Summary
CVE-2025-53771 is a flaw in on-premise Microsoft SharePoint (2016, 2019, and Subscription Edition) that lets attackers fake a login using a spoofed Referer header. This allows them to access admin functions, install web shells, and steal cryptographic keys. It’s being actively exploited, so patching, rotating keys, and reviewing systems are urgent.
Urgent Actions Required
- Apply the July 2025 security updates that patch CVE-2025-53771 on all affected on-premises SharePoint Server versions (2016, 2019, and Subscription Edition).
- Rotate ASP.NET MachineKey values (ValidationKey and DecryptionKey) on all SharePoint servers using Set-SPMachineKey or Update-SPMachineKey, then restart IIS.
- Inspect IIS logs for unauthenticated POST requests to /ToolPane.aspx with Referer: /_layouts/SignOut.aspx.
- Search the SharePoint LAYOUTS directory for unauthorized web shells (e.g., spinstall0.aspx).
- Enable AMSI Full Mode and ensure Defender Antivirus or your EDR solution is fully updated.
- If AMSI cannot be enabled or patching is delayed, remove public internet exposure from vulnerable SharePoint servers.
Which Systems Are Vulnerable to CVE-2025-53771?
Technical Overview
- Vulnerability Type: Authentication Bypass via Spoofed Referer Header
- Affected Software/Versions:
- Microsoft SharePoint Server 2016 (pre-KB5002760)
- Microsoft SharePoint Server 2019 (pre-KB5002754)
- SharePoint Server Subscription Edition (pre-KB5002768)
- Attack Vector: Network (HTTP/HTTPS)
- CVSS Score: 7.1
- Exploitability Score:
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Patch Availability: Yes, available in July 2025 security updates[2]
How Does the CVE-2025-53771 Exploit Work?
The attack typically follows these steps:
What Causes CVE-2025-53771?
Vulnerability Root Cause:
This vulnerability arises from improper validation of the Referer HTTP header in Microsoft SharePoint. The server incorrectly trusts a specially crafted Referer header spoofed to point to /layouts/SignOut.aspx, causing SharePoint to treat unauthenticated requests as authenticated. This flaw in header validation allows attackers to bypass authentication controls and perform privileged actions without valid credentials.
How Can You Mitigate CVE-2025-53771?
If immediate patching is delayed or not possible:
- Block internet access to on-prem SharePoint servers whenever possible.
- Watch for suspicious requests with fake Referer headers, especially ones pointing to /layouts/SignOut.aspx.
- Regularly change SharePoint machine keys to limit the impact if they’re stolen.
- Check systems for signs of web shells or unauthorized admin actions.
- Use firewalls or security tools to block fake or harmful SharePoint requests.
Which Assets and Systems Are at Risk?
Asset Types Affected:
- On-premises Microsoft SharePoint Servers (2016, 2019, Subscription Edition)
- SharePoint Server components handling authentication and request validation
Business-Critical Systems at Risk:
- Enterprise intranet portals relying on SharePoint for document management and collaboration
- Government, financial, healthcare, and educational institutions using on-prem SharePoint deployments
- Any SharePoint-integrated systems with privileged access or sensitive data
Exposure Level:
- Internet-facing SharePoint servers accessible externally without proper network segmentation or firewall restrictions
- Internal SharePoint servers if patching and mitigations are not applied
- SharePoint environments where cryptographic keys and session tokens are at risk of theft or misuse
Will Patching CVE-2025-53771 Cause Downtime?
Patch application impact: Low to moderate. Patching requires applying July 2025 updates and restarting IIS, which may cause brief service disruption. Using maintenance windows or load-balanced setups can reduce downtime.
Mitigation (if immediate patching is not possible): Restrict unauthenticated POST requests to /ToolPane.aspx using WAF or proxy rules. This offers partial protection but does not fully mitigate the risk—patching and machine key rotation remain essential.
How Can You Detect CVE-2025-53771 Exploitation?
Exploitation Signatures:
