Analysis: CVE-2024-11875 — “Add infos to the events calendar” (XSS)

This post provides a practical technical briefing on CVE-2024-11875 affecting the WordPress plugin “Add infos to the events calendar”. I write from the perspective of a Hong Kong security practitioner: concise, pragmatic, and focused on what administrators need to know now. No marketing, no vendor push — just clear facts and actionable remediation guidance.

कार्यकारी सारांश

CVE-2024-11875 is a reflected/stored Cross-Site Scripting (XSS) vulnerability in the plugin’s handling of event metadata or custom fields. An attacker who can supply crafted input (for example, via event creation forms or fields that accept HTML) may cause arbitrary script execution in a victim’s browser when the event is viewed. The vendor classified this issue as low urgency, primarily because exploitation requires some level of user interaction or specific site configuration; however, XSS can still enable session theft, privilege escalation via CSRF combined with the XSS, or social-engineering attacks.

Affected components & versions

• Plugin: Add infos to the events calendar
• सुरक्षा दोष का प्रकार: क्रॉस-साइट स्क्रिप्टिंग (XSS)
• Affected versions: versions prior to the vendor’s patched release (consult plugin changelog/official advisory for exact version numbers)

Technical description (high-level)

The root cause is insufficient output escaping when rendering user-supplied event fields. Fields that accept text or HTML (event descriptions, custom meta, extra info fields) are output directly into the page without proper context-aware escaping or strict sanitization. When untrusted input is reflected into an HTML page without escaping, an attacker can inject script-containing payloads that execute in the context of victims’ browsers.

प्रभाव

• Stored XSS: Malicious script persists in the database and runs for multiple visitors.
• Reflected XSS: Malicious script executed when a crafted URL is visited.
• Potential consequences: session cookie theft, unauthorized actions via authenticated users, phishing, content manipulation, and pivoting to other parts of the site.

Exploitation complexity and requirements

• Complexity: Low to moderate depending on site configuration.
• Prerequisites: ability to submit event content or fields that are later rendered to other users, or convincing a victim to click a crafted link if the flaw is reflected.
• Impact increases if unprivileged users (including subscribers) can create events or modify meta fields that render to admins or higher-privilege users.

Safe remediation steps (what administrators should do now)

The priority is to ensure the plugin is patched to a fixed version. If you cannot immediately apply the vendor patch, apply the following mitigations:

• Update: Apply the plugin update from the official plugin repository or vendor when available.
• Sanitize input at save-time: use sanitize_text_field(), wp_kses() or other appropriate sanitizers for any field that should not contain arbitrary HTML.
• Escape on output: use context-aware escaping functions in templates:
  • HTML body text: echo esc_html( $value );
  • Attribute values: echo esc_attr( $value );
  • URLs: echo esc_url( $value );
  • Trusted HTML fragments: use wp_kses( $html, $allowed_tags ) with a strict whitelist.
• Limit who can create or edit events: review roles/capabilities and restrict event creation to trusted roles.
• Disable or restrict untrusted HTML: if event fields do not require HTML, remove HTML capabilities and sanitize input to plain text.
• Harden content policies: implement a Content Security Policy (CSP) to reduce the impact of injected scripts where possible.
• Back up: take a fresh backup of the site and database before applying changes.

Secure coding guidance for plugin authors

Authors should perform context-aware escaping every time untrusted data is rendered. Never assume input is safe. Example patterns to prefer:

/* When saving text-only fields */
$clean = sanitize_text_field( $_POST['event_extra'] );
update_post_meta( $post_id, 'event_extra', $clean );

/* When rendering */
$value = get_post_meta( $post_id, 'event_extra', true );
echo esc_html( $value );               // safe for HTML body
echo esc_attr( $value );               // safe for attribute contexts
/* For limited HTML allowed */
echo wp_kses( $value, array(
  'a' => array( 'href' => true, 'title' => true ),
  'strong' => array(),
  'em' => array()
));
  

Detection & indicators of compromise

Look for unusual script tags or HTML attributes inside event descriptions or custom fields. Indicators:

• Event content containing <script> tags, inline event handlers (onerror, onclick), or <img src="…" onerror="…"> patterns.
• Access logs with suspicious payloads targeting event pages or event submission endpoints.
• Unexplained administrative actions following visits from external referrers or users.

Response checklist for incident handling

1. Identify and isolate the offending records in the database (event posts, postmeta entries).
2. Sanitize or remove malicious content from affected records (prefer restoring from recent clean backup if available).
3. Rotate sensitive credentials (administrator accounts, API keys) that may have been exposed or abused.
4. Apply the vendor patch and any recommended WordPress core updates.
5. Review user accounts and remove any suspicious accounts or unneeded elevated privileges.
6. Monitor logs for repeat attempts or related suspicious activity.

Disclosure timeline (concise)

• Discovery: (internal/private) — researcher identified insufficient escaping in event fields.
• Vendor notified & coordinated fix — vendor published update and CVE assigned (CVE-2024-11875).
• CVE published: 2026-02-03 (reference listed above).

हांगकांग सुरक्षा दृष्टिकोण से अंतिम नोट्स

In local practice we prioritise clear risk triage and timely patching. Although this XSS was rated low urgency, the practical risk depends on your site’s configuration: if untrusted users can create events or if event content is rendered to privileged users, the risk rises quickly. Apply the patch, enforce strict escaping and sanitization, and reduce the attack surface by limiting who can submit event content.