Résumé

A stored Cross‑Site Scripting (XSS) vulnerability was discovered and fixed in Koalendar versions ≤ 1.0.2 (fixed in 1.0.3). An authenticated user with Contributor privileges could inject HTML/JavaScript via the plugin’s height parameter; the content could be stored and rendered later, leading to script execution in visitors’ browsers. The issue is rated low priority (CVSS 6.5) because it requires a low‑privilege authenticated user and some user interaction, but it remains a real risk: stored XSS can lead to session theft, privilege escalation, persistent defacements, or act as an initial foothold for deeper compromise.

This post explains the vulnerability from a practical WordPress security perspective, how attackers can (and cannot) exploit it, immediate mitigations if you run the plugin, how to detect compromise, longer‑term remediation, guidance for developers to avoid the same bug, and an incident response checklist.

Table des matières

• Que s'est-il passé (en termes simples)
• Résumé technique (ce qu'est la vulnérabilité)
• Why it matters — real threats and attack scenarios
• Who is affected and how to prioritize
• Immediate steps if you run Koalendar ≤ 1.0.2
• Comment détecter si vous avez été ciblé ou compromis
• Temporary mitigations (before you can update)
• Hardening contributor roles and content workflow
• Conseils sur le WAF et le patching virtuel
• Guidance for plugin authors: secure input/output handling
• Liste de contrôle de réponse aux incidents (étape par étape)
• Long‑term prevention — processes, automation, and governance
• Notes finales et ressources

Que s'est-il passé (en termes simples)

Koalendar, a booking/events plugin for WordPress, contained a stored XSS vulnerability in versions up to 1.0.2. A Contributor‑level user could save crafted content into the plugin via a parameter called height. When that stored value was later rendered on a page without proper escaping, injected HTML/JavaScript could execute in the browser of anyone viewing the page.

The plugin author released a fix in version 1.0.3. Updating is the correct and primary remediation. If you cannot update immediately, apply the temporary mitigations and detection steps below.

Résumé technique

• Type de vulnérabilité : Cross‑Site Scripting (XSS) stocké
• Affected: Koalendar plugin versions ≤ 1.0.2
• Fixed in: 1.0.3
• Privilege required to inject: Contributor (authenticated)
• CVE: CVE‑2024‑11855
• Attack vector: A Contributor submits a crafted value to a parameter (height) that is stored and later rendered without proper output encoding, leading to script execution in the context of visitors or administrators.
• User interaction: Required — a Contributor must submit content; visitors must load the affected page.
• Severity: Low priority overall, but real impact (session theft, persistent tampering, social engineering).

Note: Contributor remains a common role in many editorial workflows (guest bloggers, external collaborators). Treat contributions as potentially hostile.

Pourquoi cela importe — scénarios d'attaque réalistes

Even “low severity” findings can be operationally harmful. Examples of abuse:

• Persistent social engineering: injected scripts modify booking confirmations, insert fake forms, or mimic admin notices to harvest credentials or payment data.
• Admin session capture: scripts executed in an admin’s browser can attempt to exfiltrate cookies or tokens if other protections are absent.
• Privilege escalation pivot: stored XSS may be chained to perform actions as the victim (CSRF‑style flows), depending on site defenses.
• Reputation and SEO damage: persistent spam, ads, or redirects harming domain reputation.
• Malware distribution: JavaScript can redirect visitors to malicious pages or load external payloads.

Because the payload is stored, a single malicious Contributor can affect many visitors over time.

Who should worry and how to prioritize

Prioritise response as follows:

• Priority 1 — Sites running Koalendar ≤ 1.0.2: update immediately.
• High concern — Sites that use Contributor accounts, accept guest authors, or have editors/admins who may view public pages while logged in.
• Lower concern — Koalendar not installed, or already updated to 1.0.3.

Stored XSS is persistent and should be treated seriously even when scored “low”.

Immediate steps if you run Koalendar ≤ 1.0.2

1. Update the plugin to version 1.0.3 immediately — this is the primary fix.
2. Si vous ne pouvez pas mettre à jour maintenant :
   • Restrict Contributor role capabilities (see section below).
   • Limit public access to Koalendar shortcodes/pages where possible (maintenance or password protection).
   • Apply temporary request‑validation rules at your edge (web server/WAF) to block non‑numeric input in numeric fields.
3. Audit recent Contributor activity:
   • Review content submitted recently for suspicious elements.
   • Check booking/event pages and any embedded widget parameters (height, custom fields).
4. Scan the site and search for suspicious HTML/JS in contenu_du_post et post_meta (examples below).
5. Rotate sensitive credentials and verify admin accounts if you find suspicious artifacts.

