<?php
// mu-plugin: foobox-mitigation.php (temporary, defensive)
add_filter('the_content', 'hk_mitigate_foobox_captions', 9);
function hk_mitigate_foobox_captions($content) {
    // Defensive: strip suspicious event handlers and script tags
    $bad_patterns = array(
        '/<script\b[^>]*>(.*?)</script>/is',
        '/(<[^>]+)on\w+\s*=([^>]+)/is'
    );
    return preg_replace($bad_patterns, '', $content);
}

Note: this is a blunt, temporary measure. Test thoroughly and remove once the plugin is patched and content is cleaned.

How to sanitise and remove existing malicious content

1. Backup your database before any changes.
2. Identify suspicious rows (see the SQL queries earlier).
3. Remove or sanitise suspect values — prefer sanitisation that retains legitimate content but strips event handler attributes and script tags.

Simple WP‑CLI replacement examples (use --dry-run first):

wp search-replace '<script' '' --dry-run
wp search-replace '</script>' '' --dry-run
wp search-replace 'onerror=' '' --dry-run
wp search-replace 'onload=' '' --dry-run

For safer, targeted sanitisation export suspect fields, review manually, and sanitise using a script that uses WordPress wp_kses() with a strict whitelist.

<?php
global $wpdb;
$rows = $wpdb->get_results(
    "SELECT meta_id, meta_value FROM {$wpdb->postmeta} WHERE meta_value LIKE '%<script%' OR meta_value LIKE '%onerror=%' LIMIT 100"
);
foreach ( $rows as $r ) {
    $clean = wp_kses( $r->meta_value, array(
        'a' => array('href' => true, 'title' => true, 'rel' => true),
        'img' => array('src' => true, 'alt' => true),
        // other tags as needed, no event attributes
    ) );
    $wpdb->update( $wpdb->postmeta, array('meta_value' => $clean), array('meta_id' => $r->meta_id) );
}

Be careful — never run destructive scripts without a tested backup.

Incident response: If you find evidence of exploitation

1. Put the site in maintenance mode and limit admin access by IP.
2. Update the vulnerable plugin immediately or deactivate it.
3. Reset passwords for all users (force password reset for authors, editors, and admins).
4. Rotate API keys, tokens, and external integration credentials.
5. Scan for and remove web shells, suspicious admin users, unknown scheduled tasks (cron), or modified core files.
6. Reinstall core WordPress and plugins from clean sources if integrity cannot be verified.
7. Review server logs to determine scope: which accounts were used, what content was changed, and any outbound exfiltration.
8. If you cannot clean with confidence — restore from a known-good backup taken prior to the compromise date.
9. Audit and document the incident and apply lessons learned.

If administrative actions were performed by an attacker (e.g., new admin user), treat it as a high-severity incident and engage experienced incident responders.

Long-term hardening and best practices

• Always run the latest stable WordPress core, themes, and plugins.
• Minimise the number of users with Author or Editor roles; use a content review workflow so only trusted users have elevated privileges.
• Enforce multi-factor authentication (MFA) for admin, editor, and author accounts.
• Limit plugins that accept and render user-submitted HTML; where needed, use wp_kses() with a strict whitelist.
• Implement virtual patching via a WAF if you maintain one, to quickly block exploit patterns for known vulnerabilities.
• Enable and monitor auditing/logging: maintain activity logs for user actions and plugin updates.
• Keep an offline backup strategy and documented quick-restore procedures.
• Periodically run vulnerability scans and code reviews on plugins you rely on heavily.

Why author-level vulnerabilities are important on multi-author sites

On sites where authors can upload media, add captions, and publish posts, the Author role is powerful enough to introduce persistent content. While Authors typically cannot delete plugins or change themes, they can inject content that, when rendered improperly, affects higher-privileged users or the visitor base.

Common mitigations for Author-level threats:

• Prevent Authors from embedding scripts or arbitrary HTML.
• Strip event handlers and dangerous tags at save time.
• Limit file uploads to trusted roles only.
• Enforce editorial review before posts go live.

Example: quick hardening snippet to limit upload capabilities

<?php
// mu-plugin: restrict-upload.php (temporary)
add_filter( 'user_has_cap', 'hk_restrict_upload_caps', 10, 4 );
function hk_restrict_upload_caps( $allcaps, $caps, $args, $user ) {
    if ( ! empty( $args[0] ) && $args[0] === 'upload_files' ) {
        // Allow only admins and editors to upload files
        if ( ! in_array( 'administrator', (array) $user->roles ) && ! in_array( 'editor', (array) $user->roles ) ) {
            $allcaps[ $args[0] ] = false;
        }
    }
    return $allcaps;
}

Use this short-term while you clean and patch. Remove it once normal operations and trust are restored.

Testing and validation after mitigation

• Re-run the database search queries to confirm no lingering <script> or event handler attributes remain in stored fields.
• Verify plugin version: wp plugin get foobox-image-lightbox --field=version should show 2.7.35 or later.
• Re-scan with malware scanners and verify server integrity (checksums for core files).
• Monitor logs for repeated attempts and tune defensive rules to reduce noise and false positives.

Prioritised remediation playbook (step-by-step)

1. Inventory: confirm FooBox plugin presence and version.
   • wp plugin get foobox-image-lightbox --field=version
2. Patch: update plugin to 2.7.35 immediately if possible.
   • wp plugin update foobox-image-lightbox
3. If you cannot update:
   • Temporarily deactivate the plugin: wp plugin deactivate foobox-image-lightbox
   • Or apply virtual patch rules (see patterns earlier).
   • Tighten upload and HTML privileges for Author-level users.
4. Clean: search for and sanitise stored payloads (using WP‑CLI/SQL or careful WordPress APIs).
5. Secure: force password resets, enable MFA, and rotate keys.
6. Monitor: keep a close eye on logs and scans for at least 30 days.
7. Review: conduct a post‑mortem and strengthen update and account policies to reduce future exposure.

Frequently asked questions

Q: My site has no Author users. Am I safe?

A: If the site truly has no users with Author (or higher) privileges, the immediate risk of this specific exploit is lower. However, attackers can obtain Author accounts via credential stuffing, weak passwords, or third-party integrations. Use this opportunity to strengthen account hygiene.

Q: Can a stored XSS lead to a full site takeover?

A: Yes. Stored XSS can be leveraged to escalate: exfiltrate admin cookies, perform actions as an admin (create users, change settings), and upload backdoors if administrative actions are possible. The scope depends on what the victim user can do inside WordPress.

Q: I updated the plugin. Do I still need to sanitise content?

A: Yes. Updating prevents future storage and rendering vulnerabilities, but it does not remove previously stored malicious payloads. Sanitize or remove suspect content after updating.

Q: How can I safely allow HTML in posts without opening XSS risk?

A: Use a strict wp_kses() whitelist, allow only needed tags and attributes, and restrict unfiltered HTML capabilities to administrators.

Final notes from a Hong Kong security expert

• Treat plugin updates seriously. Even moderate-severity issues can be combined with weak account hygiene to create larger incidents.
• The fastest effective defence is: patch, restrict, scan, and monitor. When patching is delayed, virtual patching and reducing Author capabilities buy critical time.
• If you need help implementing temporary rules, scanning for injected content, or cleaning a suspected compromise, engage experienced incident responders or a trusted local security consultant.

Stay vigilant and keep systems updated.

— Hong Kong Security Expert