In July, Microsoft announced it had 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.
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 has now shared a comprehensive technical investigation into the way attackers gained access to the Microsoft account consumer signing key.
The company discovered that threat actors stole a signing key used to breach government email accounts from a Windows crash dump after compromising a Microsoft engineer’s corporate account.
Microsoft discovered that the MSA key was accidentally leaked into a crash dump after a consumer signing system crashed in April 2021.
The investigation revealed that the system crash in April of 2021 resulted in a snapshot of the crashed process (“crash dump”). The company pointed out that crash dumps should not include the signing key, but a race condition allowed the key to be present in the crash dump (this issue has been fixed by the company). The presence of the key in the dump was not detected by Microsoft and the dump was subsequently moved from the isolated production network into the company debugging environment on the internet-connected corporate network.
“After April 2021, when the key was leaked to the corporate environment in the crash dump, the Storm-0558 actor was able to successfully compromise a Microsoft engineer’s corporate account. This account had access to the debugging environment containing the crash dump which incorrectly contained the key.” reads the analysis published by Microsoft. “Due to log retention policies, we don’t have logs with specific evidence of this exfiltration by this actor, but this was the most probable mechanism by which the actor acquired the key.”
The IT giant announced it had revoked all valid MSA signing keys to prevent attackers from accessing other compromised keys. Below are the improvements implemented after the investigation: