CVE-2026-25739: Indico Event Management XSS Vulnerability

CVE-2026-25739 Overview

CVE-2026-25739 is a Cross-Site Scripting (XSS) vulnerability affecting Indico, a Flask-based event management system that utilizes Flask-Multipass for multi-backend authentication. The vulnerability exists in versions prior to 3.3.10 and allows attackers to execute malicious scripts by uploading certain file types as materials within the platform.

Critical Impact

Authenticated users with material upload permissions can inject malicious scripts that execute in the context of other users' browsers, potentially leading to session hijacking, credential theft, or unauthorized actions within the Indico platform.

Affected Products

• Indico versions prior to 3.3.10
• Installations using nginx with STATIC_FILE_METHOD set to xaccelredirect
• Flask-Multipass integrated deployments

Discovery Timeline

• 2026-02-19 - CVE CVE-2026-25739 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2026-25739

Vulnerability Analysis

This Cross-Site Scripting vulnerability (CWE-79) resides in the material upload functionality of Indico. The application fails to properly sanitize or restrict certain file types when users upload materials, allowing attackers to upload files containing malicious JavaScript code. When other users access or download these materials, the malicious scripts execute within their browser context.

The vulnerability is network-accessible and requires the attacker to have low-level privileges (authenticated user status) along with user interaction from the victim. The changed scope characteristic indicates that the vulnerability can affect resources beyond the vulnerable component's security authority, meaning the injected scripts can potentially access data or perform actions across different security contexts within the application.

Root Cause

The root cause of this vulnerability is insufficient input validation and content sanitization during the file upload process for materials. Indico versions prior to 3.3.10 did not implement adequate Content Security Policy (CSP) restrictions for file downloads, nor did they properly validate the content type and structure of uploaded files to prevent XSS payloads from being served to other users.

Attack Vector

The attack vector for CVE-2026-25739 requires network access and exploits the material upload functionality available to authenticated users. Speakers at events, who typically have material upload permissions, can leverage this vulnerability. The attack flow involves uploading a specially crafted file containing JavaScript code, which then executes when other users interact with the uploaded material through the Indico web interface.

Since no verified code examples are available for this vulnerability, security researchers should consult the GitHub Security Advisory GHSA-jxc4-54g3-j7vp for detailed technical information about the exploitation mechanism.

Detection Methods for CVE-2026-25739

Indicators of Compromise

• Unusual file uploads containing embedded JavaScript or HTML content in material sections
• Server logs showing requests to material download endpoints with unexpected content-type responses
• User reports of unexpected browser behavior when accessing event materials
• Evidence of session token access or unauthorized actions following material downloads

Detection Strategies

• Monitor file upload activity for suspicious file types or content patterns that may contain script elements
• Implement Content Security Policy violation reporting to detect XSS attempts in real-time
• Review web server access logs for material download endpoints with anomalous referrer or user-agent patterns
• Deploy web application firewalls (WAF) with XSS detection rules targeting file upload and download paths

Monitoring Recommendations

• Enable verbose logging for material upload and download operations within Indico
• Configure CSP reporting endpoints to capture and analyze policy violations
• Implement file integrity monitoring for uploaded materials directories
• Set up alerts for users with elevated upload permissions performing bulk or unusual upload activity

How to Mitigate CVE-2026-25739

Immediate Actions Required

• Upgrade Indico to version 3.3.10 or later immediately
• Review recent material uploads for potentially malicious content
• Update nginx webserver configuration if using STATIC_FILE_METHOD set to xaccelredirect to apply strict CSP
• Audit user accounts with material upload permissions and restrict access to trusted users only

Patch Information

The Indico development team has released version 3.3.10 which addresses this vulnerability. The patch implements a strict Content Security Policy (CSP) that Indico now applies by default for file downloads. Users should upgrade to this version and consult the GitHub Indico Release v3.3.10 for complete release notes and upgrade instructions.

For deployments using nginx with STATIC_FILE_METHOD set to xaccelredirect, additional webserver configuration updates are required to fully benefit from the CSP protections. Detailed configuration guidance is available in the GitHub Security Advisory and the Indico setup documentation.

Workarounds

• Apply a strict Content Security Policy for material download endpoints at the webserver configuration level
• Restrict material upload permissions to only highly trusted users
• Disable speaker upload capabilities temporarily until the patch can be applied
• Implement additional server-side file validation to reject files with embedded script content

# Example nginx CSP header configuration for material downloads
# Add to nginx server block for Indico material endpoints
location /event/materials/ {
    add_header Content-Security-Policy "default-src 'none'; script-src 'none'; style-src 'none';" always;
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-Frame-Options "DENY" always;
}