LONDON (Reuters) -A global attack on Microsoft server software used by thousands of government agencies and businesses to share documents within organisations is likely the work of a single actor, a cybersecurity researcher said on Monday.

Microsoft on Saturday issued an alert about "active attacks" on SharePoint servers used within organisations. It said that SharePoint Online in Microsoft 365, which is in the cloud, was not hit by the exploit, also known as a "zero day" because it was previously unknown to cybersecurity researchers.

"Based on the consistency of the tradecraft seen across observed attacks, the campaign launched on Friday appears to be a single actor. However, it's possible that this will quickly change," Rafe Pilling, Director of Threat Intelligence at Sophos, a British cybersecurity firm.

That tradecraft included the sending of the same digital payload to multiple targets, Pilling added.

Microsoft said it had "provided security updates and encourages customers to install them," a company spokesperson said in an emailed statement.

It was not clear who was behind the ongoing hack. The FBI said on Sunday it was aware of the attacks and was working closely with its federal and private-sector partners, but offered no other details. Britain's National Cyber Security Centre did not immediately respond to a request for comment.

The Washington Post said unidentified actors in the past few days had exploited a flaw to launch an attack that targeted U.S. and international agencies and businesses.

According to data from Shodan, a search engine that helps to identify internet-linked equipment, over 8,000 servers online could theoretically have already been compromised by hackers.

Those servers include major industrial firms, banks, auditors, healthcare companies, and several U.S. state-level and international government entities.

"The SharePoint incident appears to have created a broad level of compromise across a range of servers globally," said Daniel Card of British cybersecurity consultancy, PwnDefend. 

"Taking an assumed breach approach is wise, and it's also important to understand that just applying the patch isn't all that is required here."