The scenario described is a classic example of a ransomware attack. Ransomware is a type of malware that encrypts the victim's data, rendering it inaccessible, and then demands payment for the decryption key. In this case, the attacker has encrypted the company's billing information and client records and is asking for money to restore access, which is characteristic of ransomware attacks.
References: This definition aligns with the information provided by sources such as IBM, which describes ransomware as malware that holds a victim's data or device hostage, threatening to keep it locked-or worse-unless a ransom is paid1. Additionally, the Wikipedia page on ransomware provides a comprehensive overview of this type of malware, including its operation and the typical demand for a ransom in exchange for decryption2.