CVE-2021-43614 in H2O
Summary
by MITRE • 09/03/2026
Error in handling the PlatformLangCodes UEFI variable could cause a buffer overflow, leading to resource exhaustion and failure.
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Analysis
by VulDB Data Team • 09/03/2026
The vulnerability described involves an improper validation or bounds checking mechanism within the implementation of the PlatformLangCodes Unified Extensible Firmware Interface (UEFI) variable. This specific configuration variable is used by firmware to define the supported languages for system interfaces, such as boot menus and setup utilities. When a malicious actor with physical access or elevated privileges can manipulate this variable, they may supply input data that exceeds the allocated memory buffer size designated for storing language codes. The core technical flaw lies in the failure of the firmware code to verify the length of the incoming string against the predefined limits before copying it into the fixed-size buffer. This lack of rigorous boundary checks allows an attacker to write beyond the end of the buffer, overwriting adjacent memory regions that may contain critical control data or other variables stored in non-volatile storage.
The operational impact of this buffer overflow is significant and multifaceted. Initially, the immediate consequence is resource exhaustion as the system attempts to process malformed data structures, potentially leading to a crash or halt during the boot sequence. More critically, if the overwritten memory contains return addresses or function pointers within the firmware execution environment, an attacker could achieve arbitrary code execution with high privileges at the pre-operating system stage. This compromises the integrity of the entire computing platform because UEFI runs before any operating system security controls are active. The failure to handle this input correctly effectively breaks the trust boundary between user-configurable settings and core firmware logic, allowing for persistent compromise that survives reboots and OS reinstalls unless the flash memory is explicitly rewritten with clean firmware images.
From a classification perspective, this vulnerability aligns closely with CWE-120 Buffer Copy without Checking Size of Input (Classic Buffer Overflow) and potentially CWE-787 Out-of-bounds Write if the overflow results in writing to protected regions. In terms of adversary tactics, exploitation would fall under MITRE ATT&CK technique T1542.003 Pre-OS Boot Compromise, specifically targeting UEFI firmware components like NVRAM variables. This allows attackers to establish persistence and evade detection by standard endpoint security solutions that operate within the operating system context. The attack vector typically requires physical access or prior compromise of a user account with permission to modify BIOS/UEFI settings, highlighting the importance of hardware-level security features such as Secure Boot and write-protection mechanisms for NVRAM variables.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. Firmware vendors should immediately release patches that enforce strict length validation on all inputs written to PlatformLangCodes and similar UEFI variables. Developers must implement safe string handling functions that explicitly check buffer boundaries before any copy operations occur. For system administrators, enabling Secure Boot is a critical defense-in-depth measure as it prevents the execution of unauthorized or modified firmware code during boot. Additionally, configuring BIOS/UEFI setup passwords restricts access to variable modification settings, thereby reducing the attack surface for local attackers. Regularly updating firmware ensures that known vulnerabilities in NVRAM handling are addressed and helps maintain the integrity of the platform's security posture against evolving threats targeting pre-boot environments.