Updated July 14, 2025
In embedded systems, especially those used in medical devices, the way you handle numbers isn’t just a matter of coding preference—it can be a matter of patient safety. One common but often overlooked issue is how software handles signed vs. unsigned integers.
Whether it’s processing temperature readings, handling timers, or validating input, the way your firmware interprets numeric values can lead to silent bugs, system crashes, or even dangerous misbehavior. This article breaks down what you need to know about these two types of integers, how mistakes happen, and how to prevent them in medical device development.
What’s the Difference?
Let’s start with a simple distinction:
- Unsigned integers can only represent zero and positive values.
Example:uint16_tcan store values from0to65,535. - Signed integers can represent both positive and negative values.
Example:int16_tranges from-32,768to+32,767.
Both types are used in medical device firmware, but using the wrong one in the wrong context can create serious problems.
Why It Matters for Medical Devices
Consider this: a wearable monitor reads a body temperature of -5°C (perhaps due to a sensor fault or a misread). If the software uses an unsigned integer to store the value, the result isn’t -5—it becomes 65,531. That could trigger a high-temperature alert, shut down the device, or worse.
This isn’t theoretical. Misused integer types have caused:
- False alarms in infusion pumps
- Incorrect vital sign logs
- Device restarts due to arithmetic errors
Common Vulnerability Examples
1. Sensor Reading Errors
A temperature sensor reports -3°C, but your code uses uint8_t instead of int8_t. The stored value becomes 253. Now your system thinks the patient is overheating.
2. Timer Overflow
A signed integer used for millisecond tracking rolls over to a negative value. The device waits millions of milliseconds before triggering a life-critical event.
3. Arithmetic Underflow
Subtracting a larger unsigned number from a smaller one wraps around to a large positive number—creating invalid memory accesses or logic loops.
How to Prevent These Bugs
🧰 Use the Right Data Types
Stick with explicitly sized types like int16_t, uint32_t, etc. Avoid generic int or short.
🔍 Initialize Every Variable
Many embedded bugs stem from uninitialized variables that default to unexpected values.
✅ Range-Check All Inputs
Validate external data—especially from sensors or user interfaces. Don’t assume it’s always in range.
🧪 Add Integer Boundary Testing
As part of software verification, test edge cases like 0, maximum, and negative values.
📋 Follow MISRA or CERT Guidelines
Use static analysis tools to enforce safe practices around integer usage.
Compliance & FDA Considerations
The FDA expects manufacturers to identify and mitigate risks associated with software behavior, especially those tied to numeric processing. According to the Secure Product Development Framework (SPDF) and latest FDA cybersecurity guidance:
- Threat models must consider logic-level faults from incorrect data types.
- Software verification plans should include testing for numeric limits.
- Signed/unsigned errors should be evaluated as potential cybersecurity and safety risks.
Neglecting these could lead to regulatory delays, rejections, or product recalls.
Final Thoughts
On the surface, choosing between signed and unsigned integers may seem like a low-level technical detail. But in medical devices, where safety and reliability are non-negotiable, even small coding oversights can have serious consequences.
By selecting the correct types, validating input, and testing rigorously, you protect not only your software—but the patients who rely on it.
Need Help Validating Your Device Firmware?
At Blue Goat Cyber, we specialize in penetration testing, code review, and FDA cybersecurity submission support for medical device firmware. We’ll help you catch these subtle bugs before they turn into real-world problems.
👉 Schedule a consultation and secure your embedded systems with confidence.