In the scenario where a hacker sends a malicious email link leading to backdoor access, what type of attack is illustrated?

The scenario describes a situation where a hacker sends a malicious email link that users might click on, leading to backdoor access into a system. This fits the characteristics of unvalidated redirects and forwards.

In this type of attack, the malicious actor manipulates the navigation of users, directing them to unintended destinations without proper validation from the application. When the victim clicks the link, they are unknowingly redirected to a site that might exploit vulnerabilities in their system or download malware, effectively creating that backdoor access. Proper validation of URLs is crucial to safeguard against this type of attack, as it ensures that users are only sent to trusted and intended locations. The unvalidated nature of the redirect allows for exploitation, which is evident in this scenario.

The other options do not align with the specifics of the situation. Pharming involves redirecting users from legitimate sites to fraudulent ones, typically without the direct action of clicking a link. Cross-site request forgery tricks a user into executing unwanted actions on a website where they are authenticated, while denial of service aims to overwhelm systems or networks, rendering them inaccessible. Each of these methods operates under different mechanisms than what is illustrated in the scenario.