Example pseudo‑rule (modsec-style semantics):

SecRule REQUEST_METHOD "POST" "chain,deny,log,status:403,msg:'Blocked possible stored XSS via slides attribute'"
  SecRule ARGS_POST "@rx (\[mpc_carousel[^\]]*slides=.*<script)|(\bslides=.*javascript:)|(\bslides=.*on\w+=)" "t:none,ctl:requestBodyProcessor=URLENCODED"

Caveats:

• Rules must be tuned to avoid false positives (some legitimate uses may include JSON-like slides data).
• Use logging-only mode first to confirm detection before blocking.
• If your WAF supports virtual patching, deploy a rule that removes <script> tokens from saved post content or rejects save requests containing script tokens in shortcodes.

Recovery and incident response playbook (if you are compromised)

If you detect that XSS payloads were executed and an admin session was likely compromised, follow this playbook:

1. Isolate and snapshot
   • Take snapshots of database and filesystem for forensic analysis. Preserve logs.
2. Reset credentials and keys
   • Reset all administrator and high‑privilege user passwords.
   • Rotate API keys, tokens, and any secrets stored on the site.
3. Remove malicious content
   • Use the SQL/WP‑CLI scans above to find and remove malicious shortcodes and script tags.
   • Restore affected posts from known-good revisions or backups.
4. Clean or reinstall modified files
   • Compare plugin and theme files with known-good copies from the WordPress.org repository or vendor archive.
   • Reinstall plugins and themes from official sources when possible; replace modified files rather than editing in place.
5. Backdoors & persistence checks
   • Search for suspicious PHP files in wp-content/uploads, mu-plugins, and theme/plugin directories.
   • Check for new admin users or unexpected scheduled tasks (wp_cron entries).
   • Review the database for unusual options and transient data.
6. Post-recovery hardening
   • Enforce least privilege and limit who can publish or insert HTML/shortcodes (see role recommendations).
   • Apply WAF virtual patches to block similar attempts.
   • Implement Content Security Policy (CSP) to make exploitation harder for future XSS.
7. Post-mortem and notification
   • Document timeline: initial injection, discovery, remediation steps.
   • Notify stakeholders and, if customer data was exposed, follow applicable breach disclosure laws.

Long-term hardening and best practices

The vulnerability highlights a few recurring themes in WordPress security. Use these to reduce risk going forward.

1. Least privilege and role separation
   • Ensure the Contributor role cannot insert raw HTML or scripts. Consider using a custom role that restricts shortcode use or requiring approval for posts.
2. Restrict plugin capabilities
   • Plugins that accept complex user input should validate on both input and output. If a plugin exposes shortcode attributes that accept HTML or structured data, the plugin author must sanitize and encode output.
3. Sanitize & escape output
   • Plugin developers must use functions such as esc_attr(), wp_kses_post(), and esc_html() when inserting attribute values into HTML. Attributes containing lists or IDs should only accept a validated whitelist (e.g., numeric IDs, comma-separated integers).
4. Use WAF / virtual patching
   • Maintain WAF rules that detect suspicious shortcode injection patterns. Virtual patches are critical when plugin maintainers are slow to release fixes.
5. Content Security Policy (CSP)
   • Enforce CSP for admin and front-end pages to limit allowed script sources. While CSP is not a panacea, it raises the exploitation cost for XSS.
6. Regular scanning & integrity checking
   • Schedule automated scans for injected content, unexpected file changes, and suspicious shortcodes. Automated integrity checks for plugin and theme files help spot tampering early.
7. Developer checklist for shortcodes
   • Validate attribute format.
   • Strip tags from attributes that must be plain text.
   • Escape before output.
   • Restrict complex or HTML attributes to trusted user roles.

How WP‑Firewall helps (and a free plan you can start with)

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Why consider this while you fix plugin issues?

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If you manage multiple WordPress sites, even the Basic plan provides a significant, immediate reduction in attack surface while you carry out the manual cleanup steps outlined above.

Appendix — Quick SQL and WP‑CLI references

A. Search posts for shortcodes containing "slides=":

SELECT ID, post_title, post_date
FROM wp_posts
WHERE post_content LIKE '%slides=%'
  AND post_status IN ('publish', 'draft', 'pending', 'future');

B. Remove script tags from post_content (dangerous — do a backup first)

UPDATE wp_posts
SET post_content = REGEXP_REPLACE(post_content, '<script[^>]*>.*?</script>', '', 'gi')
WHERE post_content REGEXP '<script[^>]*>.*?</script>';

Note: REGEXP_REPLACE availability depends on your MySQL/MariaDB version. Test on a copy first.

C. WP‑CLI: List posts with 'slides=' in content

wp post list --post_type=post,page --format=csv --field=ID,post_title | \
  while IFS=, read -r id title; do
    content=$(wp post get "$id" --field=post_content)
    echo "$content" | grep -qi "slides=" && echo "Matched: ID=$id Title=$title"
  done

D. Find revisions with risky content

SELECT p.ID, r.post_parent, r.post_modified, r.post_content
FROM wp_posts r
JOIN wp_posts p ON r.post_parent = p.ID
WHERE r.post_type = 'revision'
  AND r.post_content LIKE '%slides=%';

Final recommendations — prioritized checklist

1. Immediately identify impacted sites and plugin versions.
2. If a vendor patch is available, update right away (backup first).
3. If no patch is available, deactivate plugin or apply the temporary remove‑shortcode / strip‑script filters.
4. Implement WAF rules to block shortcode-based script payloads and javascript: occurrences in payloads.
5. Scan DB for injected shortcodes and remove malicious entries; inspect revisions and options.
6. Rotate credentials and review recent admin/editor activity.
7. Harden contributor/user roles and enforce least privilege.
8. Maintain backups and deploy ongoing scanning and monitoring.

If you need rapid help applying temporary patches or performing a clean-up, WP‑Firewall's team can assist with triage, virtual patching, and remediation workflows that reduce time-to-mitigation. Start with the free plan to get managed firewall protection, then pick the tier that matches your operational needs: https://my.wp-firewall.com/buy/wp-firewall-free-plan/

Stay safe — treat shortcodes and plugin attributes that can contain markup as untrusted input. Sanitize early, escape late, and apply layered defenses.