Authentication logs are records of sign-in activity, such as usernames, timestamps, source IP addresses, device details, success or failure results, and the method used to authenticate. They are one of the first places investigators look when they need to understand who accessed a system and how that access was obtained.
In cyber security, these logs matter because many intrusions start with stolen credentials, password spraying, brute-force attempts, or abuse of remote access tools. Review of authentication logs can reveal impossible travel, unusual login times, repeated failed attempts, new locations, or logins from unfamiliar devices. Defenders use them to confirm whether a leak-site claim, alert, or phishing report is backed by evidence of actual access. Good logging and retention also support incident response by helping teams reconstruct the timeline, identify compromised accounts, and contain further misuse.