Microsoft reveals how hackers stole its email signing key… kind of

A series of unfortunate and cascading mistakes allowed a China-backed hacking group to steal one of the keys to Microsoft’s email kingdom that granted near unfettered access to U.S. government inboxes. Microsoft explained in a long-awaited blog post this week how the hackers pulled off the heist. But while one mystery was solved, several important details remain unknown.

To recap, Microsoft disclosed in July that hackers it calls Storm-0558, which it believes are backed by China, “acquired” an email signing key that Microsoft uses to secure consumer email accounts like Outlook.com. The hackers used that digital skeleton key to break into both the personal and enterprise email accounts of government officials hosted by Microsoft. The hack is seen as a targeted espionage campaign aimed at snooping on the unclassified emails of U.S. government officials and diplomats, reportedly including U.S. Commerce Secretary Gina Raimondo and U.S. Ambassador to China Nicholas Burns.

How the hackers obtained that consumer email signing key was a mystery — even to Microsoft — until this week when the technology giant belatedly laid out the five separate issues that led to the eventual leak of the key.

Microsoft said in its blog post that in April 2021, a system used as part of the consumer key signing process crashed. The crash produced a snapshot image of the system for later analysis. This consumer key signing system is kept in a “highly isolated and restricted” environment where internet access is blocked to defend against a range of cyberattacks. Unbeknownst to Microsoft, when the system crashed, the snapshot image inadvertently included a copy of the consumer signing key 1️⃣ but Microsoft’s systems failed to detect the key in the snapshot 2️⃣.

The snapshot image was “subsequently moved from the isolated production network into our debugging environment on the internet connected corporate network” to understand why the system crashed. Microsoft said this was consistent with its standard debugging process, but that the company’s credential scanning methods also did not detect the key’s presence in the snapshot image 3️⃣.

Then, at some point after the snapshot image was moved to…