A parallel run is when an organization operates the legacy system and the new system at the same time, then compares their outputs to confirm the new platform behaves correctly before it becomes authoritative. This is common during migrations of payments, reporting, identity, and other regulated workloads where a small mismatch can create compliance, accounting, or service errors.
In cyber security, parallel runs matter because they provide validation and evidence. Teams can detect data loss, broken integrations, missing audit logs, and configuration drift before cutover. They also help prove that access controls, encryption, and logging work as intended. The risk is that both systems are active at once, which increases the attack surface and can create confusion if results diverge. Defenders therefore pair parallel runs with tight monitoring, segregated access, and clear rules for which system is the source of truth.