Microsoft said in a Friday regulatory filing that a Russian intelligence group accessed some of the software maker’s top executives’ email accounts. Nobelium, the same group that breached government supplier SolarWinds in 2020, carried out the attack, which Microsoft detected last week, according to the company.

It isn’t the first time Russian hackers have gained entry into Microsoft’s systems. State-sponsored attacks that can result in the dissemination of sensitive data becomes a greater risk during periods of armed conflict, and Russia’s war against Ukraine has been going on for almost two years now. On Thursday, Russia said Ukrainian forces conducted drone strikes in multiple Russian locations.

Microsoft’s announcement comes after new U.S. requirements for disclosing cybersecurity incidents went into effect. A Microsoft spokesperson said that while the company does not believe the attack had a material effect, it still wanted to honor the spirit of the rules.

The Cybersecurity and Infrastructure Security Agency is “closely coordinating with Microsoft to gain additional insights into this incident and understand impacts so we can help protect other potential victims,” CISA executive assistant director for cybersecurity Eric Goldstein said in a statement to CNBC. “As noted in Microsoft’s announcement, at this time we are not aware of impacts to Microsoft customer environments or products.” 

In late November, the group accessed “a legacy non-production test tenant account,” Microsoft’s Security Response Center wrote in the blog post. After gaining access, the group “then used the account’s permissions to access a very small percentage of Microsoft corporate email accounts, including members of our senior leadership team and employees in our cybersecurity, legal, and other functions, and exfiltrated some emails and attached documents,” the corporate unit wrote.

The company’s senior leadership team, including Chief Financial Offer Amy Hood and President Brad Smith, regularly meets with CEO Satya Nadella.

Microsoft said it has not found signs that Nobelium had accessed customer data, production systems or proprietary source code.

The U.S. government and Microsoft consider Nobelium to be part of the Russian foreign intelligence service SVR. The hacking group was responsible for one of the most prolific breaches in U.S. history when it added malicious code to updates to SolarWinds’ Orion software, which some U.S. government agencies were using. Microsoft itself was ensnared in the hack.

Nobelium, also known as APT29 or Cozy Bear, is a sophisticated hacking group that has attempted to breach the systems of U.S. allies and the Department of Defense. Microsoft also uses the name Midnight Blizzard to identify Nobelium.

It was also implicated alongside another Russian hacking group in the 2016 breach of the Democratic National Committee’s systems.

Last year, a vulnerability in Microsoft software allowed China-aligned hackers to access the email accounts of senior government officials, including Commerce Secretary Gina Raimondo, ahead of a critical U.S.-China meeting. The company’s “negligent cybersecurity practices” led to the attack, Sen. Ron Wyden, a Democrat from Oregon, wrote in a letter to CISA director Jen Easterly, and other federal officials.

“We are continuing our investigation and will take additional actions based on the outcomes of this investigation and will continue working with law enforcement and appropriate regulators,” the Microsoft blog post said.

The Federal Bureau of Investigation told CNBC that it knows about the attack and is working with federal partners to help.

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