CVE-2025-59245: SharePoint Online Privilege Escalation

CVE-2025-59245 Overview

CVE-2025-59245 is a critical elevation of privilege vulnerability affecting Microsoft SharePoint Online. This vulnerability stems from insecure deserialization (CWE-502), allowing unauthenticated attackers to exploit the flaw remotely over the network. Successful exploitation could enable an attacker to elevate privileges and gain unauthorized access to sensitive resources within the SharePoint Online environment.

Critical Impact

This vulnerability allows unauthenticated remote attackers to elevate privileges in Microsoft SharePoint Online through insecure deserialization, potentially compromising the confidentiality, integrity, and availability of affected systems.

Affected Products

• Microsoft SharePoint Online

Discovery Timeline

• November 20, 2025 - CVE-2025-59245 published to NVD
• November 21, 2025 - Last updated in NVD database

Technical Details for CVE-2025-59245

Vulnerability Analysis

This vulnerability is classified as an insecure deserialization flaw (CWE-502) in Microsoft SharePoint Online. The vulnerability allows attackers to exploit improper handling of serialized data, leading to elevation of privilege conditions. The attack can be executed remotely over the network without requiring authentication or user interaction, making it particularly dangerous for organizations relying on SharePoint Online for collaboration and document management.

The root cause lies in the application's failure to properly validate or sanitize serialized objects before deserialization. When an attacker submits maliciously crafted serialized data, the application processes it without adequate security checks, allowing the attacker to manipulate application logic and gain elevated privileges.

Root Cause

The vulnerability exists due to improper validation of user-supplied serialized data within Microsoft SharePoint Online. The application fails to verify the integrity and authenticity of serialized objects before deserializing them, enabling attackers to inject malicious objects that execute with elevated privileges upon deserialization.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending specially crafted HTTP requests containing malicious serialized payloads to a vulnerable SharePoint Online instance. Upon successful exploitation, the attacker can achieve privilege escalation, potentially gaining administrative access to the SharePoint environment.

The exploitation mechanism involves crafting a malicious serialized object that, when processed by the vulnerable deserialization routine, triggers unintended code paths or manipulates application state to elevate the attacker's privileges. For detailed technical information, refer to the Microsoft Security Update for CVE-2025-59245.

Detection Methods for CVE-2025-59245

Indicators of Compromise

• Unusual HTTP requests to SharePoint Online endpoints containing serialized data payloads
• Unexpected privilege changes or administrative actions performed by non-privileged accounts
• Anomalous authentication patterns or session token manipulations in SharePoint logs
• Suspicious object deserialization errors or exceptions in application logs

Detection Strategies

• Monitor SharePoint Online audit logs for unauthorized privilege escalation events or unexpected administrative actions
• Implement network-level inspection for HTTP requests containing suspicious serialized object patterns
• Deploy web application firewall (WAF) rules to detect and block known deserialization attack payloads
• Enable enhanced logging for SharePoint Online to capture detailed authentication and authorization events

Monitoring Recommendations

• Configure alerts for privilege changes or role assignments in SharePoint Online
• Monitor for unusual access patterns to sensitive document libraries or sites
• Track failed authentication attempts and correlation with subsequent successful elevated access
• Review Microsoft 365 Security Center for any related threat intelligence updates

How to Mitigate CVE-2025-59245

Immediate Actions Required

• Review and apply any available Microsoft security updates for SharePoint Online
• Audit SharePoint Online permissions and remove unnecessary elevated privileges
• Enable conditional access policies to restrict access from untrusted networks or devices
• Implement network segmentation to limit exposure of SharePoint Online services

Patch Information

Microsoft has released a security update addressing this vulnerability. Organizations should consult the Microsoft Security Update Guide for CVE-2025-59245 for specific patching instructions and update availability. As SharePoint Online is a cloud service, Microsoft manages infrastructure-level patches; however, administrators should review tenant-level configurations and ensure compliance with Microsoft's security recommendations.

Workarounds

• Restrict access to SharePoint Online through conditional access policies limiting connections to trusted IP ranges
• Enable multi-factor authentication (MFA) for all users accessing SharePoint Online
• Implement just-in-time (JIT) privileged access for administrative tasks
• Review and harden custom solutions or add-ins that process serialized data within SharePoint

# PowerShell: Review SharePoint Online external sharing settings
Connect-SPOService -Url https://yourtenant-admin.sharepoint.com
Get-SPOTenant | Select-Object SharingCapability, DefaultSharingLinkType

# Enable conditional access review (Azure AD PowerShell)
Get-AzureADMSConditionalAccessPolicy | Where-Object {$_.Conditions.Applications.IncludeApplications -contains "Office365"}