CVE-2020-13959

What it actually does

The default error page for VelocityView reflects the vm file parameter from the URL back to the user, enabling reflected XSS by embedding arbitrary JavaScript in the vm filename.

Why it matters to the business

Attackers may steal session cookies, impersonate users, or deliver phishing content, resulting in account takeovers, data exposure, and loss of user trust.

How an attack unfolds

An attacker appends a crafted .vm filename containing an XSS payload to a VelocityView endpoint URL; when a victim loads the link the error page executes the script in the victim's browser context.

How to fix it

Upgrade Apache Velocity Tools to version 3.1 or later; ensure all dependent applications are rebuilt against the patched library.

How to confirm the fix worked

Confirm the installed version is 3.1 or newer; test that supplying an arbitrary .vm filename in the URL no longer reflects executable content in the error page.

Typical attack path

1. Identify a VelocityView endpoint that triggers the default error page
2. Craft a URL embedding an XSS payload inside the vm file parameter
3. Deliver the malicious URL via phishing or other channels
4. Victim interaction causes script execution in the browser
5. Attacker harvests cookies or performs actions on behalf of the victim

How to detect it

Search web server and application logs for requests to VelocityView endpoints that contain unusual .vm filenames with HTML or script tags; inspect proxy or WAF logs for reflected XSS patterns matching the error-page behavior.

What your security team should do

• Prioritize upgrade of all Velocity Tools instances to 3.1+
• Scan codebases and dependency manifests for references to velocity-tools
• Add web application firewall rules to block reflected script patterns in URL parameters
• Review and harden error-page handling across web applications