How to Fix CVE-2026-3610: Cross-site scripting in Mailinspector

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

CVE-2026-3610 is a cross-site scripting in HSC Cybersecurity Mailinspector. The fix is to upgrade to Mailinspector 5.4.0.

What is CVE-2026-3610?

A vulnerability was found in HSC Cybersecurity Mailinspector up to 5.3.2-3. Affected by this issue is some unknown functionality of the file /mailinspector/mliUserValidation.php of the component URL Handler. The manipulation of the argument error_description results in cross site scripting. The attack may be performed from remote. The exploit has been made public and could be used. Upgrading to version 5.4.0 can resolve this issue. You should upgrade the affected component. The vendor was contacted early and responded very professional: "We have already implemented the fix and made a hotfix available to affected customers, ensuring mitigation while the official release 5.4.0 has not yet been published. This allows customers to address the issue immediately, outside the regular release cycle." The CVSS base score is 5.3 (Medium). The official advisory is at https://vuldb.com/?id.349219.

Am I affected?

Check the version of Mailinspector you are running and compare it against the Affected row above (Mailinspector 5.3.2-3). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

# Linux package check
dpkg -s mailinspector 2>/dev/null | grep -i ^Version
rpm -q mailinspector 2>/dev/null
command -v mailinspector >/dev/null && mailinspector --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*Mailinspector*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Mailinspector" 2>$null

How to fix CVE-2026-3610

Upgrade Mailinspector to a patched build: Mailinspector 5.4.0. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade mailinspector
dpkg -s mailinspector | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh mailinspector -y
# or for older releases:
sudo yum update mailinspector -y
rpm -q mailinspector

SUSE / openSUSE

sudo zypper refresh
sudo zypper update mailinspector
rpm -q mailinspector

Node.js / npm

# Update the affected package in your project
npm install Mailinspector@latest
npm ls Mailinspector
npm audit fix

PHP / Composer

composer update hsc cybersecurity/mailinspector 2>/dev/null || composer update
composer show | head -20

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-3610 affecting Mailinspector
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-3610-fix-$(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] Detect installed version"
    $pkg = Get-Package -Name "*Mailinspector*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to Mailinspector 5.4.0"
    winget upgrade --name "Mailinspector" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "Mailinspector"
    Write-Host "Patch 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-2026-3610 affecting Mailinspector
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-3610-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s mailinspector 2>/dev/null | grep -i ^Version || echo "mailinspector not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q mailinspector || echo "mailinspector not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-3610-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/mailinspector /etc/mailinspector.d /etc/mailinspector.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: Mailinspector 5.4.0)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y mailinspector
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y mailinspector
elif command -v yum >/dev/null; then
    yum update -y mailinspector
elif command -v zypper >/dev/null; then
    zypper --non-interactive update mailinspector
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s mailinspector 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q mailinspector
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)

# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)

New-NetFirewallRule -DisplayName "Block-CVE-2026-3610" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-3610"

Disable the affected service (Linux)

sudo systemctl stop mailinspector 2>/dev/null || true
sudo systemctl disable mailinspector 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.

# Linux
dpkg -s mailinspector 2>/dev/null | grep -i ^Version
rpm -q mailinspector 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true

# Windows
Get-Package -Name "*Mailinspector*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Mailinspector 5.4.0. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

Related fixes

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

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• How to Fix CVE-2026-24605: Critical Vulnerability in X Addons for Elementor , Critical Vulnerability in X Addons for Elementor
• How to Fix CVE-2026-2219: Cwe-835 loop with unreachable exit condition ('infinite loop') in dpkg , Cwe-835 loop with unreachable exit condition ('infinite loop') in dpkg
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Is CVE-2026-3610 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-3610?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-3610?

5.3 (Medium). Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P.

Where is the official advisory for CVE-2026-3610?

The vendor advisory is at https://vuldb.com/?id.349219. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-3610.

References

• Official vendor advisory: https://vuldb.com/?id.349219
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-3610
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: https://vuldb.com/?ctiid.349219
• Additional reference: https://vuldb.com/?submit.748710
• Additional reference: https://docs.google.com/document/d/1KI2SIDcVm5U3Yzo5tNT5YOwREQWe_5BJaWe-ctRmLoI/edit?usp=sharing

*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.*