CVE-2021-3431 in zephyrproject-rtos
Summary
by MITRE • 06/29/2022
Assertion reachable with repeated LL_FEATURE_REQ. Zephyr versions >= v2.5.0 contain Reachable Assertion (CWE-617). For more information, see https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-7548-5m6f-mqv9
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Analysis
by VulDB Data Team • 07/16/2022
The vulnerability identified as CVE-2021-3431 represents a critical reachability issue within the Zephyr real-time operating system where an assertion can be triggered through repeated LL_FEATURE_REQ commands. This flaw affects Zephyr versions 2.5.0 and later, making it a significant concern for embedded systems developers who rely on this lightweight operating system for IoT and edge computing applications. The vulnerability stems from improper handling of Bluetooth Low Energy feature requests within the Zephyr kernel, creating a path for attackers to potentially disrupt system operations through carefully crafted sequence of feature requests. The assertion failure occurs when the system processes multiple LL_FEATURE_REQ messages in rapid succession, leading to a condition where the software's internal checks fail and terminate execution.
The technical implementation of this vulnerability falls under CWE-617, which defines reachable assertions as a condition where an assertion can be reached through normal program execution paths, typically indicating a lack of proper input validation or error handling. In the context of Zephyr's Bluetooth stack, this occurs when the system's link layer processing logic does not adequately validate or limit the frequency of feature request messages. The flaw operates at the kernel level within the Bluetooth subsystem, specifically in the handling of Link Layer feature requests that are part of the Bluetooth specification. When an attacker sends multiple LL_FEATURE_REQ commands in quick succession, the system's internal state management becomes inconsistent, leading to assertion failures that can cause system crashes or unexpected behavior.
From an operational impact perspective, this vulnerability presents a denial of service risk that could affect embedded devices running Zephyr OS in critical applications. The reachability of this assertion means that an attacker with access to the Bluetooth interface could potentially cause system instability or complete system crashes, particularly in devices where continuous operation is essential. This vulnerability is particularly concerning for IoT devices that rely on Bluetooth connectivity for communication, as it could be exploited to disrupt services or gain unauthorized access to device functionality. The impact extends beyond simple service disruption to potentially enable more sophisticated attacks if the system is not properly secured against such assertions. The flaw also represents a potential attack vector for resource exhaustion, as repeated assertions could consume system resources and potentially lead to memory corruption or other stability issues.
The mitigation strategies for CVE-2021-3431 primarily involve updating to patched versions of the Zephyr OS where the assertion handling has been corrected to properly validate incoming LL_FEATURE_REQ messages and limit their processing frequency. Organizations should implement immediate updates to their Zephyr-based systems to address this vulnerability, particularly in production environments where device stability is paramount. Additionally, network administrators should consider implementing Bluetooth access controls and monitoring for unusual feature request patterns that could indicate exploitation attempts. The vulnerability also highlights the importance of proper input validation and error handling in real-time operating systems, as outlined in the ATT&CK framework's considerations for operating system security. Security teams should monitor for potential exploitation attempts and implement network segmentation to limit the potential impact of such vulnerabilities in their environments.