The My Calendar – Accessible Event Manager plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the `template` attribute of the `[my_calendar_upcoming]` shortcode in all versions up to, and including, 3.7.3. This is due to the use of `stripcslashes()` on user-supplied shortcode attribute values in the `mc_draw_template()` function, which decodes C-style hex escape sequences (e.g., `\x3c` to `<`) at render time, bypassing WordPress's `wp_kses_post()` content sanitization that runs at save time. 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.