Microsoft reports that it has mitigated an attack by a China-based threat actor that the tech giant tracks as Storm-0558 which targeted customer emails. Storm-0558 primarily targets government agencies in Western Europe and focuses on espionage, data theft, and credential access. The U.S. departments of State and Commerce have said they were among the agencies affected.
Based on customer reported information on June 16, 2023, Microsoft began an investigation into anomalous mail activity. Over the next few weeks, the investigation revealed that beginning on May 15, 2023, Storm-0558 gained access to email accounts affecting approximately 25 organizations including government agencies as well as related consumer accounts of individuals likely associated with these organizations. They did this by using forged authentication tokens to access user email using an acquired Microsoft account (MSA) consumer signing key. Microsoft said on July 11 that it has completed mitigation of this attack for all customers.
Microsoft’s telemetry indicates that it has successfully blocked Storm-0558 from accessing customer email using forged authentication tokens. No customer action is required. As with any observed nation-state actor activity, Microsoft has contacted all targeted or compromised organizations directly via their tenant admins and provided them with important information to help them investigate and respond. The company says it continues to work closely with these organizations and notes that if you have not been contacted, investigations indicate that you have not been impacted.
Microsoft is partnering with the Department of Homeland Security’s Cybersecurity and Infrastructure Security Agency and others to protect affected customers and address the issue. It also continues to investigate and monitor the Storm-0558 activity.
Microsoft investigations determined that Storm-0558 gained access to customer email accounts using Outlook Web Access in Exchange Online (OWA) and Outlook.com by forging authentication tokens to access user email.
The actor used an acquired MSA key to forge tokens to access OWA and Outlook.com. MSA (consumer) keys and Azure AD (enterprise) keys are issued and managed from separate systems and should only be valid for their respective systems. The actor exploited a token validation issue to impersonate Azure AD users and gain access to enterprise mail. Microsoft has no indications that Azure AD keys or any other MSA keys were used by this actor. OWA and Outlook.com are the only services where the company has observed the actor using tokens forged with the acquired MSA key.
Microsoft has mitigated the acquired MSA key and its telemetry indicates the actor activities have been blocked. The company took the following proactive steps as the investigation proceeded:
- Blocked the usage of tokens signed with the acquired MSA key in OWA preventing further threat actor enterprise mail activity.
- Completed the replacement of the key to prevent the threat actor from using it to forge tokens.
- Blocked usage of tokens issued with the key for all impacted consumer customers.
In addition, Microsoft reports that it has continuously improved the security of the MSA key management systems since the acquired MSA key was issued, as part of defense in depth, to ensure the safety and security of consumer keys.