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 to close off a vulnerability in Microsoft’s widely-used SharePoint software that hackers have exploited to carry out widespread attacks on businesses and at least some U.S. government agencies.

 to customers Saturday saying it was aware of the zero-day exploit being used to conduct attacks and that it was working to patch the issue. Microsoft updated its guidance Sunday with instructions to fix the problem for SharePoint Server 2019 and SharePoint Server Subscription Edition. Engineers were still working on a fix for the older SharePoint Server 2016 software.

“Anybody who’s got a hosted SharePoint server has got a problem,” said Adam Meyers, senior vice president with CrowdStrike, a cybersecurity firm. “It’s a significant vulnerability.”

Companies and government agencies around the world use SharePoint for internal document management, data organization and collaboration.

A zero-day exploit is a cyberattack that takes advantage of a previously unknown security vulnerability. "Zero-day" refers to the fact that the security engineers have had zero days to develop a fix for the vulnerability.

According to the U.S. Cybersecurity and Infrastructure Security Agency (CISA), the 

 is "a variant of the existing vulnerability CVE-2025-49706 and poses a risk to organizations with on-premise SharePoint servers.”

Security researchers warn that the exploit, reportedly known as “ToolShell,” is a serious one and can allow actors to fully access SharePoint file systems, including services connected to SharePoint, such as Teams and OneDrive.

 that the vulnerability may allow bad actors to "bypass future patching.”

 that it scanned over 8,000 SharePoint servers worldwide and discovered that at least dozens of systems were compromised. The cybersecurity company said the attacks likely began on July 18.

Microsoft said the vulnerability affects only on-site SharePoint servers used within businesses or organizations, and does not affect Microsoft’s cloud-based SharePoint Online service.

But Michael Sikorski, CTO and Head of Threat Intelligence for Unit 42 at Palo Alto Networks, warns that the exploit still leaves many potentially exposed to bad actors.

“While cloud environments remain unaffected, on-prem SharePoint deployments — particularly within government, schools, health care including hospitals, and large enterprise companies — are at immediate risk."

The vulnerability targets SharePoint server software so customers of that product will want to immediately follow 

Although the scope of the attack is still being assessed, CISA warned that the 

 and recommended that any servers impacted by the exploit should be disconnected from the internet until they are patched.

“We are urging organizations who are running on-prem SharePoint to take action immediately and apply all relevant patches now and as they become available, rotate all cryptographic material, and engage professional incident response. An immediate, band-aid fix would be to unplug your Microsoft SharePoint from the internet until a patch is available,” Sikorski advises.

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Microsoft is issuing an emergency fix to close off a vulnerability in Microsoft’s SharePoint software that hackers have exploited to carry out widespread attacks on businesses and at least some federal agencies. 

What to know about a vulnerability being exploited on Microsoft SharePoint servers

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