CVE-2026-2557: cskefu File Upload XSS Vulnerability

CVE-2026-2557 Overview

A Cross-Site Scripting (XSS) vulnerability has been identified in cskefu up to version 8.0.1. The vulnerability exists within the Upload function located in the file com/cskefu/cc/controller/resource/MediaController.java of the File Upload component. The manipulation of file upload parameters allows attackers to inject malicious scripts, resulting in stored or reflected XSS attacks. This vulnerability can be exploited remotely by authenticated users, and a public exploit is available.

Critical Impact

Attackers can execute arbitrary JavaScript in victim browsers, potentially leading to session hijacking, credential theft, and unauthorized actions performed on behalf of authenticated users.

Affected Products

• cskefu versions up to 8.0.1
• File Upload component (MediaController.java)
• Applications utilizing the affected cskefu library

Discovery Timeline

• 2026-02-16 - CVE-2026-2557 published to NVD
• 2026-02-18 - Last updated in NVD database

Technical Details for CVE-2026-2557

Vulnerability Analysis

This vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The flaw resides in the file upload handling mechanism within the MediaController.java component of cskefu. When users upload files through the affected endpoint, the application fails to properly sanitize or validate the uploaded content or associated metadata before rendering it in the browser context.

The network-accessible nature of this vulnerability means that any authenticated user with access to the file upload functionality can potentially exploit it. The attack requires user interaction—a victim must access a page where the malicious payload is rendered. While the confidentiality impact is limited, the integrity of user sessions and application data could be compromised through successful exploitation.

Root Cause

The root cause stems from insufficient input validation and output encoding in the file upload processing logic. The Upload function in MediaController.java does not adequately sanitize user-controlled input before incorporating it into web pages served to other users. This includes potential attack vectors through:

• File names containing malicious JavaScript
• File metadata that gets reflected in the application UI
• Uploaded file content that may be rendered without proper content-type enforcement

Attack Vector

The attack is launched remotely over the network by an authenticated attacker who uploads a specially crafted file or manipulates upload parameters to inject malicious script content. When another user views the uploaded content or related pages, the malicious JavaScript executes within their browser session. The attacker can leverage this to:

• Steal session cookies and authentication tokens
• Perform actions on behalf of the victim user
• Redirect users to phishing sites
• Deface application pages viewed by victims

The vulnerability manifests during the file upload process where user-supplied data is improperly handled. Attackers can inject XSS payloads through file names, metadata, or content that gets rendered in the browser. For detailed technical information, refer to the Feishu Document Resource and VulDB entry #346165.

Detection Methods for CVE-2026-2557

Indicators of Compromise

• Unusual file names containing JavaScript syntax such as <script>, onerror=, or javascript: patterns in upload logs
• HTTP requests to the file upload endpoint with suspicious encoded payloads (URL-encoded or HTML-encoded script tags)
• Browser console errors or unexpected script execution warnings reported by users
• Session anomalies or unauthorized actions performed without user knowledge

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect XSS payloads in file upload requests
• Deploy Content Security Policy (CSP) headers to prevent inline script execution and report violations
• Monitor application logs for file names and upload parameters containing HTML or JavaScript syntax
• Use automated security scanning tools to regularly test file upload functionality for XSS vulnerabilities

Monitoring Recommendations

• Enable detailed logging for the /resource/media upload endpoints and related controller actions
• Configure alerting for CSP violation reports that may indicate XSS exploitation attempts
• Monitor for unusual patterns in uploaded file metadata and naming conventions
• Review access logs for repeated upload attempts from single sources that may indicate attack probing

How to Mitigate CVE-2026-2557

Immediate Actions Required

• Restrict access to the file upload functionality to only trusted and necessary users
• Implement strict Content Security Policy (CSP) headers to mitigate XSS impact
• Review and sanitize all existing uploaded file names and metadata in the database
• Consider temporarily disabling the affected file upload component if not critical to operations

Patch Information

The vendor (cskefu) was contacted regarding this vulnerability but did not respond. No official patch is currently available from the vendor. Organizations should monitor the cskefu project for any future security updates. Until an official patch is released, implementing the workarounds below is strongly recommended.

Workarounds

• Implement server-side input validation to strip or encode HTML and JavaScript from file names and metadata
• Configure the web server to set Content-Disposition: attachment headers for all uploaded files to prevent inline rendering
• Apply output encoding (HTML entity encoding) when displaying any user-controlled file upload data
• Deploy a reverse proxy or WAF with XSS filtering capabilities in front of the application
• Restrict file upload functionality to administrative users only until a patch is available

# Example: Apache configuration to force downloads and add security headers
<Directory "/var/www/uploads">
    # Force all files to download rather than execute
    Header set Content-Disposition "attachment"
    
    # Add XSS protection headers
    Header always set X-Content-Type-Options "nosniff"
    Header always set X-XSS-Protection "1; mode=block"
    
    # Disable script execution
    Options -ExecCGI
    RemoveHandler .php .phtml .php3 .php4 .php5 .html .htm
</Directory>