CVE-2021-3418 in grub2
Summary
by MITRE • 03/16/2021
If certificates that signed grub are installed into db, grub can be booted directly. It will then boot any kernel without signature validation. The booted kernel will think it was booted in secureboot mode and will implement lockdown, yet it could have been tampered. This flaw is a reintroduction of CVE-2020-15705 and only affects grub2 versions prior to 2.06 and upstream and distributions using the shim_lock mechanism.
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Analysis
by VulDB Data Team • 04/01/2021
This vulnerability represents a critical flaw in the grub2 boot loader's secure boot implementation that undermines the fundamental security guarantees of UEFI secure boot mechanisms. The issue arises from a regression that re-introduces the vulnerability previously addressed in CVE-2020-15705, specifically affecting grub2 versions prior to 2.06. When certificates that sign grub are installed into the database, the boot process becomes vulnerable to manipulation that bypasses signature validation mechanisms. This creates a dangerous scenario where an attacker can install malicious certificates that will be accepted by the boot process, allowing unauthorized code execution during the boot sequence. The vulnerability operates at the intersection of firmware security and operating system boot integrity, exploiting a weakness in how grub2 handles certificate validation within the secure boot framework.
The technical flaw manifests in grub2's handling of the shim_lock mechanism, which is designed to enforce secure boot policies by validating boot components against trusted certificates. When grub2 is configured with certificates installed in the db (database) and the system uses shim_lock, the boot loader fails to properly validate kernel signatures even though it appears to be operating in secure boot mode. This creates a false sense of security where the kernel believes it was booted in a secure environment and implements kernel lockdown mechanisms, yet the actual boot process was compromised. The vulnerability specifically targets the gap between the boot loader's certificate validation and the kernel's assumption of secure boot integrity, creating a scenario where the kernel's security protections are rendered ineffective by a compromised boot process. This represents a classic case of incomplete validation where the system assumes security properties that have been bypassed.
The operational impact of this vulnerability is severe as it provides a pathway for attackers to bypass secure boot protections entirely, potentially leading to full system compromise. An attacker who can manipulate the boot process can install malicious kernels that will be executed with elevated privileges, effectively bypassing all kernel security mechanisms including lockdown, module signing, and other integrity protections. The vulnerability affects distributions that rely on shim_lock for secure boot enforcement, making it particularly concerning for enterprise environments and systems requiring strong boot integrity. This flaw allows for persistent rootkit installation and can be exploited to maintain long-term system compromise without detection, as the malicious kernel will appear to be running in a secure environment. The vulnerability also impacts the trust model of secure boot implementations, where the assumption of boot integrity is fundamentally compromised.
Mitigation strategies should focus on immediate version upgrades to grub2 2.06 or later, which contain the necessary fixes for this regression. Organizations should also implement additional verification measures such as monitoring for unauthorized certificate installations in the db database and ensuring that only trusted certificates are present. The recommended approach includes disabling the problematic certificate handling mechanisms when not required, implementing strict certificate management policies, and ensuring that systems are configured to use only verified boot loaders. Additionally, system administrators should consider implementing runtime integrity checks that can detect unauthorized boot process modifications, and organizations should review their secure boot configurations to ensure proper enforcement of certificate validation. This vulnerability highlights the importance of thorough regression testing in security-critical components and demonstrates the need for continuous monitoring of security implementations to prevent re-introduction of previously addressed flaws. The issue aligns with CWE-284 Access Control and ATT&CK techniques related to bootkits and rootkits that leverage secure boot bypass mechanisms.