IIS logs are text files that log every request made to a web server running Internet Information Services (IIS). They record who did what, when, and how on your site.
For anyone in cybersecurity, these logs are pure gold for spotting threats, troubleshooting issues, and refining defenses.
Web server logs sound boring, right? But not so fast. If you’re tasked with keeping your networks secure, ignoring IIS logs is like sealing your vault but leaving the keys on the counter. These highly detailed records aren’t just for IT admins; they’re an essential tool for detection, forensics, and ongoing cyber hygiene.
This blog answers “what are IIS logs” in plain English, shows you “how to read IIS logs,” details where to find them, what data they capture, and shares best practices for secure log management. Whether you’re prepping for an exam, leveling up your on-the-job analysis, or launching your first IIS logs tutorial for the team, this glossary guide breaks it down for every skill level.
IIS logs are records created by Microsoft’s Internet Information Services (IIS) web server software. Whenever a browser or bot visits an IIS-powered website, the server records a log entry of that request. This log is a detailed receipt of who visited, what they requested, when it happened, and how it all went down. Learn more about server logs.
Why care? These logs form a timeline of every action taken on the server. For cybersecurity pros they are rich with forensic evidence to investigate breaches, detect unauthorized activity, and meet compliance requirements.
Incident detection: Early indicators of compromise hide in the noise. IIS logs can flag brute-force attempts, weird URLs, or unknown IPs hammering away at your login page.
Forensics: Post-breach analysis is all about piecing together who did what. IIS logs allow investigators to build a minute-by-minute map of adversary actions.
Performance and troubleshooting: Not all log events mean an attack. IIS logs are vital for tracking 404 errors, latency issues, or failed requests.
Compliance: Many regulations require storing web server logs for a set period, making retention policy a must.
TL;DR: IIS logs play a starring role in both offense (detection) and defense (analysis).
The format of IIS logs matters a lot. The most common format is the W3C Extended Log File Format. It’s customizable, so you can decide which fields are logged.
date and time: When each request happened.
c-ip: The client’s IP address (who connected).
cs-username: Authenticated username, if any.
s-ip: The IIS server’s own IP.
s-port: Port number used.
cs-method: HTTP method (GET, POST, etc.).
cs-uri-stem: Requested resource (e.g., /index.html).
sc-status: HTTP response code (200, 404, 500, etc.).
sc-bytes and cs-bytes: Bytes sent/received.
cs(User-Agent): Browser or client detail.
See everything you can capture? That’s a huge amount of actionable data. Find the field cheat sheet (with explanations):
2024-07-20 12:10:15 192.168.2.23 GET /admin 404 0 2 265 101 Mozilla/5.0
This line says a client tried to access "/admin" at a specific time, but hit a 404.
Pro tip: Custom logging fields can be added for even deeper monitoring, like tracking headers for security tokens.
On Windows servers, log files are typically found at
C:\inetpub\logs\LogFiles\W3SVC#
(where “#” matches the IIS site ID)
You can pick a different directory when you configure logging settings.
Handy if you use a SIEM or want logs on a drive with more space.
IIS logs may be routed to Azure Blob Storage
Open IIS Manager on your server
Select the website or server node in the tree
Double-click the Logging icon in the Features View
Choose the log file format (W3C is recommended)
Pick which fields to capture (hint: more is usually better for security!)
Select the log file directory
Set how often new log files roll over (daily, weekly, by size)
Click Apply to save
That’s it! Your logs are now rolling. Extra: Want help automating this process? PowerShell scripts can bulk-enable logging across multiple sites.
How do you read IIS logs without going cross-eyed?
Open in a spreadsheet for easy column sorting.
Use a purpose-built IIS logs viewer (many are free or open source).
For power users, tools like Microsoft Log Parser or Sumo Logic make slicing and dicing logs a breeze.
Track down specific events (e.g., 404 errors, failed logins) by filtering with search tools or scripts.
Common use-cases for beginners
Count the number of 500 errors in a time range
Spot login attempts from odd geographic locations
Flag changes in user-agent strings (e.g., bot traffic)
Trace back what else an attacker did during an incident
Don’t suffer with Notepad forever. Here are tried-and-true tools for deeper log dives:
Microsoft Log Parser: Classic tool for querying logs with SQL-like syntax.
OpenObserve: Centralizes and visualizes logs, adds alerting, and makes it easy to search for security signals. Great for teams.
Sumo Logic and Splunk: Offer SIEM-style analytics, aggregation, and real-time alerting.
PowerShell: For automation junkies.
Open-source log viewers: Some, like LogExpert or LogViewPlus, display logs in real time.
Azure-native options: Log Analytics and Azure Monitor can ingest IIS logs.
Try them out and see which works best for your stack and security goals.
Don’t wait until your log folder eats the entire C drive!
Here’s the lowdown on keeping logs useful (and legal):
Retention policy: Set a policy for how long to keep logs (often 90 days to years, depending on compliance). Regular archiving = good; hoarding forever = bad.
Regular backups: Protect logs from loss or tampering.
Automated archiving: Move old logs to a secure location or cloud storage.
Upgrade often: New IIS versions often have better logging and security features.
Monitor for bloat: Automated alerts if disk space is running low.
For high-security environments, restrict log access and enable immutable (write-once) storage.
When an incident strikes, your IIS logs become a digital crime scene.
Forensics workflow, simplified:
Extract all logs from the relevant timeframe (and backups).
Search for suspicious requests (odd URLs, admin panel probes, unexpected POSTs).
Cross-reference user-agent strings, IP addresses, and time stamps.
Look for failed logins, privilege escalations, or rapid-fire requests.
For real-time monitoring, set up alerts (e.g., “more than 20 failed logins in 5 minutes”) with a SIEM.
Handy checklist:
Are logs being written and securely stored?
Is log access auditable?
Can you cross-link IIS logs to Windows event logs and firewall logs?
Cybersecurity standards (NIST, CIS Controls) all call for strong logging and regular review. Here’s more on that from CISA’s best practices.
Empower your team and stay up to date with the latest cybersecurity logging frameworks.
IIS logs are essential for security monitoring, incident response, and compliance.
Proper log configuration and retention is not “set and forget”—review it regularly.
Invest in analysis tools and automation for detecting attacks faster.
Don’t treat logs as boring paperwork. They’re the eyes and ears of your web security operation.
Stay current with logging best practices and new IIS features for stronger defenses.
