Urgent Advisory: wp-mpdf ≤ 3.9.1 Cross-Site Scripting Vulnerability (CVE-2025-60040) – Immediate Actions for WordPress Site Owners

Authored by Managed-WP Security Experts | Date: 2025-09-26

Executive Summary

Managed-WP’s cybersecurity team has identified a Cross-Site Scripting (XSS) vulnerability impacting the popular WordPress plugin wp-mpdf in all versions up to and including 3.9.1 (CVE-2025-60040). The vulnerability is addressed in version 3.9.2. Although classified as low urgency, XSS vulnerabilities can enable attackers to execute malicious scripts, potentially leading to session compromise, content injection, or privilege escalation.

This advisory delivers a comprehensive breakdown including risk analysis, detection strategies, immediate mitigation steps—including virtual patching and firewall rules—and developer guidance to ensure your site’s security posture remains robust.

筆記： The guidance herein assumes a working knowledge of WordPress administration and core security practices.

Vulnerability Overview

• An XSS vulnerability exists within wp-mpdf versions ≤ 3.9.1, identified as CVE-2025-60040.
• The vulnerability permits injection of malicious JavaScript in specific plugin parameters, allowing script execution in the context of site visitors or authenticated users.
• Exploitation typically requires contributor-level privileges or higher.
• Official patch released in wp-mpdf version 3.9.2; updating immediately is strongly advised.
• While public CVSS scores assign it a low priority, exploitation risk remains for targeted attacks and chained exploit scenarios.

Affected Parties

• All WordPress sites with active wp-mpdf plugin versions ≤ 3.9.1 are vulnerable.
• Sites allowing untrusted contributor roles or exposing plugin features to front-end users elevate their risk.
• Sites with editorial workflows that permit HTML content submissions by contributors are particularly at risk.

Impact Assessment

• Attack Vector: Persistent and reflected XSS that executes client-side scripts.
• Potential Consequences:
  • Session hijacking and credential theft.
  • Privilege escalation via malicious scripts within administrative interfaces.
  • Content manipulation leading to phishing or reputation damage.
• Risk Context: Lower urgency rating does not equate to negligible risk. Attackers can leverage this vulnerability in conjunction with other compromises.

Immediate Remediation Steps

1. Create a complete backup of your site files and database without delay.
2. Update the wp-mpdf plugin to version 3.9.2 immediately. If update is not possible, consider removing the plugin.
3. Apply virtual patching via Web Application Firewall (WAF) rules to block malicious requests targeting the vulnerability (rule examples provided below).
4. Audit user roles—remove or scrutinize unexpected contributor/editor accounts and reset passwords.
5. Scan for signs of compromise such as injected scripts, suspicious posts, and unexpected admin users.
6. Activate logging and monitoring on your webserver, WAF, and plugin level to detect exploit attempts.
7. For multi-site operators, deploy updates and WAF rules uniformly across your network.

Safe Update Procedure

• Via WordPress Dashboard: Navigate to Plugins → Installed Plugins, locate wp-mpdf, and select “Update now.”
• If managing multiple sites, consider enabling auto-updates specifically for wp-mpdf or leverage managed update tools.
• Command line: Use WP-CLI with wp plugin update wp-mpdf to expedite updates.
• Post-update: Clear caching layers such as page and CDN caches to prevent serving outdated code.

Virtual Patching and Firewall Guidance

When immediate plugin updates are unfeasible, a robust virtual patch via WAF can significantly reduce risk. The following are practical ModSecurity and Nginx+Lua rule examples. Test all rules in monitor mode before enforcement to minimize false positives.

ModSecurity Rule Template (adjust paths and parameters to your environment):

# Limit to wp-mpdf plugin endpoints
SecRule REQUEST_URI "@rx /wp-content/plugins/wp-mpdf/|/wp-admin/admin-ajax.php" "id:100001,phase:1,t:none,pass,initcol:global=GLOBAL_VARS,logdata:'Possible wp-mpdf XSS',chain"
    SecRule ARGS_NAMES|ARGS "@rx (title|content|mpdf_html|description|text|message)" "t:none,chain"
    SecRule ARGS|REQUEST_HEADERS|REQUEST_COOKIES "@rx (?i)(<script|javascript:|onerror\s*=|onload\s*=|eval\(|document\.cookie|window\.location|data:text/html|<img.+onerror=|<svg|<iframe)" "id:100002,phase:2,deny,status:403,log,msg:'Blocked wp-mpdf XSS attempt',severity:2"

# Block encoded script patterns
SecRule ARGS "@rx (?i)(%3Cscript|%3Csvg|%3Ciframe|%3Cimg%20).*" "id:100003,phase:2,deny,status:403,log,msg:'Blocked encoded script fragment',severity:2"
  

Nginx + Lua rule example:

local args = ngx.req.get_uri_args()
local suspicious = {"<script", "javascript:", "onerror=", "onload=", "eval(", "document.cookie", "window.location", "data:text/html"}
for k, v in pairs(args) do
  if type(v) == "table" then v = table.concat(v, " ") end
  local vs = string.lower(tostring(v))
  for _, s in ipairs(suspicious) do
    if string.find(vs, s, 1, true) then
      ngx.log(ngx.ERR, "Blocked suspicious param: ", k)
      return ngx.exit(403)
    end
  end
end
  

Recommended Developer Mitigations

1. Output Encoding: Always escape output using esc_html(), esc_attr()， 或者 wp_kses() with tightly controlled allowed tags.
2. 輸入驗證： Implement rigorous server-side validation with whitelisting strategies over blacklisting.
3. Security Nonces and Permissions: Validate nonces and user capabilities (e.g., check_admin_referer(), 當前使用者可以（）) on all actions.
4. Sanitize Stored HTML: 使用 wp_kses_post() or stricter on any stored markup outputs.
5. AJAX Endpoint Handling: Sanitize and validate all request inputs; return JSON responses safely with wp_send_json_success() / wp_send_json_error().

