CVE-2026-102866

CVE-2026-102866 is an improper cryptographic signature verification flaw (CWE-347) in Microsoft SharePoint Server. A remote, unauthenticated attacker can trigger it over the network without user interaction, though exploitation is rated high complexity. Successful abuse elevates privileges with high impact to confidentiality, integrity, and availability, and the scope stays on the vulnerable server. On-premises SharePoint Server deployments are affected; no specific version range was published in the supplied data. It is not in CISA's Known Exploited Vulnerabilities catalog, and no public proof-of-concept is known.

What to do: Install Microsoft's October 2026 security updates for SharePoint Server as soon as they are available for your build, and treat internet-facing servers first. Until patched, restrict network access to SharePoint to trusted clients and review privileged accounts for unexpected elevation. No public exploit or KEV listing is known, but the flaw is remotely reachable without credentials.