Updating to 1.0.3 is the fastest, most reliable remediation. Other measures are temporary mitigations.

Comment détecter si vous avez été ciblé ou compromis

Stored XSS can be subtle. Practical detection steps:

• Check recent changes by Contributors — use Posts/Pages revisions and plugin UI to see who made edits.
• Search the database for script tags or encoded payloads. Example WP‑CLI queries:

  wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%';"
  wp db query "SELECT meta_id, meta_key, meta_value FROM wp_postmeta WHERE meta_value LIKE '%<script%';"

• Look for HTML attributes with javascript : or event handlers (au chargement, onclick) in content fields.
• Review web server access logs for unusual requests to pages rendering Koalendar output — repeated requests from unfamiliar IPs can indicate scanning or exploitation attempts.
• Browser console anomalies: redirects, popups, or unexpected behaviour when admins/editors view pages while logged in are strong warning signs.
• Use external scanning and reputation services to monitor domain flags.
• If you use a WAF or edge filtering, check its logs for blocked XSS signatures or anomalies related to widget endpoints.

Si vous trouvez des scripts injectés, considérez le site comme potentiellement compromis et suivez la liste de contrôle de réponse aux incidents ci-dessous.

Temporary mitigations (before you can update)

If immediate update is impossible, take layered temporary steps (most effective first):

1. Disable the Koalendar plugin until you can update (if the site can tolerate downtime).
2. Restreindre l'accès :
   • Limit Contributor and higher roles to trusted accounts only.
   • Suspend or remove untrusted Contributor accounts temporarily.
3. Hide affected pages: maintenance mode or password protection for pages rendering Koalendar content.
4. Edge request filtering:
   • Block requests containing HTML tags in parameters that should be numeric (height).
   • Block values containing angle brackets (<, >), event attributes, or javascript :.
   • Tune rules to avoid false positives and consider starting in detection mode.
5. Sanitize stored content in the database — remove script tags or suspicious attributes (always backup first).
6. Audit third‑party accounts and rotate API keys if suspicious activity is discovered.
7. Monitor logs and traffic carefully for signs of exploitation.

These are stopgap measures; a plugin update to 1.0.3 is required for a permanent fix.

Conseils sur le WAF et le patching virtuel

A properly configured Web Application Firewall (WAF) can reduce risk until you update by blocking malicious payloads before they are stored or rendered. General guidance:

• Enforce numeric validation for fields that must be numbers (height) at server and edge layers (regex allowing digits only).
• Block requests where form fields contain script tags or encoded equivalents (e.g., %3Cscript%3E).
• Inspect decoded payloads to catch URL‑encoded or double‑encoded attempts.
• Flag or block suspicious attributes: onload=, onclick=, et javascript : des URI.
• Rate‑limit POST requests to widget endpoints from unknown sources and monitor for spikes.
• Start in detection/alert mode and tune rules before enabling blocking to avoid breaking legitimate use.

Virtual patching buys time but does not replace updating the plugin.

How to safely clean stored content (if you find malicious entries)

Always work from a backup. Suggested cleanup steps:

1. Mettez le site en mode maintenance.
2. Take a fresh full backup (files + database) for forensics and rollback.
3. Identify affected records:
   • Rechercher des publications : SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%<script%' ;
   • Search postmeta and options for unexpected HTML or scripts.
4. Sanitize non‑critical fields (numeric height): replace with integer or default value.
5. For content fields, remove script tags and suspicious attributes safely — use wp_kses with a strict allowlist if HTML is required.
6. Rotate passwords for accounts that may have been accessed and regenerate API keys where appropriate.
7. Scan files for modified PHP/JS files in case the compromise progressed beyond stored XSS.
8. If tampering is widespread, consider restoring from a known‑good backup.

If unsure, seek professional incident response — mistakes during cleanup can leave backdoors in place.

