CVE-2018-3968 in Das U-Boot
Summary
by MITRE
An exploitable vulnerability exists in the verified boot protection of the Das U-Boot from version 2013.07-rc1 to 2014.07-rc2. The affected versions lack proper FIT signature enforcement, which allows an attacker to bypass U-Boot's verified boot and execute an unsigned kernel, embedded in a legacy image format. To trigger this vulnerability, a local attacker needs to be able to supply the image to boot.
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Analysis
by VulDB Data Team • 08/03/2023
The vulnerability described in CVE-2018-3968 represents a critical weakness in the U-Boot bootloader's verified boot implementation that affects versions from 2013.07-rc1 through 2014.07-rc2. This flaw specifically targets the Firmware Image Tree (FIT) signature enforcement mechanism, which is a fundamental security component designed to ensure that only authenticated and authorized firmware components can execute on the system. The vulnerability exists within the Trusted Computing Base of embedded systems where U-Boot serves as the primary bootloader, making it a prime target for attackers seeking to compromise device integrity and establish persistent execution environments.
The technical implementation of this vulnerability stems from the improper enforcement of FIT signatures within the U-Boot bootloader. FIT is a standardized format for firmware images that includes cryptographic signatures to verify authenticity, but in the affected versions, the bootloader fails to properly validate these signatures before executing the loaded kernel. This allows attackers to craft malicious images that contain unsigned kernels, bypassing the intended security controls that should prevent unauthorized code execution. The vulnerability specifically affects the transition between legacy image formats and the newer FIT format, where the signature verification process becomes inconsistent or entirely bypassed. This flaw falls under CWE-327, which addresses the use of weak cryptographic algorithms and improper implementation of cryptographic functions, while also aligning with CWE-284, concerning improper access control mechanisms in bootloader environments.
The operational impact of this vulnerability is significant as it provides attackers with a direct path to bypass hardware-level security measures that are typically enforced by U-Boot's verified boot process. An attacker with local access to supply boot images can execute arbitrary code on the target system, potentially leading to complete system compromise and persistence. The vulnerability's exploitation requires only local access to provide boot images, making it particularly dangerous in environments where physical access is possible or where attackers can influence the boot process through network-based attacks. This weakness undermines the entire security architecture of embedded systems by allowing attackers to load unsigned kernels that could contain rootkits, backdoors, or other malicious components. The attack surface extends beyond simple code execution to include potential privilege escalation and lateral movement within the compromised system, as the attacker can now load code that bypasses the security controls designed to prevent such actions.
Mitigation strategies for CVE-2018-3968 should focus on upgrading to U-Boot versions that properly implement FIT signature enforcement, specifically those released after the 2014.07-rc2 version where the vulnerability was addressed. Organizations should also implement additional security measures such as secure boot policies that verify all boot components against trusted certificates and maintain proper access controls to prevent unauthorized image loading. The implementation of hardware security modules or trusted platform modules can provide additional layers of protection by ensuring that even if an attacker bypasses software-level checks, hardware-level protections remain intact. Security teams should conduct comprehensive audits of all embedded systems to identify affected U-Boot versions and ensure proper firmware updates are applied. This vulnerability demonstrates the critical importance of maintaining up-to-date firmware and the necessity of robust cryptographic implementation in bootloader environments, as it represents a fundamental failure in the security architecture that allows attackers to establish unauthorized execution environments with minimal privileges required. The remediation process should also include monitoring for suspicious boot activities and implementing proper logging of boot processes to detect potential exploitation attempts.