CISA ADDS ACTIVELY EXPLOITED MICROSOFT SHAREPOINT DESERIALIZATION VULNERABILITY TO KNOWN EXPLOITED VULNERABILITIES CATALOG

Threat Summary

Category: Active Exploitation / Remote Code Execution / Deserialization
Affected Product: Microsoft SharePoint Server
CVE: CVE-2026-45659
Primary Risk: Remote Code Execution, Unauthorized System Access, Privilege Escalation, Data Theft, Enterprise Network Compromise
Threat Status: Confirmed Active Exploitation
Affected Environment: Federal Agencies, Enterprise Networks, Organizations Operating On-Premises Microsoft SharePoint Servers
Attack Vector: Deserialization of Untrusted Data
CISA Action: Added to Known Exploited Vulnerabilities (KEV) Catalog

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The Cybersecurity and Infrastructure Security Agency (CISA) has added CVE-2026-45659, a deserialization vulnerability affecting Microsoft SharePoint Server, to its Known Exploited Vulnerabilities (KEV) Catalog after confirming evidence that the flaw is being actively exploited by threat actors.

The addition places the vulnerability among the highest-priority security issues currently facing organizations operating vulnerable SharePoint environments and signals that exploitation has moved beyond proof-of-concept research into real-world attacks.

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Vulnerability Details

According to CISA, CVE-2026-45659 is a deserialization of untrusted data vulnerability affecting Microsoft SharePoint Server.

Deserialization vulnerabilities occur when an application improperly processes serialized data received from an external source. If exploited successfully, an attacker may manipulate that data to execute unauthorized commands or code within the affected application.

Because Microsoft SharePoint frequently stores sensitive business documents, authentication data, internal communications, collaboration portals, and enterprise content management systems, successful exploitation can provide attackers with a valuable foothold inside organizational networks.

Depending on system configuration and privilege levels, exploitation may allow attackers to compromise SharePoint services, move laterally through enterprise environments, access confidential information, establish persistence, or deploy additional malware.

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Operational Impact

Microsoft SharePoint remains one of the most widely deployed collaboration platforms across government agencies, Fortune 500 companies, healthcare providers, educational institutions, financial organizations, and critical infrastructure operators.

If successfully exploited, organizations could experience:

• Remote code execution on vulnerable SharePoint servers
• Unauthorized access to sensitive organizational data
• Credential theft
• Privilege escalation
• Lateral movement throughout enterprise networks
• Deployment of ransomware or additional malware
• Long-term persistence within compromised environments
• Data exfiltration involving proprietary or regulated information
• Business disruption affecting collaboration and document management services

Because SharePoint servers frequently integrate with Microsoft Active Directory, Microsoft SQL Server, and Microsoft 365 hybrid environments, attackers who compromise a vulnerable server may be able to leverage trusted relationships to expand access across an organization’s infrastructure.

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Federal Response

CISA added the vulnerability to its Known Exploited Vulnerabilities Catalog under Binding Operational Directive (BOD) 26-04: Prioritizing Security Updates Based on Risk.

The directive requires Federal Civilian Executive Branch (FCEB) agencies to rapidly remediate KEV-listed vulnerabilities affecting publicly exposed assets capable of providing attackers with significant control following successful exploitation.

BOD 26-04 also requires agencies to determine whether systems were compromised before security updates are applied, recognizing that active exploitation may have already occurred prior to remediation.

Although the directive applies specifically to federal civilian agencies, CISA continues encouraging private-sector organizations, state and local governments, educational institutions, healthcare providers, and critical infrastructure operators to adopt the same risk-based approach by prioritizing remediation of vulnerabilities included in the KEV Catalog.

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Defensive Guidance

Organizations operating Microsoft SharePoint Server should:

• Apply Microsoft’s security updates immediately when available.
• Review SharePoint servers for indicators of compromise before and after patching.
• Examine authentication, application, and Windows event logs for suspicious activity.
• Monitor SharePoint administrative actions for unauthorized changes.
• Restrict administrative privileges using the principle of least privilege.
• Verify the integrity of SharePoint content databases and configuration files.
• Monitor network traffic for unusual outbound connections originating from SharePoint servers.
• Reset privileged credentials if unauthorized access is suspected.
• Ensure endpoint detection and response (EDR) solutions are fully updated.
• Conduct continuous vulnerability scanning of internet-facing SharePoint infrastructure.

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Forecast — 30 Days

• Increased internet-wide scanning for vulnerable Microsoft SharePoint servers.
• Continued exploitation attempts targeting unpatched enterprise environments.
• Additional ransomware groups leveraging SharePoint vulnerabilities for initial access.
• Increased incident response activity involving compromised collaboration platforms.
• Additional CISA advisories as investigators gather more information regarding observed exploitation techniques.
• Accelerated patch deployment efforts across government and private-sector organizations.

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TRJ Verdict

The addition of CVE-2026-45659 to CISA’s Known Exploited Vulnerabilities Catalog confirms that attackers are actively exploiting this Microsoft SharePoint vulnerability in operational environments. Because SharePoint often serves as a centralized repository for sensitive organizational data and integrates with numerous enterprise services, a successful compromise can provide attackers with an efficient pathway into broader corporate networks.

Organizations should view KEV catalog additions as immediate operational security alerts rather than routine vulnerability notices. Once CISA confirms active exploitation, delaying remediation significantly increases organizational risk. Security teams should not only prioritize patch deployment but also conduct thorough compromise assessments to determine whether attackers gained access before corrective actions were implemented.

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