Which of these threats is least likely to be a common email threat vector?

Cross-site scripting (XSS) is least likely to be a common email threat vector because it primarily exploits vulnerabilities in web applications and is executed in the context of a user's browser rather than through an email client. XSS attacks generally involve injecting malicious scripts into web pages that are then viewed by users. This means the attacker typically needs to lure victims to a compromised website where the scripts can run, rather than sending emails.

On the other hand, phishing directly targets users through email by tricking them into providing sensitive information, such as usernames and passwords. Malware can also be widely distributed via email attachments or links, making it a prominent email threat. Ransomware can be spread through malicious email attachments or links that, when activated, encrypt user files and demand a ransom. Therefore, these threats are more directly associated with email than cross-site scripting, which operates in a different context.