Audit Logs: The Key to Any Cybersecurity Investigation
By M. Crawley
When you get an alert about suspicious activity, the alert itself is just the starting point. The real answers live in your audit logs.
You get an alert at noon on Monday. Someone logged into your the CEO's account from an international location, but you realize that she was just pinging from the Chicago office three minutes ago. From the alert you realize that something's wrong, so the question is what actually happened during that suspicious session. Is it a false positive or is it a true positive? If its the latter, did the unauthorized user just poke around, or did they take action by setting up email forwarding and changing permissions, or did the steal any data?
When I deal with situations like this, the first thing I do is check the audit logs. I know some people freeze up here because they're not sure what they're looking for or even where to start since audit logs can be dense. But the thing is, once you get comfortable reading these logs and understand what you're seeing, they become the quickest way to get answers. If you don't understand them though, you're going to feel a little lost during investigations, and that presents a challenge for anyone working in cybersecurity.
Understanding What Audit Logs Capture
Audit logs are basically a record of everything that happens in a company's environment. When someone tries to log in, this event is recorded in the logs. When they actually succeed, that's also logged. If they access a file or change permissions or even send an email, those events also gets captured with a time of occurrence and details about which account did what and from where.
Whether its Windows Event Viewer on Windows devices, audit logs in your email server, sign-in logs for your domain controller or cloud environment, they're all capturing these types of activities constantly.
How This Plays Out in Real Situations
Let's discuss some scenarios that come up regularly and how audit logs can help you handle them.
When you get an impossible travel alert, the alert itself just tells you that the same account accessed resources from two locations that are geographically too far apart for someone to travel between in that time. But you need more information than that, right? You need to consider the possibilities of what could've happened during each of those sessions. What was accessed? Was anything deleted, moved, or altered? Your audit logs will provide you with a clear timeline of activities, and those patterns tell you whether you're dealing with a compromised account, a VPN issue, or something else.
I've dealt with some phishing compromises, and what makes them tricky is that a user will report that they clicked a suspicious link and entered their credentials. That's helpful, but what I really need to know in these moments is what the attacker did once they gained access. Did they just log in and look around or did they start setting up email forwarding rules, exfiltrating messages, or accessing file shares.
The audit logs tell you this story. You can see if emails were sent from the user's compromised account. You can also check if any mailbox rules were created to hide their activity or forward future emails to an external. You can determine whether the attacker accessed sensitive internal files. All of these curiosities determine whether you're dealing with a basic credential grab that didn't go anywhere or a full compromise that exposed company data.
Failed login attempts are another common scenario. You get alerts that someone is blasting your servers and racking up login failed attempts. Windows Event Viewer becomes critical here because the logs show you which accounts are being targeted, what IP addresses the attempts are coming from, and what authentication methods they're trying. Are they going after everyone or specifically sensitive accounts? Knowing this completely changes how you respond because random attempts hitting everything might be automated scanning, but focused attacks on privileged accounts need immediate attention.
Finding Accounts That Shouldn't Have Access
Beyond responding to active incidents, audit logs help you become aware of security problems that are just waiting to cause issues. You can run audits to find accounts that have permissions they never actually use. For example, if someone got assigned read and write access to sensitive internal files six months ago but the audit logs show they haven't touched those files since, you probably gave them more permission than they needed, so that is worth having a discussion about.
Inactive accounts with active permissions are even worse. Former employees whose accounts didn't get properly deactivated, old service accounts that are no longer being used, and test accounts from projects have already ended. Those are the accounts attackers love to target because nobody's looking at them.
Why This Matters for Compliance and Protection
If you're working with ISO 27001 or similar frameworks, audit logging isn't something you can ignore. You need to provide proof that the company is logging who's accessing sensitive data and tracking administrative actions.
Beyond just checking boxes for compliance, these logs actually protect the organization when things go wrong. When executives or legal teams ask what happened during an incident, you need real, factual answers that audit logs can provide. They show exactly what happened, when it happened, and which accounts were involved. This supports incident response activities, helps with forensic investigations, and gives you the information you need to prevent similar compromises in the future.
Getting Familiar With the Logs
If you're new to working with audit logs, the amount of information can feel overwhelming initially. Start by learning where the logs actually are in your environment. For Windows devices, Event Viewer is where you'll spend a lot of time. For email and cloud services, you'll need to know where those audit logs live in your setup.
The key is getting comfortable by knowing how to filter the logs by user, time period, and event type so you're not drowning in information. Pay attention to the activity in the logs when nothing's wrong so you can start recognizing what the logs mean when there is less pressure. The more time you spend with these logs during regular work, the faster you'll be able to identify problems and follow the crumbs when an actual incident occurs.
Understanding audit logs is fundamental to security work regardless of what level you're at. Getting comfortable with them early makes you more effective when incidents happen, and in security, incidents will happen so best to be prepared.
