Microsoft said the corporate account of one of its engineers was hacked by a highly skilled threat actor that acquired a signing key used to hack dozens of Azure and Exchange accounts belonging to high-profile users.
The disclosure solves two mysteries at the center of a disclosure Microsoft made in July. The company said that hackers tracked as Storm-0558 had been inside its corporate network for more than a month and had gained access to Azure and Exchange accounts, several of which were later identified as belonging to the US Departments of State and Commerce. Storm-0558 pulled off the feat by obtaining an expired Microsoft account consumer signing key and using it to forge tokens for Microsoft’s supposedly fortified Azure AD cloud service.
The disclosure left two of the most important questions unanswered. Specifically, how was a credential as sensitive as the consumer signing key stolen from Microsoft’s network, and how could it sign tokens for Azure, which is built on an entirely different infrastructure?
On Wednesday, Microsoft finally solved the riddles. The corporate account of one of its engineers had been hacked. Storm-0558 then used the access to steal the key. Such keys, Microsoft said, are entrusted only to employees who have undergone a background check and then only when they are using dedicated workstations protected by multi-factor authentication using hardware token devices. To safeguard this dedicated environment, email, conferencing, web research, and other collaboration tools aren’t allowed because they provide the most common vectors for successful malware and phishing attacks. Further, this environment is segregated from the rest of Microsoft’s network, where workers have access to email and other types of tools.
Those safeguards broke down in April 2021, more than two years before Storm-0558 gained access to Microsoft’s network. When a workstation in the dedicated production environment crashed, Windows performed a standard “crash dump,” in which all data stored in memory is written to disk so engineers can later diagnose the cause. The crash dump was later moved into Microsoft’s debugging environment. The hack of a Microsoft engineer’s corporate account allowed Storm-0558 to access the crash dump and, with it, the expired Exchange signing key.
Normally, crash dumps strip out signing keys and similarly sensitive data. In this case, however, a previously unknown vulnerability known as a “race condition” prevented that mechanism from working properly.