⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 7.1

How to Fix CVE-2026-39972: Cwe-1289: improper validation of unsafe equivalence in mercure

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

⚡ At a glance
Severity7.1 (High)
Actively exploited?No public listing in CISA KEV
Affecteddunglas mercure < 0.22.0
Fixed inmercure 0.22.0
Type (CWE)CWE-1289: CWE-1289: Improper Validation of Unsafe Equivalence in Input

What is CVE-2026-39972?

Mercure is a protocol for pushing data updates to web browsers and other HTTP clients in a battery-efficient way. Prior to 0.22.0, a cache key collision vulnerability in TopicSelectorStore allows an attacker to poison the match result cache, potentially causing private updates to be delivered to unauthorized subscribers or blocking delivery to authorized ones. The cache key was constructed by concatenating the topic selector and topic with an underscore separator. Because both topic selectors and topics can contain underscores, two distinct pairs can produce the same key.

Am I affected?

Run the version check that matches your platform:


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

Compare what you see against the Affected row above (dunglas mercure < 0.22.0). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-39972

The primary fix is to upgrade mercure to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: mercure 0.22.0.

Ubuntu / Debian


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

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper patch --category security
rpm -q mercure

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-39972 affecting mercure
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-39972-fix-$(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 mercure"
    $pkg = winget list --id "mercure" 2>$null
    Write-Host $pkg

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

    Write-Host "[3/4] Applying upgrade to mercure 0.22.0"
    winget upgrade --id "mercure" --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 "mercure"
    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-2026-39972 affecting mercure
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-39972-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 mercure 2>/dev/null | grep -i version || echo "mercure not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q mercure || echo "mercure not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-39972-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/mercure /etc/mercure.d /etc/mercure.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: mercure 0.22.0)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y mercure
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y mercure
elif command -v yum >/dev/null; then
    yum update -y mercure
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 mercure 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q mercure
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 mercure
sudo systemctl disable mercure

How to verify the fix worked


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


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

Expected: the reported version is at or above mercure 0.22.0. 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-2026-39972 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?

7.1 (high). 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.*