Russian hackers broke into Microsoft’s corporate email system and gained access to some accounts of the company’s leadership team

In a blog post, Microsoft said the intrusion began in late November and was discovered on Jan. 12. It said the same highly skilled Russian hacking team behind the SolarWinds breach was responsible.

“A very small percentage” of Microsoft corporate accounts were accessed, the company said, and some emails and attached documents were stolen.

A company spokesperson said Microsoft had no immediate comment on which or how many members of its senior leadership had their email accounts breached. In a regulatory filing Friday, Microsoft said it was able to remove the hackers’ access from the compromised accounts on or about Jan. 13.

“We are in the process of notifying employees whose email was accessed,” Microsoft said, adding that its investigation indicates the hackers were initially targeting email accounts for information related to their activities.

The Microsoft disclosure comes a month after a new U.S. Securities and Exchange Commission rule took effect that compels publicly traded companies to disclose breaches that could negatively impact their business. It gives them four days to do so unless they obtain a national-security waiver.

In Friday’s SEC regulatory filing, Microsoft said that “as of the date of this filing, the incident has not had a material impact” on its operations. It added that it has not, however, “determined whether the incident is reasonably likely to materially impact” its finances.

Microsoft, which is based in Redmond, Washington, said the hackers from Russia’s SVR foreign intelligence agency were able to gain access by compromising credentials on a “legacy” test account, suggesting it had outdated code. After gaining a foothold, they used the…