CVE-2026-53311: Linux Kernel FUSE Uninitialized Memory Denial of Service

Technical summary

CVE-2026-53311 addresses an uninitialized value vulnerability (CWE-908) in the Linux kernel's FUSE subsystem. The `fuse_dentry_revalidate()` function in fs/fuse/dir.c reads the `->d_time` field of a dentry structure without verifying it has been initialized. The vulnerability manifests when `lookup_open()` allocates a new dentry via `__d_alloc()` and immediately calls `d_revalidate()` before the dentry's timestamp is set. KMSAN (Kernel Memory Sanitizer) detected this pattern, which allows kernel code to operate on uninitialized stack or heap memory. The consequences depend on the garbage value: revalidation logic may fail unpredictably, leading to cache coherency issues or kernel panics.

Business impact

This vulnerability primarily threatens availability of systems running vulnerable Linux kernels. Denial of service is achievable by local, unprivileged users through targeted file operations on FUSE-mounted filesystems. While no remote exploitation path exists, systems relying on FUSE for custom storage backends (network filesystems, object storage, databases) may experience unexpected crashes or filesystem unavailability during normal operation. Repeated triggering could be weaponized for sustained service disruption on multi-user systems or containerized environments where user isolation is implemented.

Affected systems

The vulnerability affects the Linux kernel, specifically the FUSE subsystem. Any system running an affected kernel version that uses FUSE mounts is potentially impacted. This includes distributions deploying FUSE for unionfs, sshfs, encfs, or proprietary cloud storage integrations. Container orchestration platforms (Kubernetes, Docker) running on vulnerable kernels with FUSE-backed storage are at risk. The exposure depends on kernel version; verify your kernel version against vendor advisories to confirm whether the fix has been applied.

Exploitability

Exploitability is moderate in complexity but low in barrier to entry. A local, privileged user (or any user with access to FUSE mounts) can trigger the condition through ordinary file operations—opening, creating, or accessing files on a FUSE filesystem. No special tools or kernel knowledge is required; the vulnerability is triggered by normal filesystem semantics. The CVSS score of 5.5 (Medium) reflects the requirement for local access and lack of information disclosure or integrity impact, though availability impact is high. Remote exploitation is not feasible; attackers must have shell access or be able to interact with FUSE mounts directly.

Remediation

Remediation requires upgrading the Linux kernel to a version that includes the fix for this vulnerability. The kernel patch initializes the `->d_time` field during dentry allocation or adds a validity check before the field is accessed in `fuse_dentry_revalidate()`. Distributions will release patched kernel packages; check your vendor's security advisory for specific version numbers. Until patched, limiting local user access to FUSE-mounted filesystems reduces risk but does not eliminate it for privileged users.

Patch guidance

Contact your Linux distribution's security advisories and kernel maintainers for patched kernel versions. Canonical (Ubuntu), Red Hat (RHEL), Debian, and other distributors will release fixes as backports to their supported kernel series. Apply kernel updates via your package manager (`apt update && apt upgrade`, `yum update`, etc.) and reboot. Verify the running kernel version post-reboot with `uname -r`. For systems unable to reboot immediately, evaluate the risk of local FUSE access against business requirements; this is not a live-patchable vulnerability requiring kernel module reloading or restart.

Detection guidance

Detection at runtime is challenging without kernel instrumentation. Enable kernel logging with `dmesg` and watch for NULL pointer dereferences, use-after-free warnings, or

Why prioritize this

This vulnerability merits prioritization for systems with local user access and active FUSE filesystem usage. The combination of ease of exploitation (triggered by normal file operations), local-only attack surface (limiting exposure), and high availability impact justifies Medium severity. Organizations running FUSE-backed storage, shared hosting, or multi-tenant systems should prioritize patching; single-user workstations with no FUSE mounts can deprioritize. The fix is straightforward and carries low regression risk, making patching efficient.

Risk score, explained

CVSS 3.1 score of 5.5 reflects: Attack Vector Local (only local or shell access required), Attack Complexity Low (no special conditions needed), Privileges Required Low (standard user can trigger via FUSE interaction), User Interaction None (automatic upon filesystem operation), Scope Unchanged (no privilege boundary crossed), Confidentiality None (no data exposure), Integrity None (no data corruption), Availability High (denial of service via crash or hang). The score is stable and appropriate given the threat model.

Frequently asked questions