Azure Storage – 403 error notifications when attempting to make storage calls

Summary of Impact:

Starting approximately 19:00 UTC on March 15, 2021 customers may have encountered errors performing authentication operations for any Microsoft and third-party applications that depend on Azure Active Directory (Azure AD) for authentication.

The Azure Portal, Microsoft Teams, Exchange, Azure Key Vault, SharePoint and other applications have recovered. Other applications are in the process of recovering and impacted customers will continue to receive updates regarding these.

Preliminary Root Cause:

The preliminary analysis of this incident shows that an error occurred in the rotation of keys used to support Azure AD’s use of OpenID, and other, Identity standard protocols for cryptographic signing operations. As part of standard security hygiene, an automated system, on a time-based schedule, removes keys that are no longer in use. Over the last few weeks, a particular key was marked as “retain” for longer than normal to support a complex cross-cloud migration. This exposed a bug where the automation incorrectly ignored that “retain” state, leading it to remove that particular key.  

Metadata about the signing keys is published by Azure AD to a global location in line with Internet Identity standard protocols. Once the public metadata was changed at 19:00 UTC, applications using these protocols with Azure AD began to pick up the new metadata and stopped trusting tokens/assertions signed with the key that was removed. At that point, end users were no longer able to access those applications. 

Impacted Services and Regions

Service Name

Region

Storage

Central India
Southeast Asia
South India
UK South

Mitigation:

Service telemetry identified the problem, and the engineering team was automatically engaged. The key removal operation was identified as the cause, and the key metadata was rolled back to its prior state at 21:05 UTC.  

Applications need to pick up the rolled back metadata and refresh their caches with the correct metadata. Time to mitigation for individual applications varies due to a variety of server implementations that handle caching differently. The Azure Portal, Microsoft Teams, Exchange, Azure Key Vault, SharePoint and other major applications have recovered. Other applications are in the process of recovering.  

Azure AD is in a multi-phase effort to apply additional protections to the backend Safe Deployment Process (SDP) system to prevent a class of risks including this problem. The first phase does provide protections for adding a new key, but the remove key component is in the second phase which is scheduled to be finished by mid-year. A previous Azure AD incident occurred on September 28th, 2020 and both incidents are in the class of risks that will be prevented once the multi-phase SDP effort is completed.  

Next Steps:

We understand how incredibly impactful and unacceptable this is and apologize deeply. We are continuously taking steps to improve the Microsoft Azure Platform and our processes to help ensure such incidents do not occur in the future.  

In the September incident we indicated our plans to “apply additional protections to the Azure AD service backend SDP system to prevent the class of issues identified here.”  

    • The first phase of those SDP changes is finished, and the second phase is in a very carefully staged deployment that will finish mid-year. The initial analysis does indicate that once that is fully deployed, it will prevent the type of outage that happened today, as well as the related incident in September 2020. In the meantime, additional safeguards have been added to our key removal process which will remain until the second phase of the SDP deployment is completed.  
    • In that September incident we also referred to our rollout of Azure AD backup authentication. That effort is progressing well. Unfortunately, it did not help in this case as it provided coverage for token issuance but did not provide coverage for token validation as that was dependent on the impacted metadata endpoint. 

The Root Cause Analysis investigation relating to this incident is ongoing, and a full RCA will be published when this is completed, or if any other substantive details emerge in the interim.  

Disclaimer:

We do not seek any guarantee about this message. This was an automated message sent through email to Microsoft Azure customer in the above impacted regions. 

Was this Article Helpful?

Leave a Reply

%d bloggers like this: