CVE-2025-13993 — MailerLite (Signup forms) Stored XSS (≤ 1.7.16)

Authenticated (Administrator+) Stored Cross‑Site Scripting — fixed in 1.7.17

Published 12 December 2025 — A stored cross‑site scripting (XSS) vulnerability affecting the MailerLite – Signup forms plugin for WordPress (versions ≤ 1.7.16) was disclosed as CVE-2025-13993. The flaw requires administrator privileges to store a malicious payload that is later rendered and executed in the browsers of visitors or other administrators. Although exploitation requires an authenticated admin, stored XSS can be highly damaging because the payload persists and executes whenever the affected page or admin screen is loaded.

As a Hong Kong security practitioner with experience responding to WordPress incidents, this write-up explains the vulnerability, realistic risk, immediate mitigations you can apply now, detection and recovery steps, developer fixes, and recommended long‑term hardening strategies.

What is Stored XSS and why this matters

Stored (persistent) XSS occurs when an attacker injects malicious HTML/JavaScript into persistent storage (database, files) and that content is later served without proper escaping or filtering. WordPress plugins that accept HTML from administrators (form descriptions, help text, custom HTML fields) are particularly sensitive if sanitization or escaping is missing.

• Required privilege: Administrator — a malicious or compromised admin must save the payload.
• Persistence: The script is stored (plugin settings, postmeta, form definitions) and executes when the page or admin screen is viewed.
• Scope: Any user viewing the infected page — visitors, editors, or admins — may execute the payload.

Potential consequences include session cookie theft (non‑HTTPOnly), content injection, redirects to phishing domains, loading additional malware, site defacement, or performing actions on behalf of an authenticated admin via exposed JavaScript APIs.

CVSS and severity context

Reported CVSS scores for this issue are around 5.9. CVSS is useful as a baseline, but WordPress specifics matter: the need for admin privileges reduces exploitability compared with unauthenticated flaws, yet stored XSS still poses a serious risk when administrator accounts are at stake. Treat it as a medium‑high concern depending on your environment and exposure.

Realistic exploitation scenarios

1. Malicious or compromised administrator: An attacker controlling an admin account injects a persistent script into a MailerLite field that renders on the front end.
2. Third‑party access abuse: Contractors or integrators with admin access introduce malicious content.
3. Privilege escalation and persistence: Injected JS attempts to create admin users via REST API or exfiltrate tokens and perform further actions.
4. Phishing and monetization: Redirects to affiliate/adware or credential‑harvesting pages.
5. Targeted attacks on admins: If admin screens render the payload, other privileged users can be targeted.

Immediate, step‑by‑step mitigation for site owners (what to do right now)

If you run MailerLite – Signup forms and cannot immediately update to 1.7.17, follow these steps in order. These are practical actions you can do quickly to reduce exposure.

1. Update to 1.7.17 immediately. The vendor fixed the vulnerability in 1.7.17. Updating is the simplest, most reliable fix. Use a staging environment if needed, but prioritize updating exposed production sites.
2. If you cannot update, deactivate the plugin. If updating breaks critical functionality and you need to test, disable the plugin to stop the vulnerable code from running.
3. Audit admin users and rotate credentials.
   • Remove unknown or suspicious admin accounts.
   • Force password resets for all administrators.
   • Ensure privileged accounts use strong, unique passwords and two‑factor authentication (2FA).
4. Search for and remove suspicious stored scripts. Search the database for <script tags and other suspicious HTML in plugin options, posts, wp_postmeta, and wp_options. Remove or sanitize entries that are confirmed malicious.
5. Apply virtual patching / WAF rules while you update. If you use a managed Web Application Firewall (WAF) or have the ability to add request filtering, enable rules that block POSTs containing script tags to plugin settings endpoints or other obvious XSS patterns. A properly tuned WAF can reduce risk while you test the update.
6. Isolate and scan the site. Run file integrity and malware scans. If you find server‑side backdoors, treat the site as compromised: snapshot, isolate, and follow incident response procedures.
7. Harden admin access. Limit admin access by IP where feasible, enable 2FA, and apply least privilege principles (separate accounts for routine tasks and plugin management).
8. Monitor logs and alerts. Increase monitoring of access logs, admin‑ajax and REST API endpoints, and any WAF alerts for suspicious POSTs or admin login anomalies.

Detection and forensic checks

When handling stored XSS, find where the script is stored and where it renders. Use these checks as part of your investigation:

1. Search the database for script-like content. Look for <script, onerror=, onload=, javascript:, document.cookie, and suspicious base64 blobs.
2. Examine plugin-specific options and tables. Search wp_options for keys that match the plugin slug or prefixes such as mailerlite_.
3. Check post content and custom post types. Inspect wp_posts for any custom types or shortcodes used by the plugin that may contain malicious HTML.
4. Review admin activity logs. If available, correlate setting changes and content edits with suspicious DB entries.
5. Check uploads and PHP files. Stored XSS itself doesn’t modify PHP files, but an attacker with admin rights may also upload webshells. Scan wp-content/uploads for PHP files and check for modified theme/plugin files.
6. Inspect network egress. Look for unexpected outbound connections to unknown domains that may indicate exfiltration.
7. Use browser inspection safely. Load suspected pages in an isolated environment and use developer tools to inspect DOM and event handlers where scripts execute.

SQL and WP‑CLI commands to help triage

Always back up your database before performing mass changes.

