The Challenge: When Good Credentials Go Bad

How to properly invalidate assumed role sessions and prevent credential leakage

Imagine you’re the boss of a big toy store with lots of special rooms:

🏪 Your Toy Store Setup:

• You have magic wristbands that let people into different rooms
• Some rooms have video games, some have puzzles, some have art supplies
• You give out temporary wristbands that work for a few hours, then stop working

👥 The People in the Picture:

• The 3 people with different faces = your friends/workers
• Each person gets a magic wristband to visit certain rooms
• The dog = a sneaky troublemaker who shouldn’t be there

🔑 Here’s What Happens:

Step 1: You give your friends temporary wristbands using a special machine (this is like sts:AssumeRole)

Step 2: OH NO! One of your friends accidentally drops their wristband, and the sneaky dog finds it! Now the dog has a copy of the wristband.

Step 3: You notice this problem and want to stop the dog RIGHT NOW!

🚫 Why This is Tricky:

• If you change WHO can use the wristband machine, it doesn’t affect wristbands already given out
• The dog still has the copied wristband and can keep using it until it expires
• Your good friends also still have their wristbands

✅ The Smart Solution: You put up a new rule on every room’s door that says: “No wristbands made before 3 PM today can enter!”

What happens:

• ❌ The dog’s old stolen wristband stops working immediately
• ❌ Your friends’ old wristbands also stop working
• ✅ But your friends can go back to the wristband machine and get NEW wristbands that work
• ❌ The sneaky dog CAN’T use the wristband machine (because dogs aren’t allowed to use it), so they’re stuck with a broken wristband!

The Magic: Your friends just need to get new wristbands (takes 2 seconds), but the bad guy is completely locked out forever!

Understanding the Diagram:

Looking at your diagram, here’s what each part means in our toy store story:

1️⃣ The Wristband Machine (sts:AssumeRole) - This is where good people get their temporary wristbands

2️⃣ The Credential Leak — When someone drops their wristband and the dog finds it

3️⃣ The New Door Rule (AWSRevokeOlderSessions) - The sign that says "no old wristbands allowed"

4️⃣ DENY assumptions before NOW — This means any wristband made before right now doesn’t work anymore

5️⃣ The three people — They can get new wristbands easily because they’re allowed to use the machine

6️⃣ The bad dog — Stuck with a broken wristband and can’t get a new one because dogs can’t use the wristband machine

The key insight: It’s like having a master switch that breaks all old keys instantly, but only the good guys can make new keys!

Does this make more sense now? The important thing is that you can instantly stop ALL old access, but good users can quickly get new access while bad actors stay locked out permanently.

The Challenge: When Good Credentials Go Bad

In AWS Identity and Access Management (IAM), temporary credentials obtained through sts:AssumeRole provide a powerful mechanism for secure, time-limited access to AWS resources. However, what happens when you need to revoke these credentials immediately? Perhaps you've detected suspicious activity, an employee has left the company, or you've discovered a potential credential leak.

The answer isn’t as straightforward as you might think.

Trust vs. Permissions: Understanding the Distinction

AWS IAM operates on two fundamental concepts that many practitioners conflate:

Trust Policy

• Controls who can assume a role
• Lives at the role level
• Acts as the gatekeeper for sts:AssumeRole operations

Permissions Policy

• Controls what an assumed identity can do
• Attached to roles, users, or groups
• Defines access to AWS resources and actions

This distinction is crucial for understanding credential revocation behavior.

The Revocation Paradox

Here’s where things get interesting (and potentially problematic):

Modifying the Trust Policy has NO impact on existing credentials. If an identity has already successfully called sts:AssumeRole and obtained temporary credentials, changing the trust policy won't invalidate those active sessions. The credentials remain valid until their natural expiration.

Modifying the Permissions Policy impacts ALL credentials immediately. Any change to what the role can do affects both existing and future sessions instantly.

The AWSRevokeOlderSessions Condition: Your Security Lifeline

AWS provides a elegant solution through the AWSRevokeOlderSessions condition key. This inline DENY statement can be added to any permissions policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Action": "*",
      "Resource": "*",
      "Condition": {
        "DateLessThan": {
          "aws:TokenIssueTime": "2024-01-15T12:00:00Z"
        }
      }
    }
  ]
}

This condition effectively says: “Deny all actions for any session that was issued before the specified timestamp.”

Real-World Scenario: Credential Leak Response

Let’s walk through a practical example:

1. Detection: You discover that temporary credentials may have been exposed
2. Immediate Action: Add the AWSRevokeOlderSessions condition to the relevant role's permissions policy with the current timestamp
3. Result: Any sessions created before “NOW” are immediately invalidated
4. Verification: Legitimate users can simply re-assume the role to get new, valid credentials

Why Bad Actors Can’t Bypass This

The beauty of this approach lies in AWS’s security model:

• Bad actors cannot directly call sts:AssumeRole if they only have leaked temporary credentials (not the underlying identity that can assume the role)
• They remain stuck with invalidated sessions that no longer have any permissions
• Legitimate users can recover immediately by re-assuming the role

Best Practices for Credential Revocation

1. Implement Monitoring

Set up CloudTrail monitoring for unusual sts:AssumeRole patterns and failed API calls that might indicate compromised credentials.

2. Automate Response

Consider building automated responses that can quickly apply AWSRevokeOlderSessions conditions when suspicious activity is detected.

3. Regular Credential Rotation

Even without incidents, periodic rotation of long-lived credentials and role assumptions can limit exposure windows.

4. Least Privilege Permissions

Minimize the blast radius of any potential compromise by ensuring roles have only the minimum necessary permissions.

5. Session Duration Management

Use shorter session durations for sensitive roles to naturally limit the window of potential misuse.

Conclusion

Understanding the nuanced behavior of AWS temporary credential revocation is essential for maintaining robust cloud security. The AWSRevokeOlderSessions condition provides a powerful tool for immediate credential invalidation, but it requires understanding the distinction between trust and permissions policies.

By implementing proper monitoring, automated responses, and following security best practices, organizations can confidently use AWS temporary credentials while maintaining the ability to quickly respond to security incidents.

Remember: in cloud security, it’s not just about preventing breaches — it’s about having the tools and knowledge to respond effectively when they occur.