The best action that you should take in this situation is to write a high priority defect and follow the escalation communication plan as defined in your test plan. This is because a critical defect that causes data corruption is a serious issue that can affect the quality, functionality, and reliability of the system, and it needs to be reported and resolved as soon as possible. By writing a high priority defect, you can document the details and the impact of the defect, and alert the development team and the project manager about it. By following the escalation communication plan, you can ensure that the defect is communicated to the appropriate stakeholders, such as the business sponsors, the customers, or the senior management, in a timely and effective manner, and that the necessary actions are taken to address the defect. Defect Reporting - ISTQB not-for-profit association References: Certified Tester Advanced Level Test Manager (CTAL-TM) - ISTQB not-for-profit association, ISTQB Test Manager Certification - ISTQB Exams Worldwide - ISTQB Official Registration, Defect Reporting - ISTQB not-for-profit association