-- Search wp_options for <script
SELECT option_name, LENGTH(option_value) AS value_len
FROM wp_options
WHERE option_value LIKE '%<script%' OR option_value LIKE '%javascript:%';

-- Search postmeta and posts
SELECT post_id, meta_key, meta_value FROM wp_postmeta WHERE meta_value LIKE '%<script%';
SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';

WP‑CLI examples:

wp db query "SELECT option_id, option_name FROM wp_options WHERE option_value LIKE '%<script%'" --skip-column-names

wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%'" --skip-column-names

wp db query "SELECT option_name, option_value FROM wp_options WHERE option_value LIKE '%<script%'" > suspect-options.csv

# If confident the option is malicious:
wp option delete option_name_here

Do not mass-delete without manual review.

Developer recommendations: fixing XSS for good

Fixing stored XSS requires both sanitizing input at save time and escaping output at render time—trust nothing, not even admins.

Sanitize on input

Use WordPress sanitization functions according to expected types:

• Text fields: sanitize_text_field()
• URLs: esc_url_raw()
• Integers: (int)
• HTML with limited tags: wp_kses( $value, $allowed_html )

// Example: allow limited tags for a description
$allowed = array(
  'a' => array( 'href' => true, 'title' => true, 'rel' => true ),
  'br' => array(),
  'em' => array(),
  'strong' => array(),
);

$clean = wp_kses( $_POST['description'], $allowed );
update_option( 'myplugin_description', $clean );

Escape on output

Always escape according to context when printing to the page:

<?php
echo esc_html( get_option( 'myplugin_title' ) ); // plain text
echo wp_kses_post( get_option( 'myplugin_content' ) ); // safe HTML
?>

Other best practices

• Use capability checks and nonces (current_user_can, wp_nonce_field, check_admin_referer).
• For REST API fields, provide sanitize_callback and validate_callback.
• Avoid storing raw, untrusted HTML if possible—store structured data and render controlled HTML server‑side.
• Log critical setting changes and notify site owners when high‑risk changes occur.
• Add unit and integration tests that assert XSS payloads are rejected or sanitized.

Example remediation snippet (plugin author)

<?php
function myplugin_save_settings() {
    if ( ! current_user_can( 'manage_options' ) ) {
        return;
    }
    check_admin_referer( 'myplugin_save_settings', 'myplugin_nonce' );

    $title = isset( $_POST['myplugin_title'] ) ? sanitize_text_field( wp_unslash( $_POST['myplugin_title'] ) ) : '';
    $description = isset( $_POST['myplugin_description'] ) ? wp_kses_post( wp_unslash( $_POST['myplugin_description'] ) ) : '';

    update_option( 'myplugin_title', $title );
    update_option( 'myplugin_description', $description );
}
?>

Hardening site administration to reduce future risk

• Limit the number of administrators and apply least privilege.
• Use role‑specific accounts for routine work and separate accounts for plugin/theme management.
• Enable strong passwords and two‑factor authentication for all admin accounts.
• Require SSO for larger organizations and integrate with corporate identity providers where possible.
• Audit admin accounts regularly and remove dormant users.
• Enable admin activity logging and review changes to settings and user accounts.

Incident response checklist (if you believe you were exploited)

1. Isolate and snapshot: Put the site into maintenance mode or take it offline. Create a full backup (files + DB).
2. Triage: Identify malicious entries (stored scripts, suspicious posts, unknown users). Check file integrity and uploads.
3. Contain: Remove injection points where safe (after backup). Rotate all admin credentials and API keys.
4. Eradicate: Clean infected files or restore from a known‑clean backup. Reinstall core/theme/plugins from official sources.
5. Recover: Validate restores with scans and revoke unauthorized tokens.
6. Review: Determine how access was gained and patch root causes; improve policies and monitoring.

Conceptual WAF rule logic

A well‑designed WAF can block many stored XSS attempts before they are saved or executed. Conceptual rules to consider:

• Block POST requests to admin settings endpoints that contain <script or inline event attributes without a valid admin CSRF nonce.
• Reject payloads containing high‑risk tokens such as document.cookie, eval(, new Function, atob(, or excessive inline event handlers.
• Create targeted rules for plugin parameter names that tend to carry settings or HTML.

Note: WAF rules must be tested and tuned to avoid false positives.

Long‑term prevention (developer and site owner checklist)

• Keep WordPress core, plugins and themes updated.
• Run automated vulnerability and malware scans on schedule.
• Consider virtual patching via a managed WAF while testing vendor updates.
• Enforce least privilege, two‑factor authentication, and good password hygiene.
• Maintain reliable offsite backups and test restores periodically.
• Deploy Content Security Policy (CSP) headers where feasible to reduce XSS impact—deploy carefully to avoid breaking legitimate scripts.
• Adopt a secure development lifecycle for plugins/themes: code review, static analysis, and sanitization tests.

References

• CVE registry entry: CVE-2025-13993
• Vendor fixed version: 1.7.17 — update as soon as possible.

Practical priorities — concise checklist

1. Patch: Update MailerLite – Signup forms to 1.7.17 now.
2. Contain: If you cannot update, deactivate the plugin and apply WAF rules to block obvious XSS payloads.
3. Audit: Review admin accounts, rotate credentials, and search for stored <script entries.
4. Harden: Enforce 2FA and least privilege for admin roles.
5. Automate: Schedule regular scans and monitoring, and maintain tested backups.

Stored XSS vulnerabilities remind us that software will always have defects and that human processes matter. Restricting and monitoring administrative access is often the single most practical control to prevent an attacker from turning a stored vulnerability into a full compromise. Use a layered approach: timely patching, virtual patching where necessary, strict admin controls, scanning, and incident readiness.

If you need professional assistance with triage, forensic checks, or WAF tuning, engage an experienced security consultant or incident response team familiar with WordPress environments.

— Hong Kong Security Expert