Code snippet example to sanitize stored HTML:

<?php
$raw_html = get_post_meta($post_id, 'mpdf_html', true);

$allowed_tags = array(
  'a' => array('href' => true, 'title' => true, 'rel' => true),
  'p' => array(),
  'br' => array(),
  'strong' => array(),
  'em' => array(),
);

$safe_html = wp_kses($raw_html, $allowed_tags);
echo $safe_html;
?>
  

Detection: Identifying Signs of Exploitation

• Unfamiliar script or iframe tags in posts/pages.
• Unexpected or escalated admin/contributor user accounts.
• Suspicious entries in wp_posts 或者 wp_postmeta containing script tags or XSS payloads.
• Redirects, popups, or injected advertisements on frontend content.
• File changes in themes, plugins, or uploads directories.
• Firewall logs with repeated script injection attempts.
• Suspicious scheduled tasks or outbound PHP-initiated connections.

Use WP-CLI to search for suspicious content:

# Posts with script tags
wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';"

# Postmeta containing suspicious content
wp db query "SELECT meta_id, meta_key FROM wp_postmeta WHERE meta_value LIKE '%document.cookie%' OR meta_value LIKE '%<script%';"
  

Incident Response Steps

1. Isolate impacted sites: enable maintenance mode or restrict public access.
2. Preserve logs: export all relevant server, firewall, and application logs.
3. Replace compromised files with clean copies from trusted sources or backups.
4. Reset all credentials and rotate API keys.
5. Thoroughly scan uploads and database for malicious scripts or backdoors.
6. Post-remediation, update all software and enable continuous monitoring.
7. Consider professional forensic assistance for extensive breaches.

Best Practices to Mitigate Future XSS Risk

• Enforce the principle of least privilege on contributor and editor roles.
• Implement trusted content workflows and sanitize all user inputs carefully.
• Deploy strict Content Security Policies (CSP) to restrict script execution origins.
• Use secure cookie flags (HTTP-only, SameSite) to protect sessions.
• Automate regular malware scanning and integrity checks.

How Managed-WP Supports Your Security

At Managed-WP, we empower WordPress site owners and operators with advanced tools and expert guidance:

• Tailored managed WAF rules to virtually patch vulnerabilities immediately.
• Continuous malware scanning and threat detection.
• Automated plugin updates for vulnerable components.
• Real-time alerts and centralized site management for multi-site administrators.

If you have Managed-WP services active, enable the virtual patching rules for wp-mpdf now to minimize exposure as you coordinate updates.

Verifying Your Plugin Status

• Via WP Admin: Plugins → Installed Plugins → Confirm presence and version of wp-mpdf.
• Via WP-CLI:
  • wp plugin list --status=active --format=table
  • wp plugin get wp-mpdf --field=version

Update immediately if the version is 3.9.1 or below.

Recommended Timeline for Remediation

Day 0 (Disclosure): Audit plugin versions; deploy WAF rules where updates can’t be immediate.

Day 1–3: Deploy updates broadly; scan for signs of exploitation; rotate credentials if needed.

Day 4–7: Review logs for exploitation attempts; strengthen CSP and cookie policies; communicate with stakeholders.

Ongoing: Maintain proactive scanning, firewall tuning, and editorial workflow hardening.

Example SQL for Content Clean-up (Backup Before Running)

Query to locate posts with potential XSS vectors:

SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';
  

Query to locate suspicious postmeta entries:

SELECT meta_id, post_id, meta_key FROM wp_postmeta WHERE meta_value LIKE '%<script%' OR meta_value LIKE '%document.cookie%';
  

Remove or sanitize malicious content only after full backups and offline analysis.

Communication Guidelines

• Document all remediation steps internally.
• For confirmed breaches, comply with all legal disclosure requirements and engage legal counsel.
• External messaging should be clear but avoid technical details that may aid attackers.

常見問題解答

Q: The CVSS score is low; is this a serious issue?
A: Yes. XSS is often a stepping stone for more damaging attacks—and contributor-level access lowers attacker barriers significantly.

Q: Are browser protections sufficient?
A: No. Rely on server-side fixes and WAF protections for best defense.

Q: Will strict firewall rules cause site issues?
A: Potentially, so tune rules carefully and validate in monitor mode prior to enforcement.

ModSecurity Rule Tuning Notes

• Assign unique rule IDs (use 10000+ range).
• Exclude trusted IP addresses where applicable.
• Use “nolog” in monitor mode to review traffic before blocking.
• Coordinate with your hosting provider or Managed-WP support for deployment assistance.

Final Recommendations

Despite the “low” priority tag, proactively updating to version 3.9.2 and applying virtual patches are critical to safeguarding your site. Maintain vigilant user privilege management and consistently sanitize all user-generated content to drastically reduce XSS risks.

Remember: robust and timely actions post-disclosure prevent attackers from gaining a foothold and protect your WordPress ecosystem from escalating threats.

Get Started with Managed-WP’s Free Security Plan

For dependable, ongoing protection while managing updates and incident response, consider Managed-WP’s free Basic plan—featuring managed firewall (WAF), malware scanning, OWASP Top 10 mitigations, and unlimited bandwidth. Enable a strong security baseline with minimal setup. Learn more at: https://my.wp-firewall.com/buy/wp-firewall-free-plan/

Need tailored protection? Managed-WP support can help generate and deploy customized ModSecurity rules tuned specifically for your wp-mpdf usage—tested in monitor mode against your live traffic. Contact support or enable virtual patching in your Managed-WP dashboard for immediate risk reduction.