How to Fix CVE-2026-30838: Cross-site scripting in commonmark

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

CVE-2026-30838 is a cross-site scripting in thephpleague commonmark. The fix is to apply the vendor patch noted below.

What is CVE-2026-30838?

league/commonmark is a PHP Markdown parser. Prior to version 2.8.1, the DisallowedRawHtml extension can be bypassed by inserting a newline, tab, or other ASCII whitespace character between a disallowed HTML tag name and the closing >. For example, <script\n> would pass through unfiltered and be rendered as a valid HTML tag by browsers. This is a cross-site scripting (XSS) vector for any application that relies on this extension to sanitize untrusted user input. All applications using the DisallowedRawHtml extension to process untrusted markdown are affected. Applications that use a dedicated HTML sanitizer (such as HTML Purifier) on the rendered output are not affected. This issue has been patched in version 2.8.1. The CVSS base score is 5.1 (Medium). The official advisory is at https://github.com/thephpleague/commonmark/security/advisories/GHSA-4v6x-c7xx-hw9f.

Am I affected?

Check the version of commonmark you are running and compare it against the Affected row above (commonmark < 2.8.1). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

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

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

How to fix CVE-2026-30838

Upgrade commonmark 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 commonmark
dpkg -s commonmark | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

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

SUSE / openSUSE

sudo zypper refresh
sudo zypper update commonmark
rpm -q commonmark

Node.js / npm

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

PHP / Composer

composer update thephpleague/commonmark 2>/dev/null || composer update
composer show | head -20

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-30838 affecting commonmark
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-30838-fix-$(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"
    $pkg = Get-Package -Name "*commonmark*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "commonmark" --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 "commonmark"
    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-2026-30838 affecting commonmark
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-30838-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 commonmark 2>/dev/null | grep -i ^Version || echo "commonmark not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q commonmark || echo "commonmark not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-30838-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/commonmark /etc/commonmark.d /etc/commonmark.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 commonmark
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y commonmark
elif command -v yum >/dev/null; then
    yum update -y commonmark
elif command -v zypper >/dev/null; then
    zypper --non-interactive update commonmark
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s commonmark 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q commonmark
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-2026-30838" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-30838"

Disable the affected service (Linux)

sudo systemctl stop commonmark 2>/dev/null || true
sudo systemctl disable commonmark 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 commonmark 2>/dev/null | grep -i ^Version
rpm -q commonmark 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 "*commonmark*" | 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

Related fixes

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

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Is CVE-2026-30838 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-2026-30838?

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-2026-30838?

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

Where is the official advisory for CVE-2026-30838?

The vendor advisory is at https://github.com/thephpleague/commonmark/security/advisories/GHSA-4v6x-c7xx-hw9f. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-30838.

References

• Official vendor advisory: https://github.com/thephpleague/commonmark/security/advisories/GHSA-4v6x-c7xx-hw9f
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-30838
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

*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.*