⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 4

How to Fix CVE-2026-40395: Allocation of resources without limits in Varnish Enterprise

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

⚡ At a glance
Severity4 (Medium)
Actively exploited?No public listing in CISA KEV
Affectedvarnish-software Varnish Enterprise 6.0.9r5 to <6.0.16r12
Fixed inVarnish Enterprise 6.0.16r12
Type (CWE)CWE-770: Allocation of Resources Without Limits or Throttling

What is CVE-2026-40395?

Varnish Enterprise before 6.0.16r12 allows a "workspace overflow" denial of service (daemon panic) for shared VCL. The headerplus.write_req0() function from vmod_headerplus updates the underlying req0, which is normally the original read-only request from which req is derived (readable and writable from VCL). This is useful in the active VCL, after amending req, to prepare a refined req0 before switching to a different VCL with the return (vcl(<label>)) action. This is for example how the Varnish Controller operates shared VCL deployments.

Am I affected?

Run the version check that matches your platform:


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

Compare what you see against the Affected row above (varnish-software Varnish Enterprise 6.0.9r5 to <6.0.16r12). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-40395

The primary fix is to upgrade Varnish Enterprise to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: Varnish Enterprise 6.0.16r12.

Ubuntu / Debian


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

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --security varnish -y
rpm -q varnish

SUSE / openSUSE


sudo zypper patch --category security
rpm -q varnish

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-40395 affecting Varnish
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-40395-fix-$(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 Varnish"
    $pkg = winget list --id "Varnish" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to Varnish Enterprise 6.0.16r12"
    winget upgrade --id "Varnish" --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 "Varnish"
    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-2026-40395 affecting Varnish
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-40395-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 varnish 2>/dev/null | grep -i version || echo "varnish not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q varnish || echo "varnish not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-40395-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/varnish /etc/varnish.d /etc/varnish.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: Varnish Enterprise 6.0.16r12)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y varnish
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y varnish
elif command -v yum >/dev/null; then
    yum update -y varnish
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 varnish 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q varnish
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 varnish
sudo systemctl disable varnish

How to verify the fix worked


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


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

Expected: the reported version is at or above Varnish Enterprise 6.0.16r12. 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

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

Is CVE-2026-40395 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?

4 (medium). 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

*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.*