Joint controllership is a GDPR relationship where two or more parties decide together why personal data is collected and how it is processed. It is not about simple data sharing; it is about shared control over the purposes and means of processing. In practice, that can include a device maker, app provider, platform operator, or analytics partner.
This matters in cyber security because shared control creates shared responsibility. Each controller may need to support lawful processing, user notices, access and deletion requests, and security safeguards such as access control, logging, retention limits, and incident handling. When roles are unclear, attackers and defenders alike benefit from confusion: accountability gaps can hide weak consent flows, poor configuration, or risky data transfers. Clear joint-controller agreements, accurate privacy notices, and auditable settings help prove who is responsible for what and reduce compliance and security exposure.