Implantable cardiac devices are tiny computers that can be surgically installed inside a patient’s body. Each device contains a battery and a set of electrical leads that can be surgically attached to the patient’s heart muscle.

When people think about these devices, they’re probably most familiar with the cardiac pacemaker. Pacemakers issue small electrical shocks to ensure that the heart beats at an appropriate rate. However, the pacemaker is actually one of the least powerful implantable devices. A much more powerful type of device is the Implantable Cardioverter-Defibrillator (ICD). These devices are implanted in patients who have a serious risk of spontaneously entering a dangerous state in which their heart ceases to pump blood effectively. The ICD continuously monitors the patient’s heart rhythm to identify when the patient’s heart has entered this condition, and applies a series of increasingly powerful shocks to the heart muscle to restore effective heart function. Unlike pacemakers, ICDs can issue shocks of several hundred volts or more, and can both stop and restart a patient’s normal heart rhythm.

Like most computers, implantable devices can communicate with other computers. To avoid the need for external data ports – which would mean a break in the patient’s skin – these devices communicate via either a long-range radio frequency (“RF”) or a near-field inductive coupling (“EM”) communication channel, or both. Healthcare providers use a specialized hospital device called a Programmer to update therapeutic settings on the device (e.g., program the device, turn therapy off). Using the Programmer, providers can manually issue commands that cause an ICD to shock the patient’s heart. One command, called a “T-Wave shock” (or “Shock-on-T”) can be used by healthcare providers to deliberately induce ventrical fibrillation. This capability is used after a device is implanted, in order to test the device and verify it’s functioning properly.

Because the Programmer is a powerful tool – one that could cause harm if misused – it’s generally deployed in a physician office or hospital setting. Moreover, device manufacturers may employ special precautions to prevent spurious commands from being accepted by an implantable device. For example:

1. Some devices require that all Programmer commands be received over a short-range communication channel, such as the inductive (EM) channel. This limits the communication range to several centimeters.
2. Other devices require that a short-range inductive (EM) wand must be used to initiate a session between the Programmer and a particular implantable device. The device will only accept long-range RF commands sent by the Programmer after this interaction, and then only for a limited period of time.

From a computer security perspective, both of these approaches have a common feature: using either approach requires some form of close-proximity physical interaction with the patient before the implantable device will accept (potentially harmful) commands via the long-range RF channel. Even if a malicious party steals a Programmer from a hospital, she may still need to physically approach the patient – at a distance limited to perhaps centimeters – before she can use the Programmer to issue commands that might harm the patient.

In addition to the Programmer, most implantable manufacturers also produce some form of “telemedicine” device. These devices aren’t intended to deliver commands like cardiac shocks. Instead, they exist to provide remote patient monitoring from the patient’s home. Telematics devices use RF or inductive (EM) communications to interrogate the implantable device in order to obtain episode history, usually at night when the patient is asleep. The resulting data is uploaded to a server (via telephone or cellular modem) where it can be accessed by healthcare providers.

What can go wrong?

Before we get into specific vulnerabilities in implantable devices, it’s worth asking a very basic question. From a security perspective, what should we even be worried about?

There are a number of answers to this question. For example, an attacker might abuse implantable device systems or infrastructure to recover confidential patient data (known as PHI). Obviously this would be bad, and manufacturers should design against it. But the loss of patient information is, quite frankly, kind of the least of your worries.

A much scarier possibility is that an attacker might attempt to harm patients. This could be as simple as turning off therapy, leaving the patient to deal with their underlying condition. On the much scarier end of the spectrum, an ICD attacker could find a way to deliberately issue dangerous shocks that could stop a patient’s heart from functioning properly.

Now let me be clear: this isn’t not what you’d call a high probability attack. Most people aren’t going to be targeted by sophisticated technical assassins. The concerning thing about this  the impact of such an attack is significantly terrifying that we should probably be concerned about it. Indeed, some high-profile individuals have already taken precautions against it.

The real nightmare scenario is a mass attack in which a single resourceful attacker targets thousands of individuals simultaneously — perhaps by compromising a manufacturer’s back-end infrastructure — and threatens to harm them all at the same time. While this might seem unlikely, we’ve already seen attackers systematically target hospitals with ransomware. So this isn’t entirely without precedent.

Securing device interaction physically

The real challenge in securing an implantable device is that too much security could hurt you. As tempting as it might be to lard these devices up with security features like passwords and digital certificates, doctors need to be able to access them. Sometimes in a hurry.

This is a big deal. If you’re in a remote emergency room or hospital, the last thing you want is some complex security protocol making it hard to disable your device or issue a required shock. This means we can forget about complex PKI and revocation lists. Nobody is going to have time to remember a password. Even merely complicated procedures are out — you can’t afford to have them slow down treatment.

