CVE-2019-15894 in ESP-IDF
Summary
by MITRE
An issue was discovered in Espressif ESP-IDF 2.x, 3.0.x through 3.0.9, 3.1.x through 3.1.6, 3.2.x through 3.2.3, and 3.3.x through 3.3.1. An attacker who uses fault injection to physically disrupt the ESP32 CPU can bypass the Secure Boot digest verification at startup, and boot unverified code from flash. The fault injection attack does not disable the Flash Encryption feature, so if the ESP32 is configured with the recommended combination of Secure Boot and Flash Encryption, then the impact is minimized. If the ESP32 is configured without Flash Encryption then successful fault injection allows arbitrary code execution. To protect devices with Flash Encryption and Secure Boot enabled against this attack, a firmware change must be made to permanently enable Flash Encryption in the field if it is not already permanently enabled.
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Analysis
by VulDB Data Team • 09/27/2020
The vulnerability described in CVE-2019-15894 represents a critical security flaw in Espressif ESP-IDF firmware versions across multiple release streams including 2.x, 3.0.x through 3.0.9, 3.1.x through 3.1.6, 3.2.x through 3.2.3, and 3.3.x through 3.3.1. This issue stems from the system's susceptibility to fault injection attacks that can physically disrupt the ESP32 CPU during the boot process. The attack specifically targets the Secure Boot digest verification mechanism that normally ensures only authenticated code executes from flash memory. When successful, this fault injection attack bypasses the cryptographic verification checks that are fundamental to preventing unauthorized code execution, allowing malicious actors to load unverified code into the system.
The technical implementation of this vulnerability involves exploiting the physical characteristics of the ESP32 microcontroller to induce faults during critical boot operations. This type of attack falls under the category of fault injection attacks that are commonly classified under CWE-388 as "Error Handling" vulnerabilities, though more specifically relates to hardware-level fault injection techniques. The attack vector requires physical access to the device and the ability to manipulate the power supply or clock signals to create timing disruptions during the Secure Boot verification process. This approach aligns with techniques documented in various penetration testing frameworks and represents a sophisticated attack method that targets the fundamental security mechanisms of embedded systems.
The operational impact of this vulnerability is significant for devices that rely solely on Secure Boot protection without implementing additional layers of security. When Flash Encryption is not enabled, successful fault injection attacks provide attackers with complete arbitrary code execution capabilities, effectively nullifying the security protections that Secure Boot was designed to provide. This creates a pathway for persistent malware deployment, data exfiltration, and potential system compromise that could affect IoT devices, industrial control systems, and other embedded applications using affected ESP32 configurations. The vulnerability particularly affects systems where physical security measures are inadequate or where devices are deployed in environments where tampering is possible.
Organizations and developers must implement comprehensive mitigation strategies to address this vulnerability effectively. The primary recommendation involves ensuring that Flash Encryption is permanently enabled on all affected devices, as this combination of Secure Boot and Flash Encryption significantly reduces the attack surface. This requirement aligns with industry best practices for embedded security and represents a fundamental defense-in-depth approach. Additionally, the firmware must be updated to include mechanisms that prevent disabling Flash Encryption, as this protection must be permanently active to provide meaningful security. System designers should consider implementing additional physical security measures such as tamper detection circuits and secure boot monitoring to detect and prevent fault injection attacks. The vulnerability also highlights the importance of following NIST SP 800-147 guidelines for secure embedded system design and implementing proper security controls throughout the device lifecycle to prevent such attacks from compromising system integrity.