CVE-2026-45484: SharePoint Server Privilege Escalation via Unsafe Deserialization

Technical summary

This vulnerability is rooted in insecure deserialization (CWE-502), where SharePoint Server deserializes untrusted data without sufficient integrity checks or type restrictions. The flaw allows an authenticated network attacker to construct crafted serialized objects that, when processed by the application, result in privilege escalation. The attack requires valid credentials (PR:L in CVSS terms) but no user interaction, and impacts the confidentiality, integrity, and availability of affected SharePoint instances. The CVSS v3.1 base score of 8.8 reflects the high impact across all three security properties combined with low attack complexity and network accessibility.

Business impact

Privilege escalation in SharePoint creates significant operational risk. An insider or compromised lower-privileged account can gain administrative or highly elevated access to sensitive documents, user data, and collaboration content stored in SharePoint. This can lead to unauthorized data exfiltration, modification of business-critical information, malware distribution through document repositories, and compliance violations. For organizations relying on SharePoint as a central collaboration and document management platform, this flaw directly threatens data confidentiality and integrity at scale.

Affected systems

The vulnerability affects Microsoft SharePoint Server. Organizations running any affected on-premises or hybrid SharePoint Server installations are in scope. Cloud-based SharePoint Online deployments may have different exposure depending on Microsoft's patching cadence for cloud infrastructure. Verify your specific SharePoint Server version and deployment model against Microsoft's official security update guidance to determine if your environment is impacted.

Exploitability

This vulnerability requires valid authentication credentials to exploit, which significantly restricts the attack surface compared to unauthenticated flaws. However, the low attack complexity and network-accessible nature mean that any authenticated SharePoint user—including those with minimal privileges—can potentially mount the attack. The lack of user interaction required makes the attack reliable once credentials are obtained. As of the published date, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, but the high CVSS score and technical nature suggest prioritization is warranted.

Remediation

Apply security updates released by Microsoft for affected SharePoint Server versions. Verify the specific patch version numbers and applicable KB articles on the Microsoft Security Update Guide. In parallel, implement network segmentation to restrict SharePoint Server access to trusted internal networks, enforce multi-factor authentication for SharePoint users, and monitor authentication logs for anomalous privilege escalation patterns. Organizations unable to patch immediately should heighten monitoring of SharePoint administrative activities and audit serialization-related operations where possible.

Patch guidance

Consult Microsoft's official security advisory and the Security Update Guide (portal.msrc.microsoft.com) for the specific cumulative update or patch applicable to your SharePoint Server version and installation track. Apply patches in a staged manner—test in a non-production environment first. Coordinate with SharePoint farm administrators and test for any compatibility impacts on customizations or third-party solutions before production deployment. Microsoft typically releases patches on the second Tuesday of each month.

Detection guidance

Monitor SharePoint Server logs and audit trails for unusual deserialization activities, successful privilege escalations, and administrative account usage by lower-privileged users. Deploy network-level monitoring to detect outbound communications from SharePoint servers that may indicate post-exploitation activity. Security Information and Event Management (SIEM) systems should be configured to alert on unexpected elevation-of-privilege events within SharePoint application logs. Consider endpoint detection and response (EDR) solutions on SharePoint servers to detect process anomalies that may result from malicious deserialization.

Why prioritize this

This vulnerability merits high priority due to its combination of elevated access impact, network accessibility, and low attack complexity. Although authentication is required, the potential for insider threats and credential compromise is substantial in most organizations. The fact that any authenticated user can trigger the attack amplifies risk. Given SharePoint's ubiquity in enterprise environments as a collaboration hub, successful exploitation can provide an attacker with broad lateral movement capabilities and access to sensitive intellectual property and personal data.

Risk score, explained

The CVSS v3.1 score of 8.8 (HIGH severity) reflects a network-accessible privilege escalation with low complexity, occurring in a single security context, affecting all three core security properties—confidentiality, integrity, and availability. The requirement for valid credentials (PR:L) prevents a score in the CRITICAL range, but the complete compromise potential of authenticated sessions and the widespread presence of SharePoint in enterprises justify urgent remediation within 30 days of patch availability.

Frequently asked questions