Editor’s note: This piece was originally published by embedded.com
There’s no shortage of companies that need help configuring devices securely, or vendors seeking to remediate vulnerabilities. But from our vantage point at NCC Group, we mostly see devices when working directly with OEMs confronting security issues in their products — and by this point, it’s usually too late to do much. We root out as many vulnerabilities as we can in the time allotted, but many security problems are already baked in. That’s why we advocate so strongly for security early in the development process.
Product development for an embedded system has all the stages you expect to find in textbooks. While formal security assessments are most common in the quality testing phase, there is a role for security in all phases.
Figure 1: Typical product design cycle
Requirements and Threat Modelling
We see major security problems introduced even during requirements gathering. Insufficient due diligence here can cause many issues down the line. Conversely, even a little effort at this point can have a huge payoff at the end.
Functional requirements tell you everything your product is supposed to do, and how. Security requirements outline all the things your product is not supposed to do, and that’s equally important. Security testing occupies this gap, and it’s a vital part of the process.
Figure 2: Testing vs. security testing
To develop your security requirements,, you need a solid understanding of the threat model. Before you can even consider appropriate security controls and mitigations, you must define the product’s security objectives, and the types of threats your product should withstand, in as much detail as possible. This describes all the bad guys who want to compromise your systems and devices, as well as those of your customers and users. They come in many forms:
- Remote attackers using a wired or wireless network interface (if the device has such capabilities). These attacks can scale easily and affect many devices at once.
- Local attacks that require an ability to run code on the device, often as a lower privilege. Browser code or mobile apps are examples of such vectors.
- Physical attackers with possession of the hardware. Lost or stolen devices, rogue administrators, temporary access through short-term rentals, and domestic abuse, are all common examples. This issue is harder to solve, and the best recourse is to increase the cost for the attacker. These come in two forms: cost to develop an attack, and cost to execute. Increasing the first may help buy you time, but if the product is to have any longevity in the market, it’s better to concentrate on the latter. Weaknesses such as sharing secrets across a fleet of devices is an all-too-common design pattern that leads to a near-zero execution cost (once the secret is known).
A reasonable baseline for nearly all modern products is to set the initial bar at “thousands of dollars,” which implies that an attack on the core chipset of the device is required. Anything less, and your product will very likely fall victim to a very cheap circuit attack. Setting the bar this high should not reflect the product cost or price, but rather the value of the assets that the device must protect. Mass market devices like smartphones have had this level of security since at least the early 2000s. And that’s good — every aspect of our lives is accessed by our smartphones, so the cost for an attacker should be high.
A formal threat model is a living document that can be adjusted and consulted as needed throughout the product development cycle.
Next, you need to select your building blocks: the platform, processor, bootloaders and operating system.
Most embedded systems are built around a core microcontroller, system-on-chip (SoC) or other CPU. For most companies this involves outsourcing and technical debt: Building connected consumer devices, industrial control systems or vehicle ECUs typically means selecting a chipset from a third-party vendor that meets cost, performance and feature requirements. But let’s not forget security requirements: Not all components are designed with security in mind, and careful evaluation will make this clear. Make sure it has the security features you need — cryptographic accelerators, hardware random number generator, secure boot or other firmware integrity features, a modern memory management unit to implement privilege separation and memory protections, internal fuse-based security configuration, and a hardware root-of-trust. It’s also important to ensure that it doesn’t have security traps you want to avoid. For example:
- Ask the vendor to show you the security audit of the internal ROM(s).
- Get details about the security properties of the provisioning systems.
- Ask how they handle devices returned for failure analysis after debug functionality has been disabled (you’ll be surprised how many admit to having a backdoor).
- Understand specifics about how the hardware boots the software, security properties of the ROM, bootloader, and firmware reference design.
One crucial aspect of any processor is that it must form a trust anchor: a root-of-trust that can validate the integrity of the system firmware for subsequent boot stages. This typically consists of an immutable first-stage bootloader (in ROM or internal flash), and an immutable public key (commonly programmed into fuses). While all other aspects of the system firmware can be validated, the root-of-trust is trusted implicitly.
