Explanation
The finding that should be of greatest concern to an IS auditor reviewing data conversion and migration during the implementation of a new application system is that unauthorized data modifications occurred during conversion. Data conversion and migration is a process that involves transferring data from one system to another, ensuring its accuracy, completeness, integrity, and usability. Unauthorized data modifications during conversion can result in data loss, corruption, inconsistency, or duplication, which can affect the functionality, performance, reliability, and security of the new system. Unauthorized data modifications can also have serious business implications, such as affecting decision making, reporting, compliance, customer service, and revenue. The IS auditor should verify that adequate controls are in place to prevent, detect, and correct unauthorized data modifications during conversion, such as access control, data validation, reconciliation, audit trail, and backup and recovery. The other findings (A, B and D) are less concerning, as they can be mitigated by documenting the change management process, restoring the backups of the old system and data from offline storage, or automating the data conversion process. References: CISA Review Manual (Digital Version), Chapter 3: Information Systems Acquisition, Development & Implementation, Section 3.4: System Implementation

Explanation/Reference:
If an IS auditor observes that an IS department fails to use formal documented methodologies, policies, and standards, the auditor should at least document the informal standards and policies, and test for compliance. Furthermore, the IS auditor should recommend to management that formal documented policies be developed and implemented.

Without a policy defining who has the responsibility for granting access to specific systems, there is an increased risk that one could gain (be given) system access when they should not have authorization. By assigning authority to grant access to specific users, there is a better chance that business objectives will be properly supported.