At the same time, these devices obviously must perform some sort of authentication: otherwise anyone with the right kind of RF transmitter could program them — via RF, from a distance. This is exactly what you want to prevent.

Many manufacturers have adopted an approach that cut through this knot. The basic idea is to require physical proximity before someone can issue commands to your device. Specifically, before anyone can issue a shock command (even via a long-range RF channel) they must — at least briefly — make close physical contact with the patient.

This proximity be enforced in a variety of ways. If you remember, I mentioned above that most devices have a short-range inductive coupling (“EM”) communications channel. These short-range channels seem ideal for establishing a “pairing” between a Programmer and an implantable device — via a specialized wand. Once the channel is established, of course, it’s possible to switch over to long-range RF communications.

This isn’t a perfect solution, but it has a lot going for it: someone could still harm you, but they would have to at least get a transmitter within a few inches of your chest before doing so. Moreover, you can potentially disable harmful commands from an entire class of device (like telemedecine monitoring devices) simply by leaving off the wand.

St. Jude Medical and MedSec

 

So given this background, what did St. Jude Medical do? All of the details are discussed in a full expert report published by Bishop Fox. In this post we I’ll focus on the most serious of MedSec’s claims, which can be expressed as follows:

Using only the hardware contained within a “Merlin @Home” telematics device, it was possible to disable therapy and issue high-power “shock” commands to an ICD from a distance, and without first physically interacting with the implantable device at close range.

This vulnerability had several implications:

1. The existence of this vulnerability implies that – through a relatively simple process of “rooting” and installing software on a Merlin @Home device – a malicious attacker could create a device capable of issuing harmful shock commands to installed SJM ICD devices at a distance. This is particularly worrying given that Merlin @Home devices are widely deployed in patients’ homes and can be purchased on eBay for prices under $30. While it might conceivably be possible to physically secure and track the location of all PCS Programmer devices, it seems challenging to physically track the much larger fleet of Merlin @Home devices.
2. More critically, it implies that St. Jude Medical implantable devices do not enforce a close physical interaction (e.g., via an EM wand or other mechanism) prior to accepting commands that have the potential to harm or even kill patients. This may be a deliberate design decision on St. Jude Medical’s part. Alternatively, it could be an oversight. In either case, this design flaw increases the risk to patients by allowing for the possibility that remote attackers might be able to cause patient harm solely via the long-range RF channel.
3. If it is possible – using software modifications only – to issue shock commands from the Merlin @Home device, then patients with an ICD may be vulnerable in the hypothetical event that their Merlin @Home device becomes remotely compromised by an attacker. Such a compromise might be accomplished remotely via a network attack on a single patient’s Merlin @Home device. Alternatively, a compromise might be accomplished at large scale through a compromise of St. Jude Medical’s server infrastructure.

We stress that the final scenario is strictly hypothetical. MedSec did not allege a specific vulnerability that allows for the remote compromise of Merlin @Home devices or SJM infrastructure. However, from the perspective of software and network security design, these attacks are one of the potential implications of a design that permits telematics devices to send such commands to an implantable device. It is important to stress that none of these attacks would be possible if St. Jude Medical’s design prohibited the implantable from accepting therapeutic commands from the Merlin @Home device (e.g., by requiring close physical interaction via the EM wand, or by somehow authenticating the provenance of commands and restricting critical commands to be sent by the Programmer only).

Validating MedSec’s claim

To validate MedSec’s claim, we examined their methodology from start to finish. This methodology included extracting and decompiling Java-based software from a single PCS Programmer; accessing a Merlin @Home device to obtain a root shell via the JTAG port; and installing a new package of custom software written by MedSec onto a used Merlin @Home device.

We then observed MedSec issue a series of commands to an ICD device using a Merlin @Home device that had been customized (via software) as described above. We used the Programmer to verify that these commands were successfully received by the implantable device, and physically confirmed that MedSec had induced shocks by attaching a multimeter to the leads on the implantable device.

Finally, we reproduced MedSec’s claims by opening the case of a second Merlin @Home device (after verifying that the tape was intact over the screw holes), obtaining a shell by connecting a laptop computer to the JTAG port, and installing MedSec’s software on the device. We were then able to issue commands to the ICD from a distance of several feet. This process took us less than three hours in total, and required only inexpensive tools and a laptop computer.

What are the technical details of the attack?

Simply reproducing a claim is only part of the validation process. To verify MedSec’s claims we also needed to understand why the attack described above was successful. Specifically, we were interested in identifying the security design issues that make it possible for a Merlin @Home device to successfully issue commands that are not intended to be issued from this type of device. The answer to this question is quite technical, and involves the specific way that SJM implantable devices verify commands before accepting them.

