After an intrusion has been detected and contained, the system should be recovered to a known and trusted state. The best practice for ensuring the integrity of the recovered system is to install the OS, patches, and application from the original source, such as the vendor’s website or media. This way, any malicious code or backdoors that may have been inserted by the intruder can be eliminated. Restoring the OS, patches, and application from a backup may not guarantee the integrity of the system, as the backup may have been compromised or outdated. Restoring the application and data from a forensic copy may preserve the evidence of the intrusion, but it may also reintroduce the vulnerability or malware that allowed the intrusion in the first place. Removing all signs of the intrusion from the OS and application may not be sufficient or feasible, as the intruder may have made subtle or hidden changes that are difficult to detect or undo.

References =

ISACA, CISM Review Manual, 16th Edition, 2020, page 2401

ISACA, CISM Review Questions, Answers & Explanations Database - 12 Month Subscription, 2020, question ID 2132

The BEST practice for ensuring the integrity of the recovered system after an intrusion is to restore the OS, patches, and application from a backup. This will ensure that the system is in a known good state, without any potential residual malicious code or changes from the intrusion. Restoring from a backup also enables the organization to revert to a previous configuration that has been tested and known to be secure. This step should be taken prior to conducting a thorough investigation and forensic analysis to determine the cause and extent of the intrusion.