CVE-2026-31583: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31583 Overview

CVE-2026-31583 is a use-after-free vulnerability in the Linux kernel's em28xx USB media driver. The flaw resides in em28xx_v4l2_open(), which reads the dev->v4l2 pointer without holding dev->lock. This creates a race condition with em28xx_v4l2_init()'s error path and em28xx_v4l2_fini(), both of which free the em28xx_v4l2 struct and set dev->v4l2 to NULL while holding the lock. Local attackers with access to the device node can trigger memory corruption or a NULL pointer dereference. The issue is classified under CWE-416 (Use After Free) and affects multiple Linux kernel branches.

Critical Impact
Local attackers can exploit the race to trigger use-after-free in v4l2_fh_init() or a NULL pointer dereference in em28xx_resolution_set(), leading to kernel memory corruption, denial of service, or potential local privilege escalation.

Affected Products

Linux Kernel (multiple stable branches prior to the fix commits)
Distributions packaging the em28xx V4L2 media driver
Systems with em28xx-based USB video capture hardware exposed to local users

Discovery Timeline

2026-04-24 - CVE-2026-31583 published to NVD
2026-04-27 - Last updated in NVD database

Technical Details for CVE-2026-31583

Vulnerability Analysis

The em28xx driver provides V4L2 (Video4Linux2) support for Empia em28xx-based USB video capture devices. When userspace opens the V4L2 device node, the kernel invokes em28xx_v4l2_open(). This handler dereferences dev->v4l2 to initialize the file handle and configure capture resolution.

The handler reads dev->v4l2 without acquiring dev->lock first. Concurrently, two other code paths free the em28xx_v4l2 structure under the lock and reset dev->v4l2 to NULL. The error path in em28xx_v4l2_init() performs this cleanup when initialization fails. The teardown routine em28xx_v4l2_fini() does the same when the driver detaches from the device.

Two distinct issues result from this race. First, v4l2_fh_init() can dereference vdev->ctrl_handler through a video_device embedded inside the already-freed em28xx_v4l2 structure. Second, em28xx_resolution_set() can dereference v4l2->norm after dev->v4l2 has been set to NULL.

Root Cause

The root cause is missing synchronization on the read side of a shared pointer. The writers correctly serialize the free and NULL assignment under dev->lock, but the reader in em28xx_v4l2_open() proceeds without acquiring the lock. The fix moves the mutex_lock() call ahead of the dev->v4l2 read and adds a NULL check while the lock is held.

Attack Vector

Exploitation requires local access and permission to open the /dev/video* node exposed by the em28xx driver. An attacker triggers the race by repeatedly opening the device node while the driver is being torn down or while initialization is failing. Successful exploitation produces a use-after-free condition in kernel memory, which can be shaped into a denial of service or, with additional primitives, kernel memory corruption. The attack surface is constrained to systems where em28xx hardware is present and the kernel module is loaded.

No verified public proof-of-concept code is available. Technical details can be reviewed in the upstream commits referenced under Linux Kernel Commit dd2b888 and Linux Kernel Commit 871b8ea.

Detection Methods for CVE-2026-31583

Indicators of Compromise

Kernel oops or panic messages referencing em28xx_v4l2_open, v4l2_fh_init, or em28xx_resolution_set in dmesg or /var/log/kern.log.
KASAN reports identifying use-after-free reads against em28xx_v4l2 allocations.
Unexpected NULL pointer dereference traces originating from the V4L2 subsystem during device hotplug or driver unload.

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) on test kernels to surface use-after-free conditions in the em28xx driver paths.
Audit installed kernel versions against the patched commits listed in the upstream stable tree to identify unpatched hosts.
Monitor process activity for unprivileged users repeatedly opening /dev/video* nodes coincident with USB hotplug events.

Monitoring Recommendations

Centralize kernel log collection and alert on BUG:, KASAN, or general protection fault entries that name V4L2 or em28xx symbols.
Track loaded kernel modules across the fleet and flag hosts still loading vulnerable em28xx builds.
Review udev rules and device node permissions to confirm only trusted accounts can access V4L2 capture devices.

How to Mitigate CVE-2026-31583

Immediate Actions Required

Upgrade affected systems to a Linux kernel build that includes the upstream fix commits, or apply distribution-provided security updates.
Where patching is not immediately feasible, unload the em28xx and em28xx_v4l2 modules on hosts that do not require em28xx capture hardware.
Restrict access to /dev/video* device nodes to trusted users and groups using filesystem permissions or udev rules.

Patch Information

The Linux kernel maintainers resolved the race by moving the mutex_lock(&dev->lock) call before the dev->v4l2 read and adding a NULL check under the lock. Fixed builds are available in the following upstream commits: Linux Kernel Commit 5fb2940, Linux Kernel Commit 6b9e664, Linux Kernel Commit 871b8ea, Linux Kernel Commit a66485a, Linux Kernel Commit b5d141e, and Linux Kernel Commit dd2b888.

Workarounds

Blacklist the em28xx module on systems that do not use Empia-based USB capture devices to remove the vulnerable code path entirely.
Tighten device node permissions so that only privileged service accounts can open V4L2 nodes, reducing the local attack surface.
Disable USB hotplug for untrusted ports on multi-user systems to limit exposure to the race window during driver init and teardown.

bash

# Blacklist the vulnerable module until the kernel is patched
echo "blacklist em28xx" | sudo tee /etc/modprobe.d/em28xx-blacklist.conf
echo "blacklist em28xx_v4l2" | sudo tee -a /etc/modprobe.d/em28xx-blacklist.conf
sudo rmmod em28xx_v4l2 em28xx 2>/dev/null
sudo update-initramfs -u