What action most effectively minimizes the threat from cross-site scripting (XSS) attacks on a web portal?

Implementing a web application firewall (WAF) is the most effective action to minimize the threat from cross-site scripting (XSS) attacks on a web portal. A WAF is specifically designed to filter, monitor, and protect web applications from various types of attacks, including XSS. It analyzes incoming traffic and can block malicious scripts or patterns that resemble XSS payloads before they reach the application, thereby providing an essential layer of security.

While updating browser plugins, using HTTPS, and conducting user training are beneficial practices in enhancing overall security, they do not directly address the specific vulnerabilities that XSS attacks exploit. Regularly updating browser plugins helps ensure that known vulnerabilities in those plugins are patched, but it does not prevent the web application itself from being compromised through XSS. Using HTTPS secures data in transit but does not prevent attacks that exploit vulnerabilities in the web application code. Conducting user training on safe browsing practices can raise awareness about potential threats but does not provide any direct protection against XSS vulnerabilities that can be exploited through malicious scripts embedded in web applications.

Thus, utilizing a WAF focuses directly on mitigating the risk associated with XSS attacks by filtering out harmful input, making it the most effective choice for this particular threat scenario.