CVE-2026-8868: Single Mailchimp Plugin XSS Vulnerability

CVE-2026-8868 Overview

CVE-2026-8868 is a Stored Cross-Site Scripting (XSS) vulnerability in the Single Mailchimp plugin for WordPress, affecting all versions up to and including 1.4. The flaw exists in the single_mailchimp() function within shortcodes.php, which concatenates user-supplied shortcode attributes directly into HTML output without sufficient sanitization or escaping. Authenticated users with contributor-level access or above can inject arbitrary JavaScript through the single-mailchimp shortcode. The injected payload executes in any visitor's browser when they load the affected page. This vulnerability maps to CWE-79: Improper Neutralization of Input During Web Page Generation.

Critical Impact
Contributor-level attackers can persist JavaScript payloads in WordPress pages, enabling session theft, administrative action forgery, and visitor redirection.

Affected Products

Single Mailchimp plugin for WordPress, versions 1.0 through 1.4
WordPress sites permitting contributor-level (or higher) shortcode usage
Any page or post embedding the single-mailchimp shortcode

Discovery Timeline

2026-05-27 - CVE-2026-8868 published to NVD
2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-8868

Vulnerability Analysis

The Single Mailchimp plugin exposes a single-mailchimp shortcode that renders a subscription form on WordPress pages. The shortcode accepts six attributes: autocomplete, label, placeholder, btn_text, success_msg, and error_msg. The single_mailchimp() handler in shortcodes.php interpolates these attribute values directly into the HTML response without calling WordPress escaping helpers such as esc_attr() or esc_html().

An authenticated contributor can submit a post containing the shortcode with malicious attribute values. WordPress stores the post content verbatim. When any visitor loads the page, the browser parses the attacker-controlled markup as part of the document, executing arbitrary JavaScript in the visitor's session context. Because the payload persists in the database, the attack reaches every user who views the page, including administrators.

Root Cause

The root cause is missing output escaping on shortcode attributes that are written into HTML attribute and text contexts. The plugin trusts $atts values produced by shortcode_atts() and concatenates them into the response string. WordPress's shortcode_atts() only merges defaults and does not sanitize content, so attacker-supplied data flows untouched into the rendered DOM.

Attack Vector

An attacker with contributor-level access creates or edits a post containing a crafted [single-mailchimp] shortcode. The attribute payload breaks out of the intended HTML context using quote characters and injects an event handler or <script> element. Once the post is published or previewed by a higher-privileged user, the JavaScript executes with that user's browser privileges, enabling cookie theft, CSRF against the WordPress REST API, or covert administrator account creation. Exploitation requires no user interaction beyond viewing the page. Refer to the Wordfence Vulnerability Report and the WordPress Shortcode Code Review for the unsanitized sink locations.

Detection Methods for CVE-2026-8868

Indicators of Compromise

WordPress posts or pages containing [single-mailchimp] shortcodes with unusual attribute values containing <, >, ", onerror=, onload=, or javascript: substrings.
New or modified administrator accounts created shortly after a contributor publishes content using the plugin's shortcode.
Outbound requests from visitor browsers to unknown domains immediately after loading a page that embeds the shortcode.

Detection Strategies

Audit the wp_posts table for post_content rows matching the single-mailchimp shortcode and inspect attribute values for HTML metacharacters.
Review WordPress access logs for contributor accounts that edited posts containing the shortcode prior to suspicious admin activity.
Use a Content Security Policy (CSP) violation report endpoint to surface inline script execution originating from plugin-rendered pages.

Monitoring Recommendations

Alert on creation or modification of WordPress users with administrator or editor roles where the request originates from an authenticated contributor session.
Monitor edits to posts that include shortcode attributes containing angle brackets, encoded entities, or data: URIs.
Track plugin file integrity for shortcodes.php to confirm patched versions remain in place after updates.

How to Mitigate CVE-2026-8868

Immediate Actions Required

Deactivate the Single Mailchimp plugin until a patched release above version 1.4 is installed.
Audit all existing posts and pages for the [single-mailchimp] shortcode and remove any instances containing unexpected HTML or script content.
Restrict contributor-level account creation and review existing contributor accounts for legitimacy.

Patch Information

No fixed version is identified in the published advisory at the time of writing. Versions 1.0 through 1.4 remain vulnerable. Monitor the WordPress plugin repository and the Wordfence advisory for an updated release that applies esc_attr() and esc_html() to the six affected shortcode attributes.

Workarounds

Remove or disable the single-mailchimp shortcode by unregistering it via a mu-plugin that calls remove_shortcode('single-mailchimp') on the init hook.
Limit shortcode usage to administrator and editor roles by filtering the_content and stripping the shortcode for posts authored by contributors.
Deploy a web application firewall rule that blocks requests containing the single-mailchimp shortcode with HTML metacharacters in attribute values.

bash

# Configuration example: disable the vulnerable shortcode via mu-plugin
# File: wp-content/mu-plugins/disable-single-mailchimp.php
<?php
add_action('init', function () {
    if (shortcode_exists('single-mailchimp')) {
        remove_shortcode('single-mailchimp');
    }
}, 99);