CVE-2021-31571: Amazon FreeRTOS Buffer Overflow Flaw

CVE-2021-31571 Overview

CVE-2021-31571 is an integer overflow vulnerability in the Amazon Web Services FreeRTOS kernel affecting versions prior to 10.4.3. The vulnerability exists in queue.c during queue creation operations, where improper validation of arithmetic operations can lead to an integer overflow condition. This flaw could potentially allow attackers to exploit the overflow to cause memory corruption, denial of service, or potentially achieve code execution on affected embedded systems.

Critical Impact

This integer overflow vulnerability in the FreeRTOS kernel's queue creation mechanism could allow remote attackers to compromise IoT devices and embedded systems running vulnerable versions of Amazon FreeRTOS, potentially leading to complete system compromise with high impact on confidentiality, integrity, and availability.

Affected Products

• Amazon FreeRTOS versions prior to 10.4.3
• IoT devices and embedded systems using vulnerable FreeRTOS kernel
• AWS IoT implementations utilizing affected FreeRTOS versions

Discovery Timeline

• 2021-04-22 - CVE-2021-31571 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31571

Vulnerability Analysis

The vulnerability resides in the queue creation functionality within queue.c of the FreeRTOS kernel. When creating a queue, the kernel performs arithmetic operations to calculate the total memory required for the queue structure and its storage area. The flaw occurs because the code did not properly validate for addition overflow when computing the combined size of the Queue_t structure and the queue storage buffer (xQueueSizeInBytes).

An attacker could craft malicious input parameters that cause the addition operation to overflow, resulting in a smaller-than-expected memory allocation. Subsequent operations would then write beyond the allocated buffer, leading to heap corruption. This type of vulnerability is particularly dangerous in embedded systems and IoT devices where FreeRTOS is commonly deployed, as these systems often lack memory protection mechanisms present in full-featured operating systems.

Root Cause

The root cause is a missing overflow check in the queue creation code path. While the existing code included a check for multiplication overflow when calculating xQueueSizeInBytes, it lacked validation for the subsequent addition operation that combines the Queue_t structure size with the calculated buffer size. This is classified as CWE-190 (Integer Overflow or Wraparound), where integer arithmetic operations can produce values outside the expected range, wrapping around to small positive values when exceeding maximum integer bounds.

Attack Vector

The attack vector is network-based, allowing remote exploitation without requiring authentication or user interaction. An attacker could potentially trigger this vulnerability by:

1. Sending specially crafted messages to an IoT device that cause queue creation with malicious parameters
2. Exploiting application-level interfaces that pass attacker-controlled values to queue creation functions
3. Leveraging network protocols that interact with FreeRTOS task scheduling and inter-task communication mechanisms

The overflow causes undersized memory allocation, enabling heap buffer overflow conditions when the queue is subsequently used.

         /* Check for multiplication overflow. */
         configASSERT( ( uxItemSize == 0 ) || ( uxQueueLength == ( xQueueSizeInBytes / uxItemSize ) ) );
 
+        /* Check for addition overflow. */
+        configASSERT( ( sizeof( Queue_t ) + xQueueSizeInBytes ) >  xQueueSizeInBytes );
+
         /* Allocate the queue and storage area.  Justification for MISRA
          * deviation as follows:  pvPortMalloc() always ensures returned memory
          * blocks are aligned per the requirements of the MCU stack.  In this case

Source: GitHub FreeRTOS Kernel Commit

Detection Methods for CVE-2021-31571

Indicators of Compromise

• Unexpected device crashes or resets during queue-intensive operations
• Memory corruption indicators in system logs or debug output
• Abnormal network traffic patterns targeting IoT device management interfaces
• Heap corruption detected by memory debugging tools or watchdog mechanisms
• Unexplained behavior changes in FreeRTOS task scheduling

Detection Strategies

• Implement firmware version checking to identify devices running FreeRTOS versions prior to 10.4.3
• Deploy network monitoring to detect anomalous traffic patterns targeting embedded devices
• Enable FreeRTOS kernel assertions (configASSERT) to catch integer overflow conditions
• Use static analysis tools to scan for integer overflow vulnerabilities in custom FreeRTOS integrations
• Implement runtime memory integrity checks where possible

Monitoring Recommendations

• Monitor device telemetry for abnormal memory usage patterns or allocation failures
• Implement centralized logging for IoT fleet to detect widespread exploitation attempts
• Set up alerts for unexpected device reboots or watchdog timer resets
• Track firmware versions across IoT deployments to ensure patched versions are deployed
• Monitor network boundaries for suspicious traffic targeting embedded device protocols

How to Mitigate CVE-2021-31571

Immediate Actions Required

• Update Amazon FreeRTOS to version 10.4.3 or later immediately
• Inventory all devices running FreeRTOS and prioritize patching based on network exposure
• Implement network segmentation to isolate vulnerable IoT devices from untrusted networks
• Review and audit any custom code that creates queues with externally-influenced parameters
• Enable configASSERT in FreeRTOS configuration to detect overflow conditions during development

Patch Information

Amazon has released a security patch addressing this vulnerability in FreeRTOS version 10.4.3. The fix adds an explicit assertion check to verify that the addition of sizeof(Queue_t) and xQueueSizeInBytes does not overflow. Organizations should update to the patched version by applying the commit 47338393f1f79558f6144213409f09f81d7c4837 or upgrading to FreeRTOS 10.4.3 or later. The patch is available via the FreeRTOS Kernel GitHub repository.

Workarounds

• If immediate patching is not possible, isolate affected devices on segmented networks with strict access controls
• Implement input validation at application layer to prevent large or malicious queue parameters
• Manually apply the overflow check assertion to queue.c if using a custom FreeRTOS build
• Disable or restrict network interfaces on vulnerable devices where feasible
• Deploy intrusion detection systems to monitor for exploitation attempts

# Verify FreeRTOS version in your project
grep -r "tskKERNEL_VERSION" FreeRTOS/Source/include/task.h

# Update FreeRTOS to patched version
git fetch origin
git checkout V10.4.3

# Rebuild your FreeRTOS project with updated kernel
make clean && make all