Next you need to choose a software stack to run atop the hardware and boot system provided by your chip vendor. There are many commercial and open source embedded operating system vendors to choose from, with different levels of security maturity: Linux/Android, FreeRTOS, Zephyr, MbedOS, VxWorks, and more. Many companies will even roll their own. Your chipset vendor will influence the selection with a shortlist of operating systems they support, and anything else means more work for you. The key criteria here are privilege separation, memory protection, capabilities and access controls, secure storage, and modern exploit mitigations. Also important is a vendor commitment to providing ongoing support on the hardware platform you’re using.
At the application level, where you implement the bulk of the business logic, you again have choices. Most vulnerabilities are memory corruption-related, and they can be severe, even catastrophic. Consequently, these are also among the few classes of vulnerabilities we know how to eliminate, and that’s by using modern memory safe programming languages. If your platform supports such an environment, then applications should be written in Java, Go, Rust or Python. Where this is not possible, employ strong defensive programming and secure development lifecycle (SDLC) techniques to reduce the risk of developer errors ending up in the released product.
Once the requirements are laid out and major platform decisions have been made, the bulk of the design, implementation and testing phases of the product development process can move forward. Through the development cycle, continual security review with reference to the threat model (with updates as needed) will keep you on the right path.
A few other security measures deserve mention:
Patching and ongoing maintenance is crucial to the continued operation of your devices. Threats evolve rapidly as vulnerabilities are discovered, and new attacker techniques are developed. Staying ahead of the bad guys requires that firmware updates be released on a regular cadence, and that there be a high adoption and installation of these patches. Automatic updates can make this extremely practical for most connected devices. Where safety considerations prevent automatic updates, or where users are otherwise involved in the update process, regular update behavior can sometimes be incentivized (e.g.: Apple has frequently included new emoji collections with their security updates to encourage user adoption).
One challenge is in the form of that technical debt you inherited when you outsourced your board support package. The chip business is sales-driven, and they have little incentive to maintain ongoing support for old devices and BSP versions. One way to help here is to ensure that ongoing security support is enshrined in the contract; otherwise, security is an afterthought.
Manufacturing and supply chain
If you are using a general-purpose microcontroller or SoC from a common vendor, you should expect the root-of-trust to be unconfigured until you provision it. This is where your manufacturing and production processes come into play — it is absolutely vital for these steps to be performed securely if your product is to rely on these bedrock security features. However, there are strong incentives to outsource production to ODM or CM partners with low labor costs — the challenge is to ensure that your root-of-trust is securely configured even with potential threat actors in the factory.
Getting these processes in place early in the development cycle can be difficult, partly because secure firmware is likely to lag behind early hardware prototypes. Getting to them late can be equally difficult, because manufacturing is likely to resist process changes once they have a working recipe that produces widgets with the expected yield.
Repair and reverse logistics also likely require privileged access to your embedded devices. Ensuring that this privilege cannot be abused requires strong authentication on the calibration and configuration interfaces, and a careful understanding of the nuances of the production process for your specific devices.
Early threat modeling and the development of security requirements doesn’t have to be a burden, and it can save a great deal of time and effort if done at the right time. Incorporating input from your security experts will help you make the right platform choices and avoid the churn associated with repeated security fixes. Early engagement is far more effective.
|Rob Wood is the VP for the Hardware and Embedded Security Services practice at cybersecurity consultancy, NCC Group. His career in embedded devices spans two decades, having worked at both BlackBerry and Motorola Mobility in roles focused on embedded software development, product firmware and hardware security, and supply chain security. Rob is an experienced firmware developer with extensive security architecture experience. His specialty is in designing, building, and reviewing products to push the security boundaries deeper into the firmware, hardware, and supply chain. He is most comfortable working with the software layers deep in the bowels of the system, well below userland, where the lines between hardware and software begin to blur. This includes things like the bootloaders, kernel, device drivers, firmware, baseband, trusted execution environments, debug and development tools, factory and repair tools, bare-metal firmware, and all the processes that surround them.|