How to Fix CVE-2026-31421: Security vulnerability in Linux

By Sai Kiran Pandrala. Last verified: 2026-05-25.

What is CVE-2026-31421?

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_fw: fix NULL pointer dereference on shared blocks The old-method path in fw_classify() calls tcf_block_q() and dereferences q->handle. Shared blocks leave block->q NULL, causing a NULL deref when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified. Reject the configuration in fw_change() when the old method (no TCA_OPTIONS) is used on a shared block, since fw_classify()'s old-method path needs block->q which is NULL for shared blocks. The fixed null-ptr-deref calling stack: KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] RIP: 0010:fw_classify (net/sched/cls_fw.c:81) Call Trace: tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860) tc_run (net/core/dev.c:4401) __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

Am I affected?

Run the version check that matches your platform:

# Linux
dpkg -s linux-image-generic 2>/dev/null | grep -i version
rpm -q linux-image-generic 2>/dev/null
linux-image-generic --version 2>/dev/null

Compare what you see against the Affected row above (Linux 1abf272022cf1d18469405f47b4ec49c6a3125db to <d6d5bd62a09650856e1e2010eb09853eba0d64e1, 1abf272022cf1d18469405f47b4ec49c6a3125db to <febf64ca79a2d6540ab6e5e197fa0f4f7e84473e, 1abf272022cf1d18469405f47b4ec49c6a3125db to <3d41f9a314afa94b1c7c7c75405920123220e8cd, 1abf272022cf1d18469405f47b4ec49c6a3125db to <18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28, 1abf272022cf1d18469405f...). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-31421

The primary fix is to upgrade Linux to the patched build. Use the commands for your platform below; the patched versions listed in the vendor advisory are: Linux d6d5bd62a09650856e1e2010eb09853eba0d64e1; Linux febf64ca79a2d6540ab6e5e197fa0f4f7e84473e; Linux 3d41f9a314afa94b1c7c7c75405920123220e8cd; Linux 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28 (and 4 more SKUs, see vendor advisory).

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade linux-image-generic
linux-image-generic --version 2>/dev/null || dpkg -s linux-image-generic | grep -i version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --security linux-image-generic -y
rpm -q linux-image-generic

SUSE / openSUSE

sudo zypper patch --category security
rpm -q linux-image-generic

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-31421 affecting Linux kernel
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-31421-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detecting installed version of Linux kernel"
    $pkg = winget list --id "Linux_kernel" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\Linux_kernel-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Linux_kernel" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to Linux d6d5bd62a09650856e1e2010eb09853eba0d64e1; Linux febf64ca79a2d6540ab6e5e..."
    winget upgrade --id "Linux_kernel" --silent --accept-source-agreements --accept-package-agreements
    # Fallback: Windows Update for OS-level fixes
    if ($LASTEXITCODE -ne 0) {
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verifying patched build"
    winget list --id "Linux_kernel"
    Write-Host "Fix applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)

#!/usr/bin/env bash
# Fix script for CVE-2026-31421 affecting Linux kernel
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-31421-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detecting installed version"
if command -v dpkg >/dev/null; then
    dpkg -s linux-image-generic 2>/dev/null | grep -i version || echo "linux-image-generic not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q linux-image-generic || echo "linux-image-generic not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-31421-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/linux-image-generic /etc/linux-image-generic.d /etc/linux-image-generic.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: Linux d6d5bd62a09650856e1e2010eb09853eba0d64e1; Linux febf64ca79a2d6540ab6e5e...)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y linux-image-generic
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y linux-image-generic
elif command -v yum >/dev/null; then
    yum update -y linux-image-generic
elif command -v zypper >/dev/null; then
    zypper --non-interactive patch --category security
fi

echo "[4/4] Verifying patched build"
if command -v dpkg >/dev/null; then
    dpkg -s linux-image-generic 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q linux-image-generic
fi
echo "Done. Restart any running daemons that loaded the old library."

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Network restriction (Linux, nftables)

# Block inbound traffic to the affected service from untrusted networks
sudo nft add table inet filter
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }'
sudo nft 'add rule inet filter input tcp dport {443, 80} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Service-level fallback

# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop linux-image-generic
sudo systemctl disable linux-image-generic

How to verify the fix worked

# Linux
linux-image-generic --version 2>/dev/null || dpkg -s linux-image-generic | grep -i version
rpm -q linux-image-generic 2>/dev/null || true

# Windows
winget list | findstr /I "linux-image-generic"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Linux d6d5bd62a09650856e1e2010eb09853eba0d64e1; Linux febf64ca79a2d6540ab6e5e197fa0f4f7e84473e; Linux 3d41f9a314afa94b1c7c7c75405920123220e8cd; Linux 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28 (and 4 more SKUs, see vendor advisory). Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Related fixes

Other vulnerabilities in the same area that are worth patching alongside this one:

• How to Fix CVE-2026-23180: Critical Vulnerability in Linux — Critical Vulnerability in Linux
• How to Fix CVE-2026-23013: Critical Vulnerability in Linux — Critical Vulnerability in Linux
• How to Fix CVE-2026-43273: Security Vulnerability in Linux , Security Vulnerability in Linux
• How to Fix CVE-2026-31687: gpio: omap: do not register driver in Linux , gpio: omap: do not register driver in Linux
• How to Fix CVE-2026-31544: firmware: arm_scmi: Fix NULL dereference on notify error path in Linux , firmware: arm_scmi: Fix NULL dereference on notify error path in Linux

Is CVE-2026-31421 actually being exploited?

According to the data sources above, no public confirmation of in-the-wild exploitation at this time. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

The rating in the advisory. Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

• Official vendor advisory: https://git.kernel.org/stable/c/d6d5bd62a09650856e1e2010eb09853eba0d64e1
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31421
• https://git.kernel.org/stable/c/febf64ca79a2d6540ab6e5e197fa0f4f7e84473e
• https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd
• https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28
• https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*