Hardening Contributor roles and editorial workflows

Contributor is useful but can be risky when given to external parties. Practical steps:

• Grant minimum necessary privileges — only trusted people should hold Contributor or higher roles.
• Require editorial review before publishing; use an editor to preview and sanitise content.
• Limit who can add widgets or embed code; restrict plugin access.
• Use capability control to remove unfiltered_html où cela est approprié.
• Consider staging workflows for guest posts; publish to production only after full review.
• Require 2‑factor authentication (2FA) for editors and administrators.
• Log and alert on new user registrations, role changes, and sudden content changes.

Secure coding guidance for plugin authors (preventing this bug)

The root cause is insufficient input validation and output escaping. Pragmatic rules for authors:

• Validate input early: if a parameter must be an integer, cast or validate (e.g., (int)$height ou absint()).
• Escape output at render time: use esc_attr(), esc_html(), esc_url() ou wp_kses() depending on context.
• Avoid storing unsanitized HTML. If HTML is required, use a strict allowlist.
• Restrict HTML submission to users with appropriate capabilities.
• Use nonces and authenticated REST endpoints as appropriate.
• Sanitize before saving and escape before output — both are necessary.
• Utilisez les API WordPress : sanitize_text_field(), wp_kses_post(), esc_html(), esc_attr(), wp_kses() with an allowlist.

Example: sanitizing a numeric height parameter

<?php
// Example: Receive a height value from a form or widget options
$raw_height = isset($_POST['height']) ? $_POST['height'] : '';

// Strict integer approach
$height = absint( $raw_height ); // ensures 0..2^31 and strips non-numeric characters

// When outputting into an attribute, escape:
echo '<div class="koalendar-widget" style="height:' . esc_attr( $height ) . 'px;"></div>';
?>

If the parameter needs to accept a limited set of CSS values, validate against an allowlist rather than accepting freeform input.

Incident response checklist — step‑by‑step

1. Isoler — If serious, take the site offline or enable maintenance mode.
2. Sauvegarde — Take a full backup (files + database) for forensic purposes.
3. Contenir — Update Koalendar to 1.0.3 immediately; apply blocking rules; disable or restrict Contributor accounts.
4. Identifier — Search the DB for malicious stored content (script tags, encoded payloads); check user and access logs.
5. Éradiquer — Remove malicious entries or restore from a known‑good backup; verify plugin/theme files integrity.
6. Récupérer — Rotate passwords and API keys; test in staging; re‑enable production when confident.
7. Examiner — Conduct root cause analysis and harden controls (2FA, role restrictions, update schedules).
8. Surveillez — Keep an eye on logs, user behaviour, and external reputation for a period after the incident.

Professional incident response is advised for complex or persistent compromises.

Long‑term prevention — processes, automation, and governance

Robust security combines people, process, and technology. Recommended long‑term practices:

• Keep WordPress core, themes, and plugins up to date. Test updates in staging where possible.
• Minimise plugin inventory — remove unused plugins.
• Monitor vendor channels for security advisories and CVE notices.
• Use automated scanning and edge protections to reduce exposure windows.
• Implement strict user onboarding/offboarding and require 2FA for privileged accounts.
• Maintain frequent backups and test restores regularly.

Notes finales et ressources

The Koalendar stored XSS (≤ 1.0.2) reinforces two enduring lessons:

1. Low‑privilege users can be an attack vector — always treat user content as potentially hostile and apply validation and escaping.
2. Patch promptly and use protective layers (WAF/edge rules, scanning, role hardening) to reduce the window of exposure.

If you run Koalendar, update to 1.0.3 now. If you require assistance, engage a trusted security professional to audit your site and help with detection and cleanup.

Références utiles :

• CVE-2024-11855
• WordPress developer resources on data validation and escaping: esc_attr(), esc_html(), wp_kses(), absint().

Stay vigilant. If you need help assessing your site, seek experienced incident responders to ensure a thorough cleanup and restoration.

Recommended next steps checklist

• Update Koalendar plugin to 1.0.3 on all environments (test first if needed).
• Apply edge request validation for widget parameters (numeric-only rules for height).
• Audit all Contributor accounts and restrict where appropriate.
• Search for and remove suspicious script tags from posts and metadata.
• Rotate credentials where compromise is suspected.
• Implement stricter content review workflow for guest content.
• Review backups and prepare a recovery plan in case restoration is needed.