CISA urges immediate SharePoint hardening as exploits mount

The US Cybersecurity and Infrastructure Security Agency (CISA) has urged organizations to immediately secure Microsoft SharePoint deployments after warning that three vulnerabilities affecting the on-premises collaboration platform are being actively exploited.

A recent advisory from the federal cybersecurity watchdog asked administrators to patch vulnerable servers, review Microsoft’s mitigation guidance, and assume that internet-facing SharePoint instances remain attractive targets for attackers seeking an initial foothold into enterprise environments.

While applying patches remains the immediate priority, security experts caution that organizations should view the advisory as more than another Patch Tuesday exercise.

“This is what separates an IT incident from a business crisis,” said Chris Boehm, field CTO at Zero Networks. “One compromised SharePoint box is a ticket. That same box, with a clear path to your domain controllers, backups, and file shares, is how you end up with an encrypted infrastructure and a disclosure event. Segmentation stops the first from becoming the second.”

CISA’s advisory highlights CVE-2026-332201, CVE-2026-45659, and the newly added CVE-2026-56164, all of which have now been confirmed as exploited in the wild and added to the agency’s Known Exploited Vulnerabilities (KEV) catalog.

Exploitation tells a different severity story

The latest addition to CISA’s KEV catalog is CVE-2026-56164, an elevation-of-privilege vulnerability affecting Microsoft SharePoint Server. Although assigned a CVSS score of 5.3, the flaw can be exploited remotely without authentication, making it significantly more dangerous in practice than its severity rating alone suggests.

Microsoft has released security updates for supported SharePoint versions and recommended enabling the Antimalware Scan Interface (AMSI) integration to help detect malicious requests associated with exploitation attempts.

CISA also advised organizations to follow Microsoft’s incident response guidance, hunt for indicators of compromise, and rotate SharePoint machine keys where appropriate, acknowledging that patching alone may not fully remove attacker persistence from already compromised servers.

Older vulnerabilities remain active entry points

Alongside the newly disclosed flaw, CISA reiterated the urgency of addressing CVE-2026-45659, an insecure deserialization vulnerability allowing RCE that Microsoft had marked as “exploitation less likely” in its advisory in May. Another old bug CISA flagged is CVE-2026-32201, an improper input validation flaw that allows spoofing over a network.

Both of these flaws are being actively exploited in the wild.

CISA called out organizations failing to catch up with SharePoint updates, adding that attackers are increasingly targeting N-days rather than relying exclusively on newly discovered zero-days.

On concerns of patching speed, Boehm noted resilience is becoming an architectural challenge as much as an operational one.

“Stop measuring this in patch speed,” he said. “That’s a race you eventually lose. Some of these landed as zero-days with no fix on day one, and the window between disclosure and exploitation keeps shrinking. So the board-level question isn’t whether a server gets compromised. Assume one will. It’s how much of the business a single-owned system can take down with it.”

Boehm argued that limiting network reachability through segmentation should sit alongside patch management and threat hunting as a core defensive strategy. Reachability, he said, is a control that organizations own, not patch timing. CISA has given Federal Civilian Executive Branch (FCEB) agencies three days to remediate CVE-2026-56164 under Binding Operational Directive (BOD) 22-01.

The article originally appeared on CSO.