2026-009: Critical Vulnerabilities in Microsoft SharePoint

Microsoft SharePoint Server contains an improper input validation flaw (CWE-20) that can be triggered by an unauthenticated, network-based attacker submitting crafted input to the server. Successful exploitation allows the attacker to perform spoofing over the network, impersonating a trusted user or source within SharePoint; detailed impact mechanics have not been published and no CVSS score or public proof-of-concept is available. Any organization running on-premises Microsoft SharePoint Server is potentially affected, and the available data does not specify affected version ranges. CISA added the vulnerability to the Known Exploited Vulnerabilities catalog on 2026-04-14, indicating evidence of active exploitation, and EPSS assigns a 42.8% probability of exploitation within 30 days (99th percentile). Ransomware association is currently unknown.

Do: Apply Microsoft's security updates for SharePoint Server per the vendor advisory as soon as possible, and identify your SharePoint Server versions and builds since specific affected ranges are not provided here. Given the KEV listing, federal agencies must apply the vendor mitigations, follow applicable BOD 22-01 cloud guidance, or discontinue use by the established deadline. Until patched, limit network exposure of SharePoint servers and review authentication and access logs for signs of impersonation or spoofing activity.

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.

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…

CVE-2026-50522 is a deserialization of untrusted data flaw (CWE-502) in Microsoft SharePoint Server that allows an unauthenticated attacker to send maliciously crafted serialized data over the network and execute code on the server, reflected in its 9.8 critical CVSS score with no privileges or user interaction required. Successful exploitation yields full remote code execution with high impact on confidentiality, integrity, and availability, giving attackers a foothold for follow-on actions such as data theft, lateral movement, or ransomware. Any organization running on-premises SharePoint Server is in scope, particularly deployments reachable from untrusted networks; the required action notes stakeholders must evaluate each asset's internet exposure under CISA BOD 26-04. The flaw is under active exploitation: CISA added it to the Known Exploited Vulnerabilities catalog on 2026-07-22 and security news headlines describe it as a critical RCE exploited in the wild, with some reports referencing exploitation after a public proof-of-concept release and an authentication bypass. The structured record lists no public PoC as confirmed, but an EPSS of 84.6% (100th percentile) underscores a very high near-term exploitation likelihood.

Do: Apply Microsoft's security updates for SharePoint Server immediately per the vendor advisory, as required under CISA's KEV listing and BOD 26-04, and prioritize any SharePoint deployments that are internet-facing (federal/critical-infrastructure operators must follow BOD 26-04 timelines or discontinue unmitigated use). Until patched, restrict public access to SharePoint endpoints (VPN, firewall rules, or reverse proxy) and review IIS/application logs and running processes for signs of unauthenticated deserialization abuse. Because some reports reference an authentication bypass being chained, also verify authentication paths and monitor for follow-on attacker activity after patching.

Microsoft SharePoint Server contains a missing authentication for critical function vulnerability (CWE-306) that lets an unauthenticated attacker elevate privileges over a network without valid credentials. The flaw is triggered when the affected SharePoint function is accessed remotely without any authentication check, allowing an attacker to gain higher privileges than intended. Successful exploitation could enable an attacker to take elevated actions within the SharePoint environment, potentially leading to further compromise of the server and its data. All organizations running on-premises Microsoft SharePoint Server are potentially affected, though specific versions have not yet been enumerated by Microsoft or CISA. The vulnerability was added to CISA's Known Exploited Vulnerabilities catalog on 2026-07-14, indicating it is being actively exploited, and its EPSS score of 26.6% (98th percentile) reflects a high near-term exploitation risk.

Do: Apply Microsoft's security updates for SharePoint Server as soon as they are available, and check Microsoft's advisory for the specific affected version ranges once published. In the meantime, restrict network access to SharePoint servers, especially for internet-facing instances, and verify whether your environment falls under CISA BOD 26-04 requirements given the KEV listing. Monitor for updated guidance from Microsoft and CISA, as exploitation is confirmed and patching urgency is high.