CVE-2025-23150 is a vulnerability identified in the Linux kernel's ext4 filesystem implementation, specifically within the do_split function used during directory entry insertion (ext4_insert_dentry). The flaw is an off-by-one error that leads to an out-of-bounds memory access and subsequent use-after-free condition. This vulnerability was discovered through fuzz testing with Syzkaller, a kernel fuzzer, and reported by the Linux Verification Center. The root cause lies in the logic of a loop that iterates over directory entries to determine how to split directory blocks when inserting new entries. The loop counter 'i' can decrement to -1, causing the code to incorrectly handle the splitting of directory blocks when there are many long-named files in a single block. This results in memory corruption due to accessing memory beyond allocated bounds and freeing memory that is still in use. The kernel's Kernel Address Sanitizer (KASAN) detected this use-after-free during testing, indicating a serious memory safety issue. The vulnerability affects multiple recent Linux kernel versions, including various commits referenced by their hashes. The flaw is located in ext4's namei.c source file, which handles filesystem path name resolution and directory entry management. No known exploits are reported in the wild as of the publication date (May 1, 2025), and no CVSS score has been assigned yet. The vulnerability could potentially be triggered by local users or processes that perform filesystem operations involving symlinks or directory modifications on ext4 filesystems. The issue has been addressed by correcting the loop condition and splitting logic to prevent out-of-bounds access and use-after-free errors.

For European organizations, this vulnerability poses a significant risk primarily to systems running Linux with ext4 filesystems, which are widely used in servers, desktops, and embedded devices. Exploitation could allow local attackers or compromised processes to cause kernel crashes (denial of service) or potentially escalate privileges by corrupting kernel memory. This could lead to system instability, data corruption, or unauthorized access to sensitive information. Given the prevalence of Linux in critical infrastructure, cloud services, and enterprise environments across Europe, the impact could extend to service outages and compromise of confidential data. Organizations relying on ext4 for storage in data centers or cloud platforms may face operational disruptions. Moreover, the vulnerability could be leveraged as part of multi-stage attacks targeting Linux-based systems. Although no public exploits are known yet, the use-after-free nature of the bug makes it a candidate for exploitation by skilled attackers. The impact on confidentiality, integrity, and availability is therefore high, especially if combined with other vulnerabilities or misconfigurations.

1. Immediate patching: Apply the official Linux kernel patches that fix the off-by-one error in do_split and the use-after-free in ext4_insert_dentry as soon as they are released and tested for your environment. 2. Kernel version management: Upgrade to a Linux kernel version that includes the fix for CVE-2025-23150. Monitor kernel mailing lists and vendor advisories for updates. 3. Restrict local access: Limit local user privileges and access to systems running vulnerable kernels to reduce the risk of exploitation by untrusted users or processes. 4. Filesystem monitoring: Implement monitoring for unusual filesystem activity, especially related to directory modifications and symlink creation, which could indicate attempts to trigger the vulnerability. 5. Use security modules: Employ Linux Security Modules (LSMs) such as SELinux or AppArmor to enforce strict access controls on filesystem operations. 6. Testing and validation: Before deploying kernel updates, conduct thorough testing in staging environments to ensure stability and compatibility. 7. Incident response readiness: Prepare for potential exploitation by having incident response plans and forensic capabilities to detect and respond to kernel-level compromises. 8. Avoid untrusted code execution: Prevent execution of untrusted or malicious code that could attempt to exploit this vulnerability locally.