CVE-2021-31572: Amazon FreeRTOS Buffer Overflow Flaw

CVE-2021-31572 Overview

CVE-2021-31572 is an integer overflow vulnerability affecting the kernel in Amazon Web Services FreeRTOS before version 10.4.3. The vulnerability exists in stream_buffer.c where improper handling of buffer size calculations can lead to an integer overflow condition during stream buffer operations.

Critical Impact

This integer overflow vulnerability in FreeRTOS can allow remote attackers to potentially achieve arbitrary code execution, data corruption, or denial of service on embedded IoT devices running affected versions.

Affected Products

• Amazon FreeRTOS versions prior to 10.4.3
• IoT devices and embedded systems utilizing vulnerable FreeRTOS kernel

Discovery Timeline

• April 22, 2021 - CVE-2021-31572 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-31572

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw resides in the stream buffer implementation within stream_buffer.c of the FreeRTOS kernel. When a stream buffer is created, the code performs arithmetic operations on the requested buffer size without adequate overflow checks. Specifically, the original code incremented xBufferSizeBytes and added sizeof(StreamBuffer_t) to calculate the total allocation size, but failed to verify that these additions would not cause an integer wraparound.

When an attacker provides a carefully crafted buffer size value near the maximum integer value, the addition operations can cause the calculated size to wrap around to a small value. This results in a significantly undersized memory allocation via pvPortMalloc(), while the application logic continues to operate as if the full requested buffer size was allocated.

Root Cause

The root cause is insufficient validation of arithmetic operations when calculating memory allocation sizes for stream buffers. The vulnerable code path incremented the buffer size and added the structure overhead without checking whether these operations would overflow, violating secure coding practices for embedded systems.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Sending specially crafted data to an IoT device running vulnerable FreeRTOS
2. Triggering stream buffer allocation with a malicious size value
3. Causing integer overflow during size calculation
4. Exploiting the resulting heap buffer overflow condition

The security patch adds proper overflow detection before performing the allocation:

          * this is a quirk of the implementation that means otherwise the free
          * space would be reported as one byte smaller than would be logically
          * expected. */
-        xBufferSizeBytes++;
-        pucAllocatedMemory = ( uint8_t * ) pvPortMalloc( xBufferSizeBytes + sizeof( StreamBuffer_t ) ); /*lint !e9079 malloc() only returns void*. */
+        if( xBufferSizeBytes < ( xBufferSizeBytes + 1 + sizeof( StreamBuffer_t ) ) )
+        {
+            xBufferSizeBytes++;
+            pucAllocatedMemory = ( uint8_t * ) pvPortMalloc( xBufferSizeBytes + sizeof( StreamBuffer_t ) ); /*lint !e9079 malloc() only returns void*. */
+        }
+        else
+        {
+            pucAllocatedMemory = NULL;
+        }
+        
 
         if( pucAllocatedMemory != NULL )
         {

Source: GitHub FreeRTOS Commit Log

The patch introduces a conditional check that validates whether the sum of xBufferSizeBytes + 1 + sizeof(StreamBuffer_t) exceeds the original value, effectively detecting overflow conditions before the allocation occurs.

Detection Methods for CVE-2021-31572

Indicators of Compromise

• Unexpected memory allocation failures or crashes in FreeRTOS-based devices
• Anomalous stream buffer creation requests with unusually large size parameters
• Device instability or unexpected reboots in IoT deployments
• Memory corruption artifacts in device diagnostics

Detection Strategies

• Monitor network traffic to FreeRTOS devices for abnormal data patterns that could trigger buffer operations
• Implement firmware integrity verification to detect exploitation attempts
• Deploy runtime memory monitoring on IoT devices where feasible
• Review device logs for allocation failures or memory-related errors

Monitoring Recommendations

• Track FreeRTOS version deployments across your IoT fleet to identify vulnerable devices
• Implement network segmentation to limit exposure of embedded devices
• Enable verbose logging on IoT gateways to capture anomalous traffic patterns
• Establish baseline behavior for device memory usage to detect deviations

How to Mitigate CVE-2021-31572

Immediate Actions Required

• Upgrade all Amazon FreeRTOS deployments to version 10.4.3 or later immediately
• Inventory all IoT devices and embedded systems running FreeRTOS in your environment
• Apply network-level controls to restrict access to vulnerable devices pending patching
• Review device configurations for unnecessary exposed services

Patch Information

Amazon has released a security patch addressing this integer overflow vulnerability in FreeRTOS version 10.4.3. The fix introduces proper overflow validation before memory allocation in stream buffer operations. The patch is available via the official FreeRTOS Kernel commit.

Workarounds

• Implement network segmentation to isolate vulnerable IoT devices from untrusted networks
• Deploy intrusion detection/prevention systems to filter malicious traffic to embedded devices
• If firmware updates are not immediately possible, consider temporarily disconnecting affected devices from network exposure
• Apply firewall rules to limit inbound connections to vulnerable FreeRTOS devices

# Example: Restrict network access to IoT devices
# Apply on network firewall/gateway
iptables -A INPUT -d <iot_device_ip> -p tcp --dport <exposed_port> -j DROP
iptables -A INPUT -d <iot_device_ip> -s <trusted_management_ip> -p tcp --dport <exposed_port> -j ACCEPT