Client data is any record tied to a customer relationship, such as names, contact details, account information, invoices, contracts, tax documents, support tickets, or payment references. Because it can identify people or expose business activity, it often carries legal, financial, and reputational risk if it is leaked, altered, or misused.
In cyber attacks, client data is frequently highlighted in extortion notes and leak posts because it increases pressure on the victim: disclosure can lead to privacy complaints, fraud, regulatory obligations, and loss of trust. Defenders reduce that risk by classifying sensitive records, limiting access, encrypting data, logging file activity, and testing backup and incident response processes so they can contain exposure quickly if an intrusion or exfiltration event occurs.