CVE-2025-13898: Ultra Skype Button Plugin XSS Vulnerability

CVE-2025-13898 Overview

CVE-2025-13898 is a Stored Cross-Site Scripting (XSS) vulnerability [CWE-79] in the Ultra Skype Button plugin for WordPress. The flaw affects all versions up to and including 1.0. It resides in the btn_id parameter of the [ultra_skype] shortcode, where the plugin fails to properly sanitize input and escape output. Authenticated users with Contributor-level access or higher can inject arbitrary JavaScript into pages. The injected scripts execute in the browser of any user who views the affected page, enabling session theft, forced redirects, or actions performed on behalf of the victim.

Critical Impact
Authenticated contributors can persist malicious JavaScript in WordPress pages, executing in the context of every visitor, including administrators.

Affected Products

Ultra Skype Button plugin for WordPress — all versions ≤ 1.0
WordPress sites permitting Contributor-level or higher registration
Any page rendering the [ultra_skype] shortcode

Discovery Timeline

2025-12-06 - CVE-2025-13898 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-13898

Vulnerability Analysis

The Ultra Skype Button plugin registers a [ultra_skype] shortcode that accepts a btn_id attribute. This attribute is intended to assign a unique HTML id to the rendered button element. The plugin passes the attribute value directly into HTML output without applying WordPress sanitization functions such as esc_attr() or sanitize_key(). As a result, an attacker can break out of the attribute context and inject arbitrary script content.

Because the shortcode is stored in post content, the payload persists in the database. Every visit to the affected page triggers script execution in the visitor's browser. Contributors typically cannot publish content directly, but an administrator previewing or approving the submission also becomes a target.

Root Cause

The root cause is insufficient input sanitization and missing output escaping in the shortcode handler located in index.php around lines 39 and 44 of the plugin source. The btn_id shortcode attribute is concatenated into an HTML id attribute without escaping. This violates WordPress secure coding guidance requiring esc_attr() on any attribute value derived from user input.

Attack Vector

An authenticated attacker with Contributor privileges creates or edits a post containing a crafted shortcode such as [ultra_skype btn_id='"><script>...</script>']. When the post is rendered, the injected script executes with the privileges of the viewer. If an administrator opens the post, the attacker can steal session cookies, create new admin accounts, or plant backdoors via the REST API. The attack vector is network-based and requires low privileges with no user interaction beyond viewing the page.

No verified proof-of-concept code is published. See the Wordfence Vulnerability Report and plugin source at WordPress Plugin Trunk Line 39 for reference.

Detection Methods for CVE-2025-13898

Indicators of Compromise

Post or page content containing [ultra_skype] shortcodes with btn_id values that include <, >, ", ', script, onerror, or onload substrings.
Unexpected outbound requests from visitor browsers to attacker-controlled domains sourced from WordPress-rendered pages.
New administrator accounts, modified user roles, or unauthorized plugin installations following contributor submissions.

Detection Strategies

Query the wp_posts table for stored shortcode payloads: SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%[ultra_skype%btn_id=%<%'.
Deploy a Web Application Firewall (WAF) rule that inspects POST bodies to /wp-admin/post.php for btn_id attribute values containing HTML metacharacters.
Review WordPress audit logs for post creation or edits by Contributor-role users referencing the vulnerable shortcode.

Monitoring Recommendations

Monitor Content Security Policy (CSP) violation reports for inline script executions on pages hosting the plugin.
Alert on privilege escalation events, particularly role changes to Administrator, following contributor activity.
Track plugin inventory across WordPress sites and flag hosts still running Ultra Skype Button ≤ 1.0.

How to Mitigate CVE-2025-13898

Immediate Actions Required

Deactivate and remove the Ultra Skype Button plugin until a patched version is released by the vendor.
Audit existing posts and pages for the [ultra_skype] shortcode and remove or sanitize any suspicious btn_id values.
Restrict contributor and author registration on sites that do not require public content submission.

Patch Information

At time of publication, no fixed version is listed in the enriched CVE data. The vulnerability affects all versions up to and including 1.0. Monitor the Wordfence Vulnerability Report and the plugin repository for a patched release, and apply it as soon as it becomes available.

Workarounds

Remove the plugin entirely and replace it with a maintained alternative if Skype button functionality is required.
Revoke or downgrade Contributor and above roles for untrusted users to prevent authenticated exploitation.
Deploy a WAF with rules that block HTML metacharacters within WordPress shortcode attributes.
Enforce a strict Content Security Policy that disallows inline scripts on public pages.

bash

# Configuration example: disable the vulnerable plugin via WP-CLI
wp plugin deactivate ultra-skype-button
wp plugin delete ultra-skype-button

# Audit stored content for injected shortcode payloads
wp db query "SELECT ID, post_title FROM wp_posts WHERE post_content LIKE '%[ultra_skype%btn_id=%<%';"