CVE-2026-77187 in My Calendar Plugin
Summary
by MITRE • 09/09/2026
The My Calendar – Accessible Event Manager plugin for WordPress is vulnerable to Stored Cross-Site Scripting via 'before' and 'after' Shortcode Attributes in all versions up to, and including, 3.8.3 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.
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Analysis
by VulDB Data Team • 09/09/2026
The My Calendar – Accessible Event Manager plugin for WordPress contains a critical security vulnerability affecting all versions up to and including 3.8.3. This flaw is classified as a Stored Cross-Site Scripting (XSS) issue, which represents one of the most prevalent and dangerous web application vulnerabilities according to industry standards such as CWE-79. The root cause lies in insufficient input sanitization and output escaping within specific shortcode attributes named 'before' and 'after'. These attributes are designed to allow users to customize the display format of calendar events by inserting custom HTML or text before and after event details. However, the plugin fails to properly validate or escape user-supplied data when these values are processed for storage in the database and subsequently rendered on public-facing pages.
The operational impact of this vulnerability is significant because it allows authenticated attackers with contributor-level access and above to inject arbitrary web scripts into the application's content. Unlike reflected XSS, where malicious payloads must be delivered via a link, stored XSS persists within the website's data store. Consequently, every time an administrator or another user views a page containing the compromised event entry, the injected script executes in their browser context. This persistence transforms the vulnerability from a single-instance exploit into a continuous threat vector that can compromise multiple users over time without further interaction from the attacker.
From a tactical perspective, this vulnerability aligns with MITRE ATT&CK technique T1059.007, which covers JavaScript execution within web browsers. Attackers leveraging this flaw can perform various malicious actions depending on their objectives and the victim's privileges. Common exploitation scenarios include session hijacking through cookie theft, credential harvesting via fake login forms injected into the page content, or defacement of the website to display unauthorized messages. If an administrator is targeted, the attacker may gain full control over the WordPress installation, leading to complete site compromise, malware distribution to visitors, and potential lateral movement within the hosting environment.
Mitigation strategies must address both immediate remediation and long-term security posture improvements. The primary defense is to upgrade the My Calendar plugin to version 3.8.4 or later, where developers have implemented proper input sanitization using functions like wp_kses_post and output escaping with esc_attr or similar WordPress-specific helpers. For organizations unable to patch immediately due to compatibility concerns, a temporary workaround involves restricting contributor-level access to trusted individuals only and monitoring for unusual shortcode usage patterns in event entries. Additionally, implementing a Web Application Firewall (WAF) can help detect and block common XSS payloads targeting the 'before' and 'after' attributes before they are processed by the application logic. Regular security audits and adherence to OWASP guidelines for input validation will further reduce the risk of similar vulnerabilities emerging in future updates or custom code integrations.