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

Author: Hong Kong Security Expert
Date: 2025-09-26

Overview

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

This article is written from the perspective of Hong Kong security practitioners: it explains the exposure, assesses the risk, describes detection techniques, provides practical virtual-patching/WAF guidance you can apply immediately, and outlines step-by-step mitigation and clean-up if you suspect compromise. It assumes familiarity with WordPress administration and basic security operations.

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 is tracked as CVE-2025-60040.
• The plugin author released a fixed version: 3.9.2. Site owners should update as soon as possible.
• The 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 (reports indicate contributor-level privilege may be sufficient to exploit some flows).
• The public disclosure categorized the issue as Low priority (CVSS 6.5), but “Low” does not mean “ignore” — targeted or chained 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 and which user roles interact with its functionality. Contributor-level access has been reported as sufficient in some flows.
• Sites that expose plugin functionality to untrusted users (frontend forms, contributor workflows, shared editorial environments) are at higher risk.

Immediate risk assessment

Impact type: Cross-Site Scripting — client-side code execution.

Typical impacts include:

• Persistent (stored) XSS: malicious script 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.

Although public ratings list this as lower priority, sites that accept HTML from untrusted users can be heavily impacted. Attackers scan quickly; patching or applying virtual patches at the edge should be prioritised.

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 it is not needed).
3. If you cannot update immediately, apply virtual patching/WAF rules (examples below) to block known exploit patterns.
4. Review user accounts (look for unexpected contributors or editors) 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 / application level to catch attempted exploit patterns.
7. If you manage multiple sites, push the update or virtual patching across your fleet.

How to update safely

From the WordPress admin:

• Plugins → Installed Plugins → find wp-mpdf → click “Update now”.

If you prefer the command line:

wp plugin update wp-mpdf

After updating, clear page caches and CDN caches to ensure visitors receive the corrected 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. Use the examples below as templates but tune them to your site’s normal traffic to avoid false positives. Test rules in monitoring mode first.

General approach:

• Scope rules to plugin endpoints and known parameter names.
• 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, etc.

Example ModSecurity rule set (ModSecurity v2/v3 compatible)

Adapt PARAM_NAMES and URL paths 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 nginx-based sites using Lua/OpenResty:

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 rules to plugin endpoints and admin-ajax.php calls that the plugin uses; broad script-blocking can break legitimate functionality (e.g., HTML editors).
• If your site allows trusted users to post HTML, prioritise updating the plugin rather than aggressive blocking.
• Monitor false positives and add trusted parameter or endpoint exceptions where 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 appropriate. For controlled HTML outputs, use wp_kses() with a strict allowed tags/attributes list.
2. Input validation: Validate input types and lengths server-side. Whitelist acceptable tags and content rather than blacklisting.
3. Nonces and capability checks: Verify nonces for form submissions and AJAX endpoints (check_admin_referer(), wp_verify_nonce()). Check user capabilities with current_user_can() and do not rely on client-side controls.
4. Avoid direct echo of user-submitted HTML: If the plugin must store and output HTML, sanitise with wp_kses_post() at minimum or provide a WYSIWYG editor that sanitises input.
5. Content storage: Store raw content if necessary and apply escaping on output, rather than storing already-escaped content.
6. AJAX endpoints: Sanitize and validate all $_REQUEST, $_POST, $_GET values. Prefer wp_send_json_success() / wp_send_json_error() for returning JSON safely.

Sample PHP fix pattern

// 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.
• Database entries containing <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 or webserver logs showing repeated attempts to pass script fragments to plugin endpoints.
• Changes in scheduled tasks (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 the database.
4. Rotate secrets: Reset passwords for all admin/editor/contributor users. Rotate API keys and secrets in configuration and integrated services.
5. Scan and clean: Scan uploads/wp-content for web shells and suspicious files. Inspect the 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. Seek professional help for serious incidents: Forensic investigators can validate scope and advise on legal or 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.
• Content Security Policy (CSP): implement a strict CSP that disallows inline scripts and restricts script sources. CSP is not a replacement for escaping/sanitizing but provides defence-in-depth.
• Use HTTP-only and Secure cookies with appropriate SameSite attributes to reduce session theft risk.
• Regular automated scanning: run malware and integrity scans for files and database content.

Practical examples: checking for the vulnerable plugin and versions

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

If the plugin is active and version ≤ 3.9.1 — update immediately.

Preventing future plugin-based XSS exposures

• 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 installation/activation to site administrators or dedicated maintenance windows.
• Subscribe to vulnerability intelligence feeds to be notified when plugins you use are disclosed as vulnerable.
• Test plugin updates on a staging environment before deploying to production, then patch quickly after validation.

Example remediation timeline (recommended)

Day 0 (disclosure day)

• Audit plugin versions across all sites.
• Update high-priority sites first.
• If update impossible, deploy targeted WAF rules and enable additional logging.

Day 1–3

• Update remaining sites to 3.9.2.
• Scan for indicators of compromise in database and files.
• Reset passwords for elevated users if suspicious activity is 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 remediation steps taken.

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 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 steps taken (backups, updates, WAF rules applied, scans run).
• External: If customer data may be affected, follow applicable legal and contractual disclosure requirements and engage legal/compliance teams early.
• Public messaging: Be transparent, explain remediation, and avoid technical details that assist attackers.

Frequently asked questions

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

Q: Can I rely on browser extensions or client-side protections?
A: No. Client-side protections are inconsistent and not under your control. Server-side fixes and edge filtering are the correct approach.

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

Appendix — ModSecurity rule tuning notes

• Use unique rule IDs and maintain a clear changelog for WAF rules.
• Add exclusions for trusted admin IPs only after careful consideration.
• Use monitor mode (phase:2, pass, nolog or log-only) to evaluate rules before blocking.
• If using a managed WAF from a hosting provider, ask them to push a targeted rule for wp-mpdf endpoints.

Closing thoughts

Even when a vulnerability is labeled as “low priority”, proactive patching and defence-in-depth are the best strategies. Update wp-mpdf to 3.9.2 now. If you manage multiple sites and cannot update immediately, deploy targeted virtual patches / WAF rules to reduce exposure and monitor for exploitation. Tighten editor privileges and ensure user-provided content is sanitised and escaped.

Security is a continuous process — fast, coordinated action after disclosure is what prevents opportunistic attackers from causing damage. If you need help, engage experienced security professionals or forensic investigators to validate scope and guide recovery.