Sophisticated hackers have accessed email accounts of organizations and government agencies via authentication tokens they forged by using an acquired Microsoft account (MSA) consumer signing key, the company has revealed on Tuesday.
“The threat actor Microsoft links to this incident is an adversary based in China that Microsoft calls Storm-0558. We assess this adversary is focused on espionage, such as gaining access to email systems for intelligence collection.”
This specific hacking group primarily targets government agencies in Western Europe, the company added. But according to The Washington Post, these latest attacks also compromised a number of unclassified U.S. email accounts.
The hackers exploited a token validation issue
Microsoft began investigating anomalous mail activity on June 16, 2023, after being alerted by customers.
They ultimately established that the account compromises started the day before, and that the attackers managed to access email accounts of employees at 25 organizations and some consumer accounts of individuals associated with those organizations.
The attackers gained access via Outlook Web Access in Exchange Online (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 explained.
“We have 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 we have observed the actor using tokens forged with the acquired MSA key.”
Microsoft says customers don’t have to do anything to protect themselves against this attack – the company has implemented mitigations (blocked the usage of maliciously signed tokens issued with the key and replaced it). There is no mention of them fixing the exploited token validation issue, though.
All targeted or compromised organizations have been contacted by Microsoft directly via their tenant admins and and have been provided with information to help them investigate and respond. “If you have not been contacted, our investigations indicate that you have not been impacted,” the company added, and promised to share “new details and recommendations as appropriate.”
Microsoft has also shared on Tuesday that attackers have been exploiting its Microsoft Windows Hardware Developer Program (MWHDP) to sign malicious drivers, and has released fixes for various zero-days actively exploited in the wild.