Urgent: wp-mpdf <= 3.9.1 XSS (CVE-2025-60040) — What Site Owners Need to Know and Do Now

Written by WP‑Firewall Security Team | Date: 2025-09-26

Aperçu

A Cross-Site Scripting (XSS) vulnerability was disclosed for the WordPress plugin wp‑mpdf affecting versions <= 3.9.1 (CVE-2025-60040). The issue has been fixed in version 3.9.2. As WordPress site owners and security operators, you should treat XSS issues seriously — even lower-severity XSS can be chained into more impactful attacks (session theft, administrative account takeover via CSRF+XSS, content injection, or phishing).

This article is written from the WP‑Firewall perspective: we explain the exposure, risk, detection, practical mitigations — including virtual patching/WAF rules you can apply immediately — and recommended clean-up if you suspect your site was targeted. The goal is to give site owners and developers clear, pragmatic steps to secure sites quickly.

NOTE: This guide assumes familiarity with WordPress administration and basic security operations. If you manage multiple sites, follow the quick remediation checklist first, then apply the deeper developer guidance.

What was reported (short summary)

• A Cross-Site Scripting (XSS) vulnerability exists in wp‑mpdf versions up to and including 3.9.1.
• The vulnerability has been assigned CVE‑2025‑60040.
• The plugin author published a fixed release: 3.9.2. Site owners should update as soon as possible.
• The reported vulnerability allows injection of arbitrary script/HTML payloads in certain plugin inputs or outputs, enabling execution in the context of site visitors or authenticated users with appropriate privileges (the report indicated a “Contributor” privilege level was required to exploit some functionality).
• Patch priority was categorized as Low (CVSS 6.5) by the public disclosure, but “Low” does not mean “ignore” — it means the window for immediate mass exploitation is smaller, but targeted or combined attacks remain possible.

Who is affected?

• Any WordPress site running the wp‑mpdf plugin at version 3.9.1 or earlier.
• Attack surface depends on how the plugin is used on the site and which user roles interact with plugin functionality. Reports indicate contributor-level privilege is sufficient to trigger the vulnerability in some flows.
• Sites exposing plugin functionality to untrusted users (frontend forms, contributor-level posting workflows, shared editorial environments) are higher risk.

Immediate risk assessment

• Impact type: Cross-Site Scripting — client-side code execution.
• Typical impacts:
  • Persistent (stored) XSS: malicious script is stored and executed for other visitors.
  • Reflected XSS: attacker entices a user to open a crafted URL or submit a payload; script executes in victim browser.
  • Privilege escalation chains: with access to contributor/editor accounts it’s possible to inject scripts that perform privileged actions inside the admin UI.
• CVSS and public ratings mark this issue as lower priority; however:
  • If a site allows untrusted contributors, stored XSS could be leveraged to harvest credentials, plant backdoors, or push malicious content.
  • Attackers scan quickly; patching or applying virtual patches via WAF should be prioritized.

What to do right now (quick action checklist — follow this first)

1. Backup your site now (files + database).
2. Update wp‑mpdf to version 3.9.2 (or remove the plugin if you do not need it).
3. If you cannot update immediately, apply virtual patching/WAF rules (examples below) to block known exploit patterns.
4. Check user accounts (look for unexpected contributors/editor users, and reset passwords as needed).
5. Scan the site for indicators of compromise (malicious posts, modified theme/plugin files, unknown admin users, suspicious scheduled tasks).
6. Enable logging/alerting at the web server / WAF / plugin level to catch attempted exploit patterns.
7. If you host multiple sites, push the update or WAF rule to all sites centrally.

How to update safely

• From WP Admin:
  • Go to Plugins → Installed Plugins → find wp‑mpdf → click “Update now”.
• If WP Auto‑Update is disabled and you are responsible for many sites, consider enabling updates for this plugin only (or use a managed update tool).
• If you prefer CLI:
  • Use WP‑CLI: wp plugin update wp-mpdf
• After updating, clear any page caches and CDN caches to ensure visitors receive updated code.

Virtual patching and WAF guidance (apply immediately if you cannot update)

Virtual patching with a Web Application Firewall (WAF) mitigates attacks by blocking exploit attempts at the edge. Below are practical WAF rules to help block common XSS vectors originating from this disclosure. Use them as templates — tune to your site’s normal traffic to avoid false positives.

Important: test these rules in “monitor” mode first.

General approach:

• Block requests containing suspicious script markers in parameters used by the plugin.
• Block common XSS payload patterns like <script>, javascript:, onerror=, onmouseover=, data:text/html, eval(, document.cookie accessors, etc.
• Limit these rules to URLs/parameters associated with the plugin.

