CVE-2026-26399: Arduino Core STM32 Use-After-Free Bug

CVE-2026-26399 Overview

A stack-use-after-return vulnerability has been identified in the Arduino_Core_STM32 library prior to version 1.7.0. This memory corruption flaw occurs in the pwm_start() function, which allocates a TIM_HandleTypeDef structure on the stack and passes its address to HAL initialization routines. The address is then stored in a global timer handle registry. After the function returns, interrupt service routines (ISRs) may dereference this dangling pointer, leading to unpredictable behavior and memory corruption.

Critical Impact

Memory corruption via dangling pointer dereference can lead to system instability, data corruption, or potential code execution in embedded systems running on STM32 microcontrollers.

Affected Products

• Arduino_Core_STM32 library versions prior to 1.7.0
• STM32-based embedded devices and IoT applications using the vulnerable library
• Arduino-compatible development boards with STM32 microcontrollers

Discovery Timeline

• 2026-04-20 - CVE-2026-26399 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-26399

Vulnerability Analysis

This vulnerability is classified under CWE-562 (Return of Stack Variable Address), a common but dangerous programming error in C/C++ embedded development. The flaw exists because the pwm_start() function creates a local TIM_HandleTypeDef structure on the stack rather than allocating it statically or dynamically. When this stack-allocated structure's address is stored in a global registry for later use by interrupt handlers, it creates a dangling pointer condition.

After pwm_start() returns, the stack frame is deallocated, but the global registry still holds a reference to the now-invalid memory location. When a timer interrupt fires and the ISR attempts to access the timer handle through this stored pointer, it dereferences memory that may have been overwritten by subsequent function calls. This results in unpredictable behavior, including memory corruption, system crashes, or potentially exploitable conditions in embedded environments.

Root Cause

The root cause is improper memory management within the pwm_start() function. The developer allocated the TIM_HandleTypeDef structure as a local stack variable rather than using static allocation, heap allocation, or a globally-scoped variable. When this local variable's address was passed to HAL routines that persist the pointer beyond the function's lifetime, a classic stack-use-after-return vulnerability was introduced. This type of error is particularly insidious because it may not manifest immediately—the corruption only occurs when an ISR fires after the original stack frame has been reused.

Attack Vector

This vulnerability requires local access to exploit. An attacker with the ability to execute code on an affected STM32 device could potentially trigger conditions that exploit the memory corruption. The attack would involve:

1. Triggering the vulnerable pwm_start() function to create the dangling pointer condition
2. Manipulating subsequent stack usage to control the contents of the memory location pointed to by the dangling pointer
3. Waiting for or triggering a timer interrupt that dereferences the corrupted pointer
4. Leveraging the memory corruption to achieve information disclosure, denial of service, or potentially code execution

The vulnerability manifests when the pwm_start() function allocates a TIM_HandleTypeDef structure locally and stores its address in a global timer handle registry. After the function returns, any timer interrupt attempting to access this handle will dereference memory that no longer contains valid timer configuration data. For detailed technical analysis, refer to the CVE-2026-26399 Disclosure repository.

Detection Methods for CVE-2026-26399

Indicators of Compromise

• Unexpected system crashes or resets in STM32-based devices, particularly during PWM operations
• Erratic behavior in timer-dependent functionality such as motor control or LED dimming
• Stack corruption patterns detected in firmware debugging sessions
• Memory access violations logged by hardware debuggers when ISRs execute

Detection Strategies

• Audit source code for stack-allocated structures passed to HAL initialization functions that persist pointers
• Enable AddressSanitizer (ASan) with stack-use-after-return detection during development and testing
• Review timer interrupt handlers for potential dereference of handles that may have originated from stack variables
• Implement runtime memory monitoring in debug builds to detect access to deallocated stack regions

Monitoring Recommendations

• Deploy firmware integrity monitoring to detect unexpected crashes or resets in production devices
• Implement telemetry for exception handlers to capture and report memory access violations
• Monitor device behavior for symptoms of memory corruption such as intermittent failures in PWM-dependent features
• Establish baseline operational parameters and alert on deviations that may indicate exploitation attempts

How to Mitigate CVE-2026-26399

Immediate Actions Required

• Upgrade Arduino_Core_STM32 library to version 1.7.0 or later immediately
• Review any custom firmware that may have adopted similar patterns from the vulnerable code
• Test PWM functionality thoroughly after applying the update to ensure proper operation
• Audit other HAL integration points for similar stack-use-after-return patterns

Patch Information

The vulnerability has been addressed in Arduino_Core_STM32 version 1.7.0. The fix involves changing the TIM_HandleTypeDef structure allocation from stack-based to static or properly managed memory that persists beyond the function's lifetime. Organizations should upgrade to the latest release available through the official STM32duino repository.

Workarounds

• If immediate upgrade is not possible, modify the pwm_start() function locally to use static allocation for the TIM_HandleTypeDef structure
• Implement a wrapper function that manages timer handles in persistent memory before calling HAL routines
• Disable timer interrupts during critical sections if PWM functionality is not essential
• Consider recompiling with stack protector options enabled to detect corruption earlier

Workaround implementation requires modifying the timer handle allocation to ensure persistence beyond function scope:

// Workaround: Declare TIM_HandleTypeDef as static to ensure
// the structure persists after pwm_start() returns
static TIM_HandleTypeDef htim_persistent;

// Use &htim_persistent instead of a local stack variable
// when registering with HAL initialization routines