Microsoft Discovers GRUB2, U-Boot, and Barebox Bootloader Flaws with Copilot

Microsoft has disclosed the discovery of multiple critical vulnerabilities within the GRUB2, U-Boot, and Barebox bootloaders, leveraging its AI-driven Security Copilot platform for advanced threat analysis.

These bootloaders, integral to the Unified Extensible Firmware Interface (UEFI) Secure Boot framework and widely deployed in embedded systems, were found to contain exploitable flaws that could compromise system integrity, enable privilege escalation, and bypass Secure Boot protections.

The findings have significant implications for device security across Linux-based systems and embedded environments.

Technical Analysis of Vulnerabilities

The vulnerabilities uncovered span critical areas of bootloader functionality, particularly in filesystem parsing routines.

In GRUB2, integer overflow vulnerabilities were identified in symbolic link handling within filesystem modules such as JFS, UDF, and HFS.

These flaws could allow attackers to craft malicious filesystems that trigger memory corruption or arbitrary code execution during bootloader execution.

Exploitation of these vulnerabilities poses a direct threat to Secure Boot mechanisms by enabling attackers to inject unauthorized code into the boot sequence or deploy persistent malware that survives system reinstallation.

Similarly, U-Boot and Barebox were found to share code-level vulnerabilities due to their reliance on overlapping codebases with GRUB2.

For instance, U-Boot exhibited a critical flaw (CVE-2025-26726) in its SquashFS directory parsing logic that could lead to buffer overflows under certain conditions.

Barebox inherited similar filesystem-related weaknesses due to shared architectural components.

While exploitation of these vulnerabilities in U-Boot and Barebox typically requires physical access to the device, their presence underscores systemic risks associated with code reuse across open-source projects.

Microsoft’s Security Copilot played a pivotal role in identifying these vulnerabilities by automating the analysis of high-risk code segments.

The AI-driven platform leveraged natural language processing (NLP) and machine learning models trained on vulnerability patterns to pinpoint exploitable areas within bootloader source code.

This approach significantly reduced manual auditing time while uncovering additional flaws that may have otherwise gone unnoticed.

In adherence to responsible disclosure practices, Microsoft engaged directly with the maintainers of GRUB2, U-Boot, and Barebox to facilitate remediation efforts.

Security patches addressing these vulnerabilities were released on February 18-19, 2025.

GRUB2 maintainers implemented additional security measures by disabling certain OS modules when Secure Boot is enabled and enhancing revocation management via updates to the SBAT (Secure Boot Advanced Targeting) mechanism.

The disclosed vulnerabilities are tracked under multiple CVEs, including CVE-2025-0677 for GRUB2’s integer overflow issue and CVE-2025-26726 for U-Boot’s SquashFS parsing flaw.

These updates underscore the importance of robust patch management practices within the open-source ecosystem.

Key Findings: Filesystem Vulnerabilities

Microsoft focused its analysis on filesystem functionalities within GRUB2 after Security Copilot flagged them as high-risk areas for potential vulnerabilities.

Using the JFFS2 filesystem as a test case, Security Copilot identified multiple security issues, including an integer overflow vulnerability that was confirmed through manual review.

This vulnerability allowed an attacker to manipulate symbolic link resolution in the JFS module, leading to memory corruption. Specifically:

• The size variable in the JFS symbolic link resolution function was vulnerable to overflow due to its definition as a 64-bit unsigned integer (uint64_t).
• An attacker could supply a malicious filesystem image with a maximum value for size (0xFFFFFFFFFFFFFFFF), causing an integer overflow during the size+1 calculation.
• This resulted in an allocation of a zero-byte memory chunk, which was subsequently overwritten with attacker-controlled data, enabling arbitrary memory corruption.

Similar vulnerabilities were found across other GRUB2 filesystem modules:

Module	Vulnerability	CVE
UFS	Integer overflow in symbolic link handling	CVE-2025-0677
Squash4	Integer overflow in file reads	CVE-2025-0678
ReiserFS	Integer overflow in symbolic link handling	CVE-2025-0684
JFS	Integer overflow in symbolic link handling	CVE-2025-0685
RomFS	Integer overflow in symbolic link handling	CVE-2025-0686
UDF	Out-of-bounds block reads	CVE-2025-0689
HFS	Wild strcpy usage on non-NUL-terminated strings during mounting	CVE-2024-56737

Microsoft also reported a cryptographic side-channel attack vulnerability (CVE-2024-56738) due to non-constant time memory comparisons in the grub_crypto_memcmp function.

Extending Analysis to Other Bootloaders

Variant analysis revealed that U-Boot and Barebox shared similar vulnerabilities due to code reuse from GRUB2. For example:

• U-Boot: SquashFS directory table parsing (CVE-2025-26726) and nested file reading buffer overflows were identified.
• Barebox: EXT4 symlink resolution (CVE-2025-26723) and CramFS symlink parsing flaws were detected.

While exploitation of these vulnerabilities often requires physical access in embedded systems, their presence underscores systemic risks associated with shared open-source codebases.

Vulnerabilities at this level can undermine critical security layers such as UEFI Secure Boot, which is designed to validate cryptographic signatures of bootloader binaries before execution.

Microsoft emphasized that while AI-driven tools like Security Copilot enhance defenders’ capabilities in identifying threats, they also raise concerns about adversarial use for vulnerability exploitation.

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