CVE-2020-13959: 
Java vulnerability analysis and mitigation

The vulnerability (CVE-2020-13959) affects Apache Velocity Tools versions prior to 3.1. The default error page for VelocityView reflects back the vm file that was entered as part of the URL, creating a cross-site scripting (XSS) vulnerability. This security flaw was discovered and reported by Jackson Henry, a member of Sakura Samurai (Openwall, Debian Security).

The vulnerability stems from the default error page functionality in VelocityView, where the system reflects back the vm file specified in the URL without proper sanitization. An attacker can exploit this by setting an XSS payload file as the vm file parameter in the URL, resulting in the execution of arbitrary JavaScript code in the context of the attacked website and user (Openwall).

When successfully exploited, this XSS vulnerability enables attackers to execute arbitrary JavaScript in the context of the attacked website and the targeted user. The impact includes potential theft of session cookies, unauthorized request execution on behalf of the victim, and the possibility of conducting phishing attacks (Debian LTS, Openwall).

The vulnerability has been addressed in Apache Velocity Tools version 3.1, which implements proper escaping of the reflected text on the default error page to prevent JavaScript execution. Organizations using affected versions are strongly recommended to upgrade to version 3.1 or later (Openwall). Various Linux distributions have also released security updates, including Debian which fixed the issue in version 2.0-6+deb9u1 for Debian 9 'Stretch' (Debian LTS).