● High · CVSS 8

How to Fix CVE-2026-28405: Cross-site scripting in Markus

Q: What is the CVSS score for CVE-2026-28405?

8 (High). Vector: `CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H`.

Q: Where is the official advisory for CVE-2026-28405?

The vendor advisory is at [https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893](https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893). The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-28405.

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

CVE-2026-28405 is a cross-site scripting in MarkUsProject Markus. The fix is to apply the vendor patch noted below.

⚡ At a glance


Severity	8 (High)
Actively exploited?	No public listing in CISA KEV
Affected	Markus < 2.9.1
Fixed in	See vendor advisory
Type (CWE)	CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

What is CVE-2026-28405?

MarkUs is a web application for the submission and grading of student assignments. Prior to version 2.9.1, the courses/<:course_id>/assignments/<:assignment_id>/submissions/html_content route reads the contents of a student-submitted file and renders them without sanitization. This issue has been patched in version 2.9.1. The CVSS base score is 8 (High), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893.

Am I affected?

Check the version of Markus you are running and compare it against the Affected row above (Markus < 2.9.1). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


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


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

How to fix CVE-2026-28405

Upgrade Markus to a patched build: See vendor advisory. 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 markus
dpkg -s markus | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


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

SUSE / openSUSE


sudo zypper refresh
sudo zypper update markus
rpm -q markus

Node.js / npm


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

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-28405 affecting Markus
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-28405-fix-$(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"
    $pkg = Get-Package -Name "*Markus*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "Markus" --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 "Markus"
    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-2026-28405 affecting Markus
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-28405-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 markus 2>/dev/null | grep -i ^Version || echo "markus not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q markus || echo "markus not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-28405-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/markus /etc/markus.d /etc/markus.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

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

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s markus 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q markus
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-2026-28405" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-28405"

Disable the affected service (Linux)


sudo systemctl stop markus 2>/dev/null || true
sudo systemctl disable markus 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 markus 2>/dev/null | grep -i ^Version
rpm -q markus 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 "*Markus*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

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

Frequently asked questions

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

How to Fix CVE-2026-3904: CWE-366 Race condition within a thread in glibc — CWE-366 Race condition within a thread in glibc
How to Fix CVE-2026-33156: Critical Vulnerability in ScreenToGif — Critical Vulnerability in ScreenToGif
How to Fix CVE-2026-26328: Access Control Bypass in openclaw , Access Control Bypass in openclaw
How to Fix CVE-2026-34408: Weak Password Recovery Mechanism for Forgotten Password in the affected product , Weak Password Recovery Mechanism for Forgotten Password in the affected product
How to Fix CVE-2026-3393: Heap buffer overflow in soloud , Heap buffer overflow in soloud

Is CVE-2026-28405 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-2026-28405?

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-2026-28405?

8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H.

Where is the official advisory for CVE-2026-28405?

The vendor advisory is at https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-28405.

References

Official vendor advisory: https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893
NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-28405
CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
Additional reference: https://github.com/MarkUsProject/Markus/commit/55d74f2ddb72d2ec2f29aa2b4cb6b2da10755036
Additional reference: https://github.com/MarkUsProject/Markus/releases/tag/v2.9.1

*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.*