Microsoft 365 Error Codes

How to Resolve Office 365 Sign-In Error CAA20008

Huma Ashraf Ch Oct 21, 2026 869 views

Question details

The user is unable to sign into Office 365 applications and encounters error code CAA20008, which prevents access to their Microsoft account.

How to Resolve Office 365 Sign-In Error CAA20008

Product: Microsoft Office 365
Device & OS: not provided
Scenario: Attempting to authenticate and sign into Microsoft 365 desktop applications.
Observed behavior: The sign-in process fails with error CAA20008, typically due to outdated TLS configurations, AD FS federation certificate expiration, or organization-wide network login issues.

Before you start

Ensure you have administrative privileges on your Windows device, as modifying Internet Options and TLS protocols requires admin rights.

Solution 1Recommended

Update Internet Options to Enable Modern TLS Protocols

Modifying your internet security settings to use TLS 1.2 and 1.3 usually resolves the CAA20008 sign-in error by meeting Microsoft's modern security requirements.

Microsoft 365 no longer supports older security protocols. If your computer is configured to use legacy TLS versions, the connection to Microsoft authentication servers will be rejected, resulting in the CAA20008 error.

Open Internet Properties

Press the Windows Key + R on your keyboard to open the Run dialog box. Type inetcpl.cpl and press Enter.

Navigate to Advanced Settings

In the Internet Properties window, click on the 'Advanced' tab located at the top right.

Disable Outdated Protocols

Scroll down the settings list to the 'Security' section. Uncheck the boxes for 'Use SSL 3.0', 'Use TLS 1.0', and 'Use TLS 1.1'.

Enable Modern Protocols

Check the boxes for 'Use TLS 1.2' and 'Use TLS 1.3' (if available). Click 'Apply' and then 'OK' to save the changes.

Restart and Sign In

Restart your computer to ensure the network settings take effect, then attempt to sign into Office 365 again.

Update Internet Options to Enable Modern TLS Protocols

Restore Advanced Settings: If the issue persists, return to the Advanced tab in Internet Properties and click the 'Restore advanced settings' button before reapplying the TLS 1.2 and 1.3 checkboxes.

Solution 2

Verify AD FS Federation Status with Your Administrator

If you are on an enterprise network, expired Active Directory Federation Services (AD FS) certificates or update glitches can trigger this authentication error.

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Frequently Asked Questions

What does error code CAA20008 mean in Office 365?

Error CAA20008 typically indicates a network communication or authentication failure. It is often caused by outdated Transport Layer Security (TLS) settings on your local machine or an expired federation certificate on your organization's network.

Why do I need to enable TLS 1.2 to fix this Office sign-in error?

Microsoft has deprecated older, less secure protocols like SSL 3.0, TLS 1.0, and TLS 1.1 across its cloud services. Enabling TLS 1.2 ensures your device meets the modern cryptographic standards required to securely communicate with Office 365 authentication servers.

How do I know if my organization uses AD FS?

You generally will not be able to verify this configuration on your personal workstation. You must contact your company's IT department or system administrator to confirm if Active Directory Federation Services (AD FS) is actively managing your Office 365 logins.

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