Example ModSecurity rule set (ModSecurity v2/v3 compatible)
(Note: adapt PARAM_NAMES or URL path to match your environment)

# 1) Limit scope to plugin endpoints (adjust the path)
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:'Potential wp-mpdf XSS attempt',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 pattern',severity:2"

# 2) Block attempts with encoded script fragments
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 (OpenResty) example for sites using nginx:

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
  

Important tuning notes:

• Limit these rules to plugin endpoints and admin-ajax.php calls that the plugin uses. Wide-ranging script-block rules can break legitimate use (e.g., HTML editors).
• If you operate a public content-editing environment where contributors post HTML intentionally, prefer plugin update over aggressive WAF blocking.
• Keep monitoring false positives and add trusted parameter/endpoint exceptions if needed.

Developer guidance — how the plugin should be fixed

If you maintain code that interacts with untrusted input, follow these secure development principles:

1. Output encoding:
   • Escape all data before output.
   • Use WordPress functions: esc_html(), esc_attr(), esc_url() where applicable.
   • For HTML allowed outputs, use wp_kses() with strict allowed tags and attributes.
2. Input validation:
   • Validate input types and lengths server-side.
   • Whitelist acceptable tag sets instead of blacklisting.
3. Nonces and capability checks:
   • Verify nonces for form submissions and AJAX endpoints: check_admin_referer() ou wp_verify_nonce() as appropriate.
   • Check user capabilities: current_user_can('edit_posts') etc. Do not rely solely on client-side controls.
4. Avoid direct echo of user-submitted HTML:
   • If the plugin must store and output HTML, sanitize with wp_kses_post() at minimum; better to provide user roles a WYSIWYG editor that sanitizes.
5. Content Storage:
   • When storing user content that may contain markup, store the raw content and apply escaping on output (not the opposite).
6. AJAX endpoints:
   • Sanitize and validate all $_REQUEST, $_POST, $_GET values before use.
   • Prefer wp_send_json_success() / wp_send_json_error() for returning JSON safely.

Sample PHP fix pattern:

<?php
// Before outputting a saved HTML fragment:
$raw_html = get_post_meta($post_id, 'mpdf_html', true);

// Safer: restrict allowed tags and attributes
$allowed_tags = array(
  'a' => array('href' => true, 'title' => true, 'rel' => true),
  'p' => array(),
  'br' => array(),
  'strong' => array(),
  'em' => array(),
  // add only what is necessary
);

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

Detection: signs your site was targeted or exploited

Look for these indicators:

• New posts or pages containing unfamiliar script tags, iframe tags, or encoded payloads.
• Unexpected admin/contributor users or role escalations.
• Unusual database entries: search wp_posts et wp_postmeta for <script, javascript:, eval(, document.cookie.
• Frontend content that redirects visitors, shows popups, or injects ads.
• Unexpected modifications to theme files, uploads directory, or plugin files.
• WAF logs showing repeated attempts to pass script fragments to plugin endpoints.
• Changes in scheduled tasks (options_wp cron entries) or strange outbound connections initiated by PHP.

Use WP‑CLI to search for suspicious patterns:

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

# Search postmeta for suspicious payloads
wp db query "SELECT meta_id, meta_key FROM wp_postmeta WHERE meta_value LIKE '%document.cookie%' OR meta_value LIKE '%<script%';"
  

Incident response — if you find evidence of compromise

1. Isolate: put the site into maintenance mode or temporarily block public access if a live exploit is causing damage.
2. Preserve logs: export webserver, WAF, and application logs for analysis.
3. Replace compromised files: restore from a known good backup. If no clean backup exists, rebuild core, plugin, and theme files from official sources after scanning uploads and database.
4. Rotate secrets:
   • Reset passwords for all admin/editor/contributor users.
   • Rotate API keys and secret keys in your config or services that integrate with your site.
5. Scan and clean:
   • Scan uploads/ wp-content for web shells and suspicious files.
   • Inspect database for injected scripts in posts, pages, options, and user metadata.
6. Post-clean actions:
   • Update all plugins/themes/core to latest versions.
   • Implement WAF rules and enable monitoring.
   • Run a post-incident security audit to identify root cause and attack vectors.
7. Consider professional help for serious incidents: forensic investigators can validate scope and advise on disclosure obligations.

Controls to reduce the impact of future XSS

• Principle of least privilege: grant contributor/editor roles only the capabilities they truly need. Disable file uploads for low-privilege roles if not necessary.
• Harden editorial workflows: use trusted HTML editors, moderate user submissions, or filter content before publishing automatically.
• CSP (Content Security Policy): implement a strict CSP that disallows inline scripts and restricts script sources. CSP is not a replacement for escaping/sanitizing but provides a valuable defense-in-depth layer.
  • Example header:
    
    Content-Security-Policy: default-src 'self'; script-src 'self' 'nonce-<RANDOM>'; object-src 'none'; frame-ancestors 'self';
  • Note: CSP requires careful testing to avoid breaking site functionality.
• HTTP-only secure cookies and SameSite attributes to reduce session theft risk.
• Regular scanning: run automated malware and integrity scans for files and database content.

