CVE-2026-14827: Calendar WordPress Plugin XSS Vulnerability

CVE-2026-14827 Overview

CVE-2026-14827 is a stored Cross-Site Scripting (XSS) vulnerability in the Calendar WordPress plugin before version 1.3.18. The plugin fails to properly escape a user-supplied event field before outputting it inside an HTML attribute on a public-facing page. Authenticated users holding the Contributor role can inject arbitrary JavaScript that executes in the browser of any visitor viewing the calendar. The flaw is tracked as CWE-79 (Improper Neutralization of Input During Web Page Generation).

Critical Impact
Contributor-level attackers can persist JavaScript payloads in calendar events, enabling session hijacking, credential theft, or administrator account takeover when higher-privileged users view the affected page.

Affected Products

Calendar WordPress plugin versions prior to 1.3.18
WordPress sites permitting Contributor-role registration or accounts
Public-facing calendar pages rendering user-submitted event data

Discovery Timeline

2026-07-27 - CVE-2026-14827 published to NVD
2026-07-27 - Last updated in NVD database

Technical Details for CVE-2026-14827

Vulnerability Analysis

The Calendar plugin accepts event field input from authenticated Contributor accounts and stores it for later rendering on public calendar pages. When the plugin outputs the stored value, it places the content inside an HTML attribute without applying an attribute-context escaping function such as esc_attr(). An attacker can break out of the attribute and inject a JavaScript event handler or a full <script> payload.

Exploitation requires a Contributor account and user interaction to view the calendar page. The stored nature of the payload means it triggers automatically for every visitor, including administrators. Successful execution runs attacker JavaScript in the origin of the WordPress site, exposing session cookies, nonces, and privileged admin actions to the attacker.

The attack vector is network-based over HTTP or HTTPS. EPSS data reports a probability of 0.235% at percentile 14.67 as of 2026-07-30.

Root Cause

The root cause is missing output encoding in an HTML attribute context. WordPress provides esc_attr() for this purpose, but the vulnerable code path emits stored event data directly into the rendered markup. The plugin also trusts the Contributor role to submit safe HTML, which contradicts WordPress guidance for role-based input handling.

Attack Vector

An attacker with a Contributor account authors an event and injects a crafted string into the affected event field. The payload closes the surrounding HTML attribute and introduces a JavaScript handler such as onmouseover or onerror. When any visitor loads the public calendar page, the browser parses the injected attribute and executes the attacker's script under the site's origin. The vulnerability mechanism is documented in the WPScan Vulnerability Report.

Detection Methods for CVE-2026-14827

Indicators of Compromise

Calendar event records containing HTML control characters such as ", <, >, or onerror= in event fields
Unexpected outbound requests from browser sessions of administrators after viewing the public calendar
New administrator accounts, plugin installations, or theme edits following Contributor activity
Contributor-role accounts submitting events with abnormally long or encoded payloads

Detection Strategies

Query the WordPress wp_posts and plugin-specific tables for event fields containing script tags or JavaScript event handlers
Review web server access logs for POST requests to the Calendar plugin's event submission endpoints from Contributor accounts
Compare installed Calendar plugin version against 1.3.18 across managed WordPress sites
Inspect rendered calendar pages with a headless browser to detect unauthorized script execution

Monitoring Recommendations

Alert on creation or promotion of WordPress accounts to Editor or Administrator following Contributor submissions
Monitor plugin file integrity and database changes for the Calendar plugin
Log and review all event submissions containing angle brackets, quotes, or javascript: URIs
Enforce Content Security Policy (CSP) reporting to capture inline script violations on calendar pages

How to Mitigate CVE-2026-14827

Immediate Actions Required

Upgrade the Calendar WordPress plugin to version 1.3.18 or later
Audit existing calendar event records for injected HTML or JavaScript payloads and sanitize or remove them
Review Contributor accounts, disable unused accounts, and rotate credentials for any account suspected of abuse
Force session invalidation for administrators who may have viewed the calendar during the exposure window

Patch Information

The vendor has released Calendar plugin version 1.3.18, which addresses the missing attribute-context escaping. Refer to the WPScan Vulnerability Report for advisory details and version references.

Workarounds

Restrict Contributor account creation and require manual approval for new registrations
Remove or deactivate the Calendar plugin until the patched version is deployed
Deploy a Web Application Firewall (WAF) rule to block event submissions containing HTML control characters
Apply a strict Content Security Policy that disallows inline JavaScript on calendar pages

bash

# Update the Calendar plugin using WP-CLI
wp plugin update calendar --version=1.3.18

# Verify the installed version
wp plugin get calendar --field=version

# Audit Contributor accounts
wp user list --role=contributor --fields=ID,user_login,user_email,user_registered