MedSec described to us the operation of SJM’s command protocol as part of their demonstration. They also provided us with Java JAR executable code files taken from the hard drive of the PCS Programmer. These files, which are not obfuscated and can easily be “decompiled” into clear source code, contain the software responsible for implementing the Programmer-to-Device communications protocol.

By examining the SJM Programmer code, we verified that Programmer commands are authenticated through the inclusion of a three-byte (24 bit) “authentication tag” that must be present and correct within each command message received by the implantable device. If this tag is not correct, the device will refuse to accept the command.

From a cryptographic perspective, 24 bits is a surprisingly short value for an important authentication field. However, we note that even this relatively short tag might be sufficient to prevent forgery of command messages – provided the tag ws calculated using a secure cryptographic function (e.g., a Message Authentication Code) using a fresh secret key that cannot be predicted by an the attacker.

Based on MedSec’s demonstration, and on our analysis of the Programmer code, it appears that SJM does not use the above approach to generate authentication tags. Instead, SJM authenticates the Programmer to the implantable with the assistance of a “key table” that is hard-coded within the Java code within the Programmer. At minimum, any party who obtains the (non-obfuscated) Java code from a legitimate SJM Programmer can gain the ability to calculate the correct authentication tags needed to produce viable commands – without any need to use the Programmer itself.

Moreover, MedSec determined – and successfully demonstrated – that there exists a “Universal Key”, i.e., a fixed three-byte authentication tag, that can be used in place of the calculated authentication tag. We identified this value in the Java code provided by MedSec, and verified that it was sufficient to issue shock commands from a Merlin @Home to an implantable device.

While these issues alone are sufficient to defeat the command authentication mechanism used by SJM implantable devices, we also analyzed the specific function that is used by SJM to generate the three-byte authentication tag.  To our surprise, SJM does not appear to use a standard cryptographic function to compute this tag. Instead, they use an unusual and apparently “homebrewed” cryptographic algorithm for the purpose.

Specifically, the PCS Programmer Java code contains a series of hard-coded 32-bit RSA public keys. To issue a command, the implantable device sends a value to the Programmer. This value is then “encrypted” by the Programmer using one of the RSA public keys, and the resulting output is truncated to produce a 24-bit output tag.

The above is not a standard cryptographic protocol, and quite frankly it is difficult to see what St. Jude Medical is trying to accomplish using this technique. From a cryptographic perspective it has several problems:

1. The RSA public keys used by the PCS Programmers are 32 bits long. Normal RSA keys are expected to be a minimum of 1024 bits in length. Some estimates predict that a 1024-bit RSA key can be factored (and thus rendered insecure) in approximately one year using a powerful network of supercomputers. Based on experimentation, we were able to factor the SJM public keys in less than one second on a laptop computer.
2. Even if the RSA keys were of an appropriate length, the SJM protocol does not make use of the corresponding RSA secret keys. Thus the authentication tag is not an RSA signature, nor does it use RSA in any way that we are familiar with.
3. As noted above, since there is no shared session key established between the specific implantable device and the Programmer, the only shared secret available to both parties is contained within the Programmer’s Java code. Thus any party who extracts the Java code from a PCS Programmer will be able to transmit valid commands to any SJM implantable device.

Our best interpretation of this design is that the calculation is intended as a form of “security by obscurity”, based on the assumption that an attacker will not be able to reverse engineer the protocol. Unfortunately, this approach is rarely successful when used in security systems. In this case, the system is fundamentally fragile – due to the fact that code for computing the correct authentication tag is likely available in easily-decompiled Java bytecode on each St. Jude Medical Programmer device. If this code is ever extracted and published, all St. Jude Medical devices become vulnerable to command forgery.

How to remediate these attacks?

To reiterate, the fundamental security concerns with these St. Jude Medical devices (as of 2016) appeared to be problems of design. These were:

1. SJM implantable devices did not require close physical interaction prior to accepting commands (allegedly) sent by the Programmer.
2. SJM did not incorporate a strong cryptographic authentication mechanism in its RF protocol to verify that commands are truly sent by the Programmer.
3. Even if the previous issue was addressed, St. Jude did not appear to have an infrastructure for securely exchanging shared cryptographic keys between a legitimate Programmer and an implantable device.

There are various ways to remediate these issues. One approach is to require St. Jude implantable devices to exchange a secret key with the Programmer through a close-range interaction involving the Programmer’s EM wand. A second approach would be to use a magnetic sensor to verify the presence of a magnet on the device, prior to accepting Programmer commands. Other solutions are also possible. I haven’t reviewed the solution SJM ultimately adopted in their software patches, and I don’t know how many users patched.

Conclusion

Implantable devices offer a number of unique security challenges. It’s naturally hard to get these things right. At the same time, it’s important that vendors take these issues seriously, and spend the time to get cryptographic authentication mechanisms right — because once deployed, these devices are very hard to repair, and the cost of a mistake is extremely high.