Linux Kernel Out-of-bounds Write Vulnerability Let Attackers Escalate Privileges

A severe vulnerability in the Linux kernel has remained undetected for nearly two decades, allowing local users to gain root privileges on affected systems. 

Designated as CVE-2025-0927, this out-of-bounds write vulnerability in the Linux kernel’s HFS+ filesystem driver affects systems running kernels up to version 6.12.0, with Ubuntu 22.04 with Linux Kernel 6.5.0-18-generic confirmed vulnerable.

Linux Kernel Out-of-Bounds Write Vulnerability 

According to SSD advisory, the flaw exists in the HFS+ driver, which supports Apple’s legacy file system format that was the primary MacOS X filesystem until replaced by APFS in 2017. 

The vulnerability has persisted since the initial git repository build 1da177 in 2005, remaining undetected in Linux kernels since version 2.6.12-rc2.

At the core of this vulnerability is a buffer overflow in B-tree node processing. The vulnerable function hfs_bnode_read_key in fs/hfsplus/bnode.c fails to properly validate boundary conditions regarding key sizes:

In this code, the function does not implement proper bounds checking, allowing attackers to trigger an out-of-bounds write that can corrupt kernel memory.

What makes this vulnerability particularly concerning is its accessibility to standard users. 

While mounting filesystems typically requires elevated privileges, modern Linux distributions like Ubuntu come with default polkit rules that allow users with active local sessions to mount filesystems through the udisks2 service.

An attacker exploiting this vulnerability would:

• Create a specially crafted HFS+ filesystem with intentionally malformed B-tree structures.
• Mount this malicious filesystem using standard user permissions.
• Trigger the vulnerability by setting an extended attribute (using setxattr).
• Use sophisticated heap manipulation techniques to achieve memory corruption.
• Bypass KASLR (Kernel Address Space Layout Randomization) by leaking kernel addresses.
• Ultimately achieve privilege escalation by overwriting sensitive kernel structures, such as modprobe_path.

Mitigations

This vulnerability affects numerous Linux distributions running vulnerable kernel versions. Users with active local sessions can exploit it to gain root privileges, potentially compromising the entire system.

Ubuntu has released a security advisory and fix for CVE-2025-0927. System administrators are strongly encouraged to apply all available security updates immediately.

Security experts note that this vulnerability highlights the importance of rigorous code auditing, even in legacy components. 

Despite previous fuzzing efforts, this particular vulnerability remained undetected, demonstrating that manual code analysis continues to be essential for identifying certain classes of security flaws.

This newly discovered vulnerability represents a significant security concern for Linux systems. Users and administrators should ensure their systems are updated with the latest security patches to mitigate this threat.

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