CVE-2020-13959: Apache Velocity Tools XSS Vulnerability

CVE-2020-13959 Overview

CVE-2020-13959 is a reflected Cross-Site Scripting (XSS) vulnerability affecting the default error page for VelocityView in Apache Velocity Tools prior to version 3.1. The vulnerability occurs because the error page reflects back the vm file parameter from the URL without proper sanitization. An attacker can craft a malicious URL containing an XSS payload as the vm file parameter, which then gets executed in the context of the victim's browser session when the error page is rendered.

XSS vulnerabilities of this nature allow attackers to execute arbitrary JavaScript code within the security context of the affected website and the victim's browser session. This can be leveraged to steal session cookies, perform unauthorized actions on behalf of the victim, conduct phishing attacks, or redirect users to malicious websites.

Critical Impact

Attackers can execute arbitrary JavaScript in victims' browsers, potentially stealing session cookies, performing unauthorized actions, or conducting phishing attacks against users of applications built with vulnerable Apache Velocity Tools.

Affected Products

• Apache Velocity Tools versions prior to 3.1
• Debian Linux 9.0
• Applications using VelocityView with the default error page configuration

Discovery Timeline

• 2021-03-10 - CVE-2020-13959 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-13959

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting. The root cause lies in how VelocityView's default error page handles user-supplied input from URL parameters.

When a VelocityView application encounters an error, it displays an error page that includes the name of the requested vm (Velocity template) file. The vulnerability exists because this vm file name is extracted directly from the URL and reflected back into the HTML response without proper output encoding or sanitization. This creates a classic reflected XSS attack vector where malicious scripts embedded in the URL parameter are executed by the victim's browser.

The attack requires user interaction, as victims must click on a crafted malicious link for the payload to execute. However, once triggered, the JavaScript runs with full access to the domain's cookies, DOM, and other client-side resources.

Root Cause

The root cause is insufficient input validation and output encoding in the VelocityView error page rendering logic. When the error page reflects the vm file parameter back to the user, it fails to properly escape HTML special characters, allowing embedded script tags or JavaScript event handlers to be interpreted as executable code by the browser.

Attack Vector

The attack is network-based and requires no prior authentication or privileges. An attacker crafts a malicious URL targeting a vulnerable application and distributes it through phishing emails, social engineering, or by embedding it on other websites. When a victim clicks the link, their browser sends a request to the vulnerable application with the XSS payload embedded in the URL. The server responds with an error page that includes the unescaped payload, which the browser then executes.

A typical attack URL would include JavaScript code as the vm file parameter, such as script tags or event handlers that trigger when the error page renders. The payload could exfiltrate session tokens via external requests, modify page content for phishing purposes, or perform actions using the victim's authenticated session.

Detection Methods for CVE-2020-13959

Indicators of Compromise

• Unusual URL patterns in web server logs containing JavaScript code or HTML tags in the vm file parameter
• Error page requests with encoded or obfuscated script payloads in query strings
• Outbound requests from client browsers to suspicious external domains following error page access
• User reports of unexpected browser behavior after clicking links to your application

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing script tags or event handlers in URL parameters
• Configure intrusion detection systems to alert on URL patterns matching known XSS payload signatures
• Enable detailed access logging to capture full request URLs for forensic analysis
• Deploy content security policy (CSP) headers to mitigate the impact of successful XSS attacks

Monitoring Recommendations

• Monitor web server access logs for requests to error pages with unusually long or encoded URL parameters
• Set up alerts for error rate spikes that could indicate exploitation attempts
• Track CSP violation reports to detect blocked XSS attempts
• Review application logs for patterns indicating automated scanning or exploitation

How to Mitigate CVE-2020-13959

Immediate Actions Required

• Upgrade Apache Velocity Tools to version 3.1 or later immediately
• Implement a custom error page that does not reflect user input without proper encoding
• Deploy Content Security Policy headers to restrict inline script execution
• Review and update WAF rules to block XSS payloads in URL parameters

Patch Information

Apache has addressed this vulnerability in Velocity Tools version 3.1. Organizations should upgrade to this version or later to receive the fix. Detailed information about the security update is available through the Apache Velocity User Thread and the Apache Velocity Announce Thread.

Debian users running version 9.0 should consult the Debian LTS Announcement for package updates. Gentoo users can reference the Gentoo GLSA 2021-07-52 advisory for patching guidance.

Workarounds

• Implement a custom error handler that sanitizes all reflected input using proper HTML encoding
• Configure web application firewall rules to strip or block suspicious characters in the vm file parameter
• Deploy strict Content Security Policy headers with script-src 'self' to prevent inline script execution
• Consider disabling the default error page and implementing a static error response that does not reflect any request parameters

# Example: Add Content Security Policy header in Apache configuration
# Add to httpd.conf or .htaccess
Header set Content-Security-Policy "default-src 'self'; script-src 'self'; object-src 'none'"

# Enable mod_headers if not already enabled
a2enmod headers
systemctl restart apache2