CVE-2021-3981 in Grub2
Summary
by MITRE • 03/10/2022
A flaw in grub2 was found where its configuration file, known as grub.cfg, is being created with the wrong permission set allowing non privileged users to read its content. This represents a low severity confidentiality issue, as those users can eventually read any encrypted passwords present in grub.cfg. This flaw affects grub2 2.06 and previous versions. This issue has been fixed in grub upstream but no version with the fix is currently released.
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Analysis
by VulDB Data Team • 05/20/2025
The vulnerability identified as CVE-2021-3981 represents a significant configuration flaw within the GRand Unified Bootloader version 2 implementation that impacts system security posture through improper file permission handling. This issue specifically targets the grub.cfg configuration file which serves as the primary boot configuration file for systems utilizing GRUB2, making it a critical component in the system initialization process. The flaw exists in versions 2.06 and earlier, indicating that a substantial portion of systems deployed with GRUB2 would be potentially affected by this weakness. The vulnerability stems from the bootloader's failure to properly set file permissions during the creation of the grub.cfg file, creating an access control gap that allows unauthorized users to read sensitive system information.
The technical nature of this vulnerability aligns with CWE-732, which addresses improper permission assignment, and demonstrates how inadequate file system permission management can create security exposures within critical system components. The flaw operates at the operating system level where the GRUB2 bootloader creates its configuration file without establishing appropriate access controls, typically defaulting to world-readable permissions. This misconfiguration allows any user account on the system to access the grub.cfg file contents, which may contain encrypted password references, boot parameters, and other sensitive configuration data. The vulnerability is classified as low severity in terms of direct impact but represents a confidentiality issue that can lead to information disclosure, potentially exposing system administrators to credential exposure and attack surface expansion.
From an operational perspective, this vulnerability creates a pathway for attackers to gain insights into system boot configurations and potentially extract encrypted passwords stored within the grub.cfg file. The attack surface is particularly concerning as it affects systems that utilize full disk encryption, where the bootloader configuration might contain references to encryption keys or passphrases that could be exploited by adversaries with access to the system. The issue is particularly relevant in enterprise environments where multiple users may have access to systems running GRUB2, and where the exposure of bootloader configuration information could lead to privilege escalation attempts or targeted attacks against system encryption mechanisms. The fact that this vulnerability exists in widely deployed versions of GRUB2 means that organizations implementing full disk encryption solutions may be unknowingly exposing sensitive configuration data.
The mitigation approach for CVE-2021-3981 requires system administrators to implement immediate remediation measures by either upgrading to a patched version of GRUB2 once available or applying manual permission fixes to existing grub.cfg files. The vulnerability demonstrates the importance of proper access control implementation at all levels of system software, particularly in bootloaders that serve as gatekeepers to system initialization processes. Organizations should also consider implementing monitoring solutions to detect unauthorized access attempts to bootloader configuration files and establish baseline configurations that enforce appropriate file permissions. The issue highlights the necessity of maintaining updated bootloaders and understanding the security implications of configuration file creation processes. Additionally, system administrators should review their current bootloader configurations to ensure that no sensitive information is exposed through improper file permissions, and consider implementing additional security controls such as secure boot mechanisms or hardware-based protections to mitigate potential exploitation scenarios. The vulnerability serves as a reminder of the critical importance of file system permission management in maintaining system confidentiality and overall security posture.