Versions of handlebars prior to 4.3.0 are vulnerable to Prototype Pollution leading to Remote Code Execution. Templates may alter an Object's __proto__ and __defineGetter__ properties, which may allow an attacker to execute arbitrary code through crafted payloads.

The handlebars package before 4.0.0 for Node.js allows remote attackers to conduct cross-site scripting (XSS) attacks by leveraging a template with an attribute that is not quoted.