Microsoft mitigated an attack by Chinese threat actor Storm-0558

Microsoft announced it has mitigated a cyber attack by a China-linked threat actor, tracked as Storm-0558, which targeted customer emails.

Microsoft announced it has mitigated an attack conducted by a China-linked threat actor, tracked as Storm-0558, which targeted customer emails.

Storm-0558 threat actors focus on government agencies in Western Europe and were observed conducting cyberespionage, data theft, and credential access attacks. The attack was reported by a customer on June 16, 2023. The investigation revealed that the attack began on May 15, 2023, when 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.

The attackers forged authentication tokens to access user email using an acquired Microsoft account (MSA) consumer signing key.

“Our telemetry indicates that we have successfully blocked Storm-0558 from accessing customer email using forged authentication tokens. No customer action is required.” reads the post published by Microsoft. “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.”

Microsoft researchers discovered that the threat actors 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 attackers used an acquired MSA key to forge the tokens to access OWA and Outlook.com. The attackers exploited a token validation issue to impersonate Azure AD users and gain access to enterprise mail.

Microsoft added that it is not aware of any abuses of Azure AD keys or any other MSA keys by this threat actor.

“We assess this adversary is focused on espionage, such as gaining access to email systems for intelligence collection. This type of espionage-motivated adversary seeks to abuse credentials and gain access to data residing in sensitive systems.” concludes Charlie Bell, executive vice president of Microsoft Security, said.

Follow me on Twitter: @securityaffairs and Facebook and Mastodon

Pierluigi Paganini

(SecurityAffairs – hacking, Microsoft)