Phishing attempts that target users with fake usage bills from a cloud provider are examples of a cloud to user attack surface. This type of attack surface refers to the potential vulnerabilities and entry points that exist between the cloud service provider and the user. In this scenario, the attacker is exploiting the trust relationship between the user and the cloud service provider by presenting a fraudulent bill, hoping the user will reveal sensitive information or make a payment based on the fake bill.
References: The explanation is consistent with the Certified Network Defender (CND) curriculum, which includes understanding various attack surfaces, including cloud and user-related surfaces, and how they can be exploited through phishing and other social engineering attacks12.