CVE-2026-55392: NILFS Utilities Superblock Validation DoS Vulnerability

Technical summary

The nilfs_sb_is_valid() function in NILFS utilities through version 2.3.0 fails to properly validate the s_log_block_size field within the NILFS2 superblock structure before using it in bit-shift operations. An attacker crafting a malicious NILFS2 image with an out-of-range or oversized value for this field can trigger undefined behavior—either through bit shifts that exceed safe boundaries or by causing memory allocation failures. This results in crashes of tools such as nilfs-tune and dumpseg. The issue was addressed in commit 26efb5d.

Business impact

Organizations using NILFS2 as a filesystem face operational risk during routine maintenance and forensic analysis. System administrators running tools like nilfs-tune or dumpseg could experience unexpected service interruptions if they inadvertently mount or analyze a malicious NILFS2 volume. The impact is primarily availability-focused; no data corruption, unauthorized access, or information disclosure occurs. However, repeated crashes during critical administrative tasks could complicate disaster recovery and backup validation workflows.

Affected systems

NILFS utilities versions 2.3.0 and earlier are affected. NILFS2 is a log-structured filesystem primarily used in specialized and legacy Linux deployments, making this a relatively narrow but important exposure for affected organizations. Any system with NILFS utilities installed and exposed to untrusted disk images is at risk.

Exploitability

Exploitation requires local access or the ability to supply a crafted NILFS2 image to a targeted system. The CVSS vector reflects local attack surface (AV:L), low complexity (AC:L), no privilege requirements (PR:N), and user interaction (UI:R) in the form of mounting or processing the malicious filesystem. Remote exploitation is not feasible; this is fundamentally a local denial-of-service issue triggered by administrator actions on untrusted media.

Remediation

Update NILFS utilities to a version newer than 2.3.0 that includes commit 26efb5d or later. Consult your Linux distribution's security advisories and package repositories for patched releases. In the interim, exercise caution when mounting or analyzing NILFS2 volumes from untrusted sources, and avoid running NILFS administration tools against unverified filesystem images.

Patch guidance

Verify your current NILFS utilities version with `nilfs-tune --version` or `dumpseg --version`. Check your distribution's package repositories (e.g., apt, yum, zypper) for updates beyond version 2.3.0. If your vendor has not yet released a patched package, monitor their security advisories closely. Users of source-based distributions should cherry-pick commit 26efb5d or rebuild from a newer upstream release. Test any update in a non-production environment first to ensure compatibility with your NILFS2 deployments.

Detection guidance

Monitor system logs for unexpected crashes of nilfs-tune, dumpseg, and other NILFS utilities, particularly when processing newly discovered or external NILFS2 images. Kernel logs may also show relevant error messages. If you manage NILFS2 filesystems, maintain an inventory of which systems have NILFS utilities installed and verify their versions against your patch timeline. Consider restricting which users and systems can process untrusted NILFS2 volumes.

Why prioritize this

Although this vulnerability carries a MEDIUM CVSS score and is not yet listed in CISA's Known Exploited Vulnerabilities catalog, it merits timely attention because administrative tools are often run with elevated privileges and crashes in filesystem utilities can disrupt critical operations. Organizations relying on NILFS2 should patch within their standard maintenance windows. Those without active NILFS2 deployments may defer patching but should not ignore it indefinitely.

Risk score, explained

The CVSS 3.1 score of 5.5 (MEDIUM) reflects high availability impact (A:H) but is tempered by local-only attack surface (AV:L), the requirement for user interaction (UI:R), and the absence of confidentiality or integrity violations (C:N, I:N). The vulnerability does not grant code execution or privilege escalation, limiting its severity. However, in environments where NILFS2 is operationally critical, the denial-of-service impact may warrant faster remediation than the base score alone suggests.

Frequently asked questions