PatchSiren

PatchSiren cyber security CVE debrief

CVE-2026-45915 Linux CVE debrief

A vulnerability in the Linux kernel's FAT filesystem implementation could allow corrupted FAT images to trigger a kernel warning or potentially cause undefined behavior. The issue occurs when a directory inode has an incorrect link count (i_nlink) due to filesystem corruption. When rmdir is called on such a directory, the kernel unconditionally decrements the link count, which can drive i_nlink to zero and trigger a WARN_ON assertion in drop_nlink(). The fix adds a sanity check in vfat_rmdir() and msdos_rmdir() to only decrement the parent link count when it is at least 3, otherwise reporting a filesystem error. This prevents the underflow condition and provides proper error handling for corrupted filesystem states.

Vendor
Linux
Product
Unknown
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-27
Original CVE updated
2026-06-24
Advisory published
2026-05-27
Advisory updated
2026-06-24

Who should care

Linux system administrators, embedded device manufacturers using FAT filesystems, security teams monitoring kernel vulnerabilities, and organizations relying on FAT/VFAT/MS-DOS filesystem support in production environments

Technical summary

The vulnerability exists in the FAT filesystem driver's rmdir implementation. Corrupted FAT images can leave directory inodes with incorrect i_nlink values (e.g., 2 when subdirectories exist). The rmdir operation unconditionally calls drop_nlink(dir), which decrements the link count without bounds checking. This can cause i_nlink to underflow to zero, triggering WARN_ON in drop_nlink(). The fix introduces a sanity check: before decrementing, verify the parent directory's link count is at least 3. If not, report a filesystem error instead of proceeding with the underflow-prone operation. This affects both vfat_rmdir() and msdos_rmdir() code paths.

Defensive priority

medium

Recommended defensive actions

  • Apply kernel updates containing the referenced stable commits when available from your Linux distribution
  • Monitor filesystem integrity on systems using FAT/VFAT/MS-DOS filesystems
  • Consider using filesystem integrity checking tools (fsck) before mounting untrusted FAT images
  • Review system logs for FAT filesystem errors that may indicate corruption
  • If running custom kernel builds, cherry-pick the relevant stable commits for your kernel version

Evidence notes

CVE published 2026-05-27. Multiple stable kernel commits provided indicating backports to various kernel versions. No CVSS score or severity assigned by NVD at time of disclosure. Not listed in CISA KEV.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-45915 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-45915

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-45915 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-45915

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/17866f8a0822d414cb02e621cf003a7d04396ef8

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/7fe0de287e931e07cb96ecf1f449b2ebdb0e1115

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/8cafcb881364af5ef3a8b9fed4db254054033d8a

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/955c5d670b5ae07c78f4345e23a895638db96ce1

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/9894c79fd9466612d0514be157b5c30cd93aa645

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/cd569b87378b9c33ae13c23d6bb9d205d66f7c4b

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

  • Source reference

    Unverified legacy reference

    URL: https://git.kernel.org/stable/c/d0bb592fa9def2bace90ac8926c0a1d6fa8c1aa0

    416baaa9-dc9f-4396-8d5f-8c081fb06d67

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.