Protecting Against SharePoint Vulnerabilities: Lessons from CVE-2025-53770

Strategies for Safeguarding SharePoint: Insights from CVE-2025-53770

Introduction

On July 18, 2025, a critical remote code execution (RCE) vulnerability, CVE-2025-53770, was identified in Microsoft SharePoint Server, with a CVSS score of 9.8, marking it as one of the most severe threats to on-premises SharePoint environments. Actively exploited across over 54 organizations—including banks, universities, government entities, and healthcare institutions—this vulnerability, part of the "ToolShell" exploit chain, has exposed the fragility of public-facing SharePoint servers. As attackers leverage this flaw to extract cryptographic secrets, deploy malicious payloads, and achieve persistent access, organizations must adopt robust security measures. This blog post explores the exploitation of CVE-2025-53770, its impact, and actionable recommendations rooted in Zero Trust (ZT) principles to secure SharePoint environments and prevent future breaches.

Understanding the CVE-2025-53770 SharePoint Vulnerability Exploit

How It’s Being Exploited

The CVE-2025-53770 vulnerability stems from a deserialization of untrusted data flaw in SharePoint Server, allowing unauthenticated attackers to execute arbitrary code via crafted HTTP POST requests to the /_layouts/15/ToolPane.aspx?DisplayMode=Edit endpoint, often paired with a specific HTTP referer like /_layouts/SignOut.aspx. This exploit chain, initially demonstrated at Pwn2Own Berlin 2025, combines CVE-2025-49706 (authentication bypass) and CVE-2025-49704 (code injection) to achieve RCE. Attackers deploy a malicious .aspx file, such as spinstall0.aspx, designed to extract cryptographic secrets like the ValidationKey and DecryptionKey. These keys enable attackers to forge __VIEWSTATE payloads, maintaining access even after patches are applied, as unrotated keys remain valid.

The attack is stealthy, avoiding traditional webshell behavior and focusing on extracting sensitive configuration data. Observed IP addresses involved in exploitation attempts include 107.191.58.76, 104.238.159.149, and 96.9.125.147, with large-scale campaigns targeting public-facing SharePoint servers. The impact is severe: attackers gain full system access, compromising SharePoint content, file systems, and connected services like Microsoft Teams and OneDrive, while also enabling lateral movement to systems like Exchange servers.

Current Mitigation Efforts

Microsoft released emergency patches for SharePoint Server 2019 and Subscription Edition on July 20, 2025, addressing CVE-2025-53770 and CVE-2025-53771 (a related spoofing flaw). However, SharePoint Server 2016 remains unpatched as of July 21, 2025. Organizations are urged to enable Antimalware Scan Interface (AMSI) integration, deploy Microsoft Defender AV, rotate ASP.NET machine keys using the Update-SPMachineKey PowerShell cmdlet, and monitor for malicious .aspx files. Blocking traffic from known malicious IPs and disconnecting public-facing servers without AMSI are also critical steps. Despite these measures, the public availability of proof-of-concept (PoC) exploits post-Pwn2Own has accelerated attacks, underscoring the need for proactive security.

Recommendations for Securing SharePoint Environments

The exploitation of CVE-2025-53770 highlights a critical lesson: public-facing systems are prime targets for cyberattacks. If a system is publicly accessible, it’s only a matter of time before it’s compromised. To mitigate this risk and limit the blast radius of future vulnerabilities, organizations should adopt the following strategies, grounded in Zero Trust principles.

1. Block Public-to-Internal Traffic

To reduce the attack surface, block all traffic from public-facing SharePoint servers to internal networks. This containment strategy limits the blast radius, preventing attackers from moving laterally to sensitive systems like Exchange or Active Directory. Use network segmentation and firewalls to enforce strict traffic controls, allowing only necessary communication between public and internal systems.

2. Implement Just-In-Time (JIT) Provisioning

Adopt JIT provisioning to manage access to SharePoint servers. Instead of keeping servers always accessible, use a bastion host or jump box (e.g., an AWS EC2 instance) that is activated only when needed. Configure the host to block Remote Desktop Protocol (RDP) and Secure Shell (SSH) by default, enabling them only for authorized sessions. All other traffic should be blocked until explicitly required. This approach minimizes exposure, as the server remains offline or inaccessible when not in use.

