Microsoft users are, once again, under attack. This time, the threat is not restricted to Outlook users, or involves a Windows browser-based security bypass, and unlike the recent Windows authentication relay attack vulnerability, there is no patch, no magic update, to remedy this one. Which is bad news for Microsoft SharePoint Server users, as CVE-2025-53770 is currently under confirmed “mass attack” and on-premises servers across the world are being compromised. Here’s what you need to know and do.
Microsoft Confirms CVE-2025-53770 SharePoint Server Attacks
It’s been quite the few weeks for security warnings, what with Amazon informing 220 million customers of Prime account attacks, and claims of a mass hack of Ring doorbells going viral. The first of those can be mitigated by basic security hygiene, and the latter appears to be a false alarm. The same cannot be said for CVE-2025-53770, a newly uncovered and confirmed attack against users of SharePoint Server which is currently undergoing mass exploitation on a global level, according to the Eye Research experts who discovered it. Microsoft, meanwhile, has admitted that not only is it “aware of active attacks” but, worryingly, “a patch is currently not available for this vulnerability.”
CVE-2025-53770, which is also being called ToolShell, is a critical vulnerability in on-premises SharePoint. The end result of which is the ability for attackers to gain access and control of said servers without authentication. If that sounds bad, it’s because it is. Very bad indeed.
“The risk is not theoretical,” the researchers warned, “attackers can execute code remotely, bypassing identity protections such as MFA or SSO.” Once they have, they can then “access all SharePoint content, system files, and configurations and move laterally across the Windows Domain.”
And then there’s the theft of cryptographic keys. That can enable an attacker to “impersonate users or services,” according to the report, “even after the server is patched.” So, even when a patch is eventually released, and I would expect an emergency update to arrive fairly quickly for this one, the problem isn’t solved. You will, it was explained, “need to rotate the secrets allowing all future tokens that can be created by the malicious actor to become invalid.”
And, of course, as SharePoint will often connect to other core services, including the likes of Outlook and Teams, oh and not forgetting OneDrive, the threat, if exploited, can and will lead to “data theft, password harvesting, and lateral movement across the network,” the researchers warned.
Mitigating The Microsoft SharePoint Server Attacks
While the Microsoft Security Response Center has stated that it is “actively working to release a security update,” and will “provide additional details as they are available,” there is no patch at the time of writing. In the meantime, it advised that customers should apply the following mitigations:”
Configure Antimalware Scan Interface integration in SharePoint and deploy Defender AV on all SharePoint servers. “If you cannot enable AMSI,” Microsoft said, “we recommend you consider disconnecting your server from the internet until a security update is available.”
I have approached Microsoft for a statement and will update this story with any further developments.
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