CVE-2026-45659

CVE-2026-45659 is a deserialization-of-untrusted-data vulnerability (CWE-502) in Microsoft SharePoint Server in which an authorized (authenticated, low-privilege) attacker can submit crafted serialized data over the network, with no user interaction required, to execute code on the server. Successful exploitation carries high impact on confidentiality, integrity, and availability within the SharePoint service context, giving attackers a foothold for follow-on activity, and CISA notes that ransomware use is known. Organizations running on-premises Microsoft SharePoint Server are affected; the source data lists no specific version ranges, and the CPE scope (sharepoint server) points to the on-premises product rather than the Microsoft-managed SharePoint Online service. The flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2026-07-01 after active exploitation, and its EPSS score of 76.1% (100th percentile) indicates a high probability of near-term exploitation. The CVE record lists no public proof-of-concept, though related reporting describes exploitation activity following a public PoC release for a SharePoint authentication bypass.

What to do: Apply Microsoft's current security updates for SharePoint Server following vendor instructions, prioritizing internet-facing servers, and comply with CISA BOD 26-04, which requires applying mitigations per vendor guidance (including the cited Forensics Triage Requirements) or discontinuing use of the product if mitigations are unavailable. Because in-the-wild exploitation and ransomware use are confirmed, triage exposed servers for compromise: review IIS/SharePoint logs for unexpected authenticated requests, look for webshells or newly modified files in SharePoint web roots, and check for unusual child processes spawned by the SharePoint application pool. Given related reporting on an authentication-bypass PoC, also verify that any related SharePoint authentication-bypass patches are…