3. Leverage Privileged Access Management (PAM) Tools

Integrate PAM tools with JIT provisioning to enforce strict access controls. PAM solutions, such as Microsoft Privileged Access Management or third-party tools like CyberArk, ensure that only authorized users can access SharePoint servers, and only for the duration required. PAM enforces least privilege, granting users the minimum permissions needed for their tasks, and logs all access attempts for auditing. Combining PAM with JIT reduces the window of opportunity for attackers to exploit vulnerabilities like CVE-2025-53770.

4. Assume Breach and Protect Data

Operate under the assumption that a breach is inevitable. To mitigate damage, implement robust authorization controls at the data, identity, and encryption levels:

• Data-Level Controls: Use role-based access control (RBAC) to restrict access to sensitive SharePoint content. Encrypt sensitive data at rest and in transit using strong algorithms (e.g., AES-256).

• Identity-Level Controls: Enforce multifactor authentication (MFA) and continuous authorization, revalidating user identities during sessions to detect anomalies.

• Encryption-Level Controls: Rotate cryptographic keys regularly, especially after a breach or patch application. Use tools like Microsoft’s Update-SPMachineKey to invalidate stolen keys, preventing persistent access.

Additionally, maintain regular, secure backups of SharePoint data and configurations. Test restore processes to ensure rapid recovery without relying on compromised systems. Even if attackers gain access, encrypted data and restricted permissions limit their ability to extract meaningful information.

5. Adopt Zero Trust Principles

Zero Trust (ZT) is a security framework that assumes no trust, verifies everything, and enforces least privilege. Apply these ZT principles to SharePoint environments:

• Least Privilege: Grant users and systems only the permissions required for their roles, minimizing the impact of compromised accounts.

• Continuous Authorization: Continuously validate user and device identities throughout sessions, using behavioral analytics to detect suspicious activity.

• JIT Access: Provision access dynamically, only when needed, as described above.

• Privileged Access Management: Use PAM tools to manage and monitor privileged accounts, reducing the risk of unauthorized access.

• Assume Breach: Design systems with the expectation of compromise, implementing controls to limit damage and ensure rapid recovery.

Practical Steps for Implementation

1. Network Configuration:

   • Deploy firewalls to block public-to-internal traffic.

   • Configure a bastion host with JIT provisioning, disabling RDP/SSH by default.

   • Use intrusion prevention systems (IPS) and web application firewalls (WAF) to filter malicious requests, such as those targeting /_layouts/15/ToolPane.aspx.

2. Monitoring and Detection:

   • Scan for malicious .aspx files (e.g., spinstall0.aspx) in SharePoint directories.

   • Monitor IIS logs for POST requests with /_layouts/SignOut.aspx referers.

   • Use Microsoft 365 Defender to detect and respond to compromise indicators.

3. Patching and Hardening:

   • Apply Microsoft’s July 2025 patches for SharePoint Server 2019 and Subscription Edition.

   • Enable AMSI and deploy Microsoft Defender AV on all SharePoint servers.

   • Rotate machine keys post-patch using Update-SPMachineKey and restart IIS.

4. Backup and Recovery:

   • Schedule automated backups of SharePoint databases and configurations.

   • Store backups in a secure, offline location to prevent ransomware attacks.

   • Test restore processes quarterly to ensure operational readiness.

Conclusion

The active exploitation of CVE-2025-53770 underscores the dangers of public-facing SharePoint servers and the need for a proactive, Zero Trust-based security posture. By blocking public-to-internal traffic, implementing JIT provisioning, leveraging PAM tools, and applying data, identity, and encryption controls, organizations can significantly reduce the risk of compromise. Assuming a breach is inevitable, robust backups and continuous authorization ensure rapid recovery and minimal data exposure. As cyber threats evolve, adopting these measures will not only mitigate current vulnerabilities but also future-proof SharePoint environments against emerging exploits. For further guidance, refer to Microsoft’s advisory at https://msrc.microsoft.com/blog/2025/07/customer-guidance-for-sharepoint-vulnerability-cve-2025-53770/ and CISA’s alert at https://www.cisa.gov/news-events/alerts/2025/07/20/microsoft-releases-guidance-exploitation-sharepoint-vulnerability-cve-2025-53770.