What is the primary security risk associated with implementing custom HTML tags that execute arbitrary JavaScript code, and how can it be mitigated?

The primary security risk associated with implementing custom HTML tags that execute arbitrary JavaScript code is the potential for introducing malicious code or vulnerabilities that can compromise website security, user data, or website functionality. Arbitrary JavaScript means the code can perform almost any action on the page, including reading cookies, modifying the DOM (Document Object Model), redirecting users, or injecting malware. If a custom HTML tag contains poorly written or intentionally malicious JavaScript, it can be exploited to perform cross-site scripting (XSS) attacks, steal sensitive information, or deface the website. Mitigation strategies include rigorous code review, sandboxing, and Content Security Policy (CSP). Code review involves carefully examining the JavaScript code for potential vulnerabilities, such as insecure DOM manipulation or reliance on untrusted user input. Sandboxing limits the permissions granted to the custom HTML tag, preventing it from accessing certain resources or performing sensitive operations. CSP is an HTTP header that allows you to define a whitelist of sources from which the browser is allowed to load resources, reducing the risk of injecting malicious scripts from unauthorized domains. Additionally, regularly auditing custom HTML tags and limiting the number of users with permissions to create or modify them can further reduce the risk. Using custom templates over custom HTML tags reduces the attack surface, because the GTM template sandbox limits which functions can be called. Never include sensitive data, like API keys, directly in the custom HTML tag. Use server-side processing to retrieve such keys and make them available as variables.