CVE-2025-26939: Counters Block WordPress Plugin XSS Flaw

CVE-2025-26939 Overview

CVE-2025-26939 is a stored Cross-Site Scripting (XSS) vulnerability in the bPlugins Counters Block plugin for WordPress. The flaw affects all versions up to and including 1.1.2. It stems from improper neutralization of user-supplied input during web page generation, classified under [CWE-79].

An authenticated contributor-level attacker can inject malicious JavaScript that persists in the plugin's block output. The payload executes in the browser of any user, including administrators, who views the affected page. Because the vulnerability has a scope change, injected scripts can impact resources beyond the vulnerable component.

Critical Impact

Stored XSS enables session hijacking, administrative account takeover, and arbitrary actions in the context of authenticated WordPress users.

Affected Products

• bPlugins Counters Block (counters-block) WordPress plugin
• All versions from initial release through 1.1.2
• WordPress sites with contributor-level or higher user accounts

Discovery Timeline

• 2025-02-25 - CVE-2025-26939 published to the National Vulnerability Database
• 2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-26939

Vulnerability Analysis

The Counters Block plugin adds a Gutenberg block that renders numeric counters within WordPress posts and pages. The vulnerability resides in how the plugin processes and renders block attributes supplied by authenticated users. Input is stored without adequate sanitization and rendered without proper output encoding.

An attacker with at least contributor privileges can craft a block containing JavaScript payloads in attributes such as labels, URLs, or styling fields. When an editor or administrator previews or publishes the content, the script executes under the site's origin. The CVSS vector indicates user interaction is required, consistent with an admin opening the attacker-controlled post.

The EPSS score of 0.256% places this vulnerability in the 16th percentile for likelihood of exploitation. However, WordPress plugin XSS flaws are routinely weaponized for mass defacement and credential theft campaigns.

Root Cause

The root cause is missing or insufficient input sanitization and output escaping in the plugin's block rendering logic. WordPress provides helpers such as wp_kses_post(), esc_attr(), and esc_html(), but the vulnerable code paths do not apply them consistently to user-supplied block attributes.

Attack Vector

Exploitation requires network access to the WordPress site and an account with low-privilege authoring capabilities. The attacker inserts a Counters Block containing a JavaScript payload within one of the writable attributes. The payload is stored in the post database and executed each time the content is rendered in a victim's browser. Successful exploitation can hijack administrative sessions, inject SEO spam, or pivot to installing malicious plugins.

See the Patchstack XSS Vulnerability Report for additional technical details.

Detection Methods for CVE-2025-26939

Indicators of Compromise

• Posts or pages containing Counters Block markup with inline <script> tags, javascript: URIs, or on* event handlers in attribute values
• Unexpected outbound requests from administrator browsers to attacker-controlled domains after viewing content containing Counters Block instances
• New administrator accounts, modified user roles, or unauthorized plugin installations following content review by privileged users

Detection Strategies

• Query the wp_posts table for Counters Block shortcodes or block comments (<!-- wp:counters-block/) containing suspicious strings such as <script, onerror=, or onload=
• Review WordPress audit logs for contributor or author accounts creating or editing Counters Block content shortly before privilege escalation events
• Monitor web server access logs for unusual POST requests to /wp-json/wp/v2/posts or /wp-admin/post.php containing encoded script payloads

Monitoring Recommendations

• Deploy a Web Application Firewall (WAF) rule set that inspects WordPress REST API and admin-ajax payloads for XSS signatures
• Enable WordPress security plugin logging for content changes made by low-privilege users
• Alert on administrator session anomalies such as new IP geolocations or token reuse following content preview actions

How to Mitigate CVE-2025-26939

Immediate Actions Required

• Audit installed WordPress plugins and identify any Counters Block (counters-block) version 1.1.2 or earlier
• Deactivate the plugin until a patched release is confirmed and deployed
• Review existing Counters Block content for injected JavaScript and remove malicious payloads
• Rotate administrator passwords and invalidate active sessions if exploitation is suspected

Patch Information

At the time of publication, the vendor advisory tracked by Patchstack lists versions through 1.1.2 as affected. Site operators should monitor the Patchstack advisory and the plugin's WordPress.org listing for a fixed release and upgrade immediately upon availability.

Workarounds

• Remove or deactivate the Counters Block plugin until a patched version is released
• Restrict content creation to trusted administrator and editor accounts by revoking contributor and author privileges
• Deploy a WAF with rules blocking HTML and JavaScript in Gutenberg block attribute payloads
• Implement a Content Security Policy (CSP) header that disallows inline scripts to limit the impact of stored XSS

# Example CSP header to reduce stored XSS impact
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'; base-uri 'self'"