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.

Executive summary

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
• Vulnerability type: Cross-Site Scripting (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.

Impact

• 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
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