A top security researcher claims the massive SharePoint zero-day attack was fueled by a leak from a Microsoft partner program, giving hackers a critical head start.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA), in an alert, said it's aware of active exploitation of CVE-2025-53770, which enables unauthenticated access to SharePoint systems and arbitrary code execution over the network.

Security researchers say Microsoft customers should take immediate action to defend against the ongoing cyberattacks, and must assume they have already been compromised.

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.

Active SharePoint exploits since July 7 target governments and tech firms globally, risking key theft and persistent access.

The hackers behind the initial wave of attacks exploiting a zero-day in Microsoft SharePoint servers have so far primarily targeted government organizations, according to researchers and news reports.

Microsoft also has issued a patch for a related SharePoint vulnerability — CVE-2025-53771; Microsoft says there are no signs of active attacks on CVE-2025-53771, and that the patch is to provide more robust protections than the update for CVE-2025-49706.

Hackers have been exploiting two zero-day vulnerabilities in on-premises installations of Microsoft SharePoint to gain remote access, and steal cryptographic keys

More information has emerged on the ToolShell SharePoint zero-day attacks, including impact, victims, and threat actors.