How I Finally Set Up Real-Time Alerts for Unauthorized AWS S3 Access (And Why You Should Too)

Your AWS S3 bucket is wide open — and you might not even know it. Here’s how I finally stopped flying blind and started sleeping better at night.

The Wake-Up Call

It was 1:07 AM when I got the email. Someone — from an IP address in a country I’ve never been to — accessed a file in my S3 bucket.

The worst part? It wasn’t even an alert. It was a billing spike. From AWS. That’s how I found out something was wrong. AWS told me about my security breach by charging me more.

That night, I swore I’d never let that happen again. So I rolled up my sleeves and built a real-time alert system for unauthorized S3 access — no third-party magic, no overpriced security tool, just native AWS and a little glue logic. Here’s how to do it — and why it matters more than you think.

Why You Might Already Be Compromised

S3 buckets are deceptively simple. You upload, store, and retrieve. That’s it, right?

• One wrong policy and your files are public.
• One shared link, and you’ve leaked sensitive data.
• One unmonitored bucket, and you’re blind to breaches.

And unless you’ve explicitly built a monitoring system, AWS won’t tell you. No push alert, text, or slack ping, just silence.

What “Real-Time Alerting” Really Means

We’re not talking about a daily digest or a CloudTrail log you’ll never check.

We’re talking:

• Immediate alert when someone accesses a bucket they shouldn’t
• Push notification to Slack, email, or PagerDuty
• Source IP, object accessed, and user identity — all visible
• Automated logs you can trace, audit, and respond to

What You’ll Need

1. S3 Bucket (obviously)
2. AWS CloudTrail — for tracking API calls
3. CloudWatch Logs & Alarms — to catch suspicious events
4. AWS SNS — to push alerts via email, SMS, or Lambda
5. Lambda — for Slack/Discord/Teams integrations

Step-by-Step: How I Set Up Real-Time Unauthorized Access Alerts

Step 1: Enable CloudTrail for S3 Data Events

CloudTrail is your security camera — but it doesn’t track S3 object-level access by default.

1. Go to AWS CloudTrail → Trails
2. Edit your trail → enable “Data Events.”
3. Select your S3 buckets and check both:

• Read events (GetObject)
• Write events (PutObject, DeleteObject)

You can apply filters by prefix (folder paths) to narrow the scope.

Step 2: Filter for Suspicious Access

CloudTrail dumps logs into an S3 bucket or CloudWatch. We want the latter.

1. Set up a CloudWatch Log Group for your CloudTrail.
2. Create a metric filter for something like:

{ $.eventName = "GetObject" && $.sourceIPAddress != "YOUR_WHITELISTED_IP" }

3. Set this filter to trigger an alarm whenever it matches.

Add conditions to ignore known services (e.g., AWS Lambda IPs) and focus on unknown actors.

Step 3: Push Alerts with SNS

1. In CloudWatch, set the alarm’s action to send a notification to SNS.
2. In SNS, create a topic like s3-security-alerts.
3. Subscribe to your email, Slack webhook, or use Lambda to forward it to any service.

You’ll now get pinged the moment a suspicious access event occurs.

Use Lambda to Send Beautiful Slack Alerts

Slack alerts > boring emails. Here’s how:

• Create a Lambda function in Python or Node.
• Parse the CloudTrail event from SNS.
• Format it with emoji, colors, and a link to the CloudTrail console.
• Send it to a Slack webhook.

Or plug it into a PagerDuty incident flow if you’re extra serious.

I Wish I Knew Earlier

1. CloudTrail logs aren’t immediate. There’s a slight delay (usually <5 min).
2. CloudWatch filtering syntax is finicky. Double-check with test logs.
3. SNS has quotas. Don’t DDoS yourself with alert spam.
4. Be intentional about what’s “unauthorized.” Whitelist IPs and roles.

Final Result: I Sleep Again

Now, if someone touches a file they shouldn’t — I know. If someone enables public access — I know. If an API key is abused — I know, and I’m ready. All are built with AWS-native tools, no external services, and no $299/mo “cloud security suite.”