The best way to provide assurance of the integrity of a firewall log is to ensure that the log cannot be modified. A firewall log is a record of the traffic and events that occur at the firewall, which is a device or software that controls and filters the incoming and outgoing network traffic based on predefined rules and policies. The integrity of a firewall log means that the log is accurate, complete, consistent, and valid, and that it has not been altered, deleted, or corrupted by unauthorized or malicious parties. The IS auditor should verify that the firewall log has adequate controls to prevent or detect any modification of the log, such as encryption, hashing, digital signatures, write-once media, or tamper-evident seals. The other options are not as effective as ensuring that the log cannot be modified, because they either do not address the integrity of the log data, or they are monitoring or retention measures rather than preventive or detective controls. References:
CISA Review Manual (Digital Version)1, Chapter 5, Section 5.2.4