How WP‑Firewall helps (our role in situations like this)

As the team behind WP‑Firewall, our focus is to give WordPress site owners practical, fast protection when vulnerabilities are disclosed:

• Managed WAF rules: we can create and deploy targeted virtual patches that block exploit patterns for a specific plugin while you plan the update.
• Malware scanning and mitigation: continuous scanning of files and the database to detect injected scripts and suspicious behavior.
• Auto-updates for vulnerable plugins (where safe): an option that applies updates automatically for specific vulnerable plugins.
• Monitoring and alerts: real-time detection of suspicious patterns so you can react fast.
• For teams: central management for many sites so you can push updates or WAF rules to all sites in minutes.

If you have WP‑Firewall installed, turning on the virtual patch for wp‑mpdf endpoints and enabling the managed rule set is the fastest way to reduce risk immediately while you validate and apply the upstream plugin update.

Practical examples: searching your site for the vulnerable plugin and versions

• In WP Admin:
  • Plugins → Installed Plugins → Look for “wp-mpdf” and confirm the version number.
• WP‑CLI:
  • wp plugin list --status=active --format=table
  • Or: wp plugin get wp-mpdf --field=version

If the plugin is active and version <= 3.9.1 — update now.

Preventing future plugin-based XSS exposures

• Only install plugins from reputable sources and maintain an inventory of installed plugins.
• Regularly review plugin activity and permissions (who can install/activate plugins).
• Limit plugin install/activation to site administrators or dedicated maintenance windows.
• Subscribe to a vulnerability intelligence/feed to be notified when a plugin you use is disclosed as vulnerable.
• Test plugin updates on a staging environment before pushing to production — then patch quickly after successful validation.

Example remediation timeline (recommended)

Day 0 (disclosure day)

• Audit plugin versions across all sites.
• Update heavily used sites first.
• If update impossible, deploy WAF rule to block exploit patterns and enable additional logging.

Day 1–3

• Update remaining sites to 3.9.2.
• Scan for IOCs in database and files.
• Reset passwords for elevated users if suspicious activity observed.

Day 4–7

• Review logs for post-update exploit attempts (to identify compromised sites).
• Harden CSP and cookie settings where appropriate.
• Communicate with stakeholders about the update.

Ongoing

• Maintain scheduled scans and WAF tuning.
• Consider role hardening and editorial workflow changes to reduce attack surface.

Example content search and clean-up SQL queries (use with caution, backup first)

Search posts containing script tags:

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

Search meta for suspicious content:

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

If you find malicious content, export the affected rows for offline analysis, then carefully remove the script portion and re-import sanitized content. Do not run destructive deletes without a tested backup.

Communication guidance for site owners

• Internal: document the steps you took (backup, update, WAF rules applied, scans run).
• External: if your site was compromised and customer data may be affected, follow applicable legal and contractual disclosure requirements. Engage legal/compliance teams early.
• Public messaging: be transparent, explain remediation, and avoid technical details that can help attackers.

Frequently asked questions

Q: The public report lists CVSS 6.5 (low). Should I still be worried?
A: Yes. CVSS is one measure of impact and exploitability. XSS can be combined with other weaknesses to produce significant outcomes. If your site exposes contributor-level interfaces, treat this seriously.

Q: Can I rely on browser extensions or client-side protections?
A: No. Client-side protections are not under your control and can vary. Server-side fixes + WAF are the right approach.

Q: Will a strict firewall rule break my site?
A: Aggressive rules can cause false positives. Always test rules in monitor mode and scope them to plugin endpoints or parameter names to reduce collateral damage.

Appendix — sample ModSecurity rule tuning notes

• Use unique rule IDs (above examples used 10000+ range).
• Add exclusions for trusted admin IPs if necessary (# only after careful consideration).
• Use “nolog” with “phase:2,pass” in monitor mode to see what requests would have been blocked.
• If your hosting provider uses a managed WAF, request they push a targeted rule for wp‑mpdf endpoints.

Closing thoughts

Even when a vulnerability is labeled as “low priority”, proactive patching and defense‑in‑depth are the best strategies. Update the plugin to 3.9.2 now. If you manage multiple sites and cannot update on all of them immediately, deploy the virtual patch / WAF rule templates above to reduce exposure and monitor attempts. Tighten editor privileges and sanitize/escape all user-provided content.

Security is a continuous process — quick, well-coordinated actions after disclosure are what stop opportunistic attackers from causing damage.

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If you want, we can generate a tailored ModSecurity rule set specifically tuned to your site’s wp‑mpdf usage and test it in monitor mode against your production traffic. Contact WP‑Firewall support or enable virtual patching in your WP‑Firewall dashboard and we’ll deploy targeted rules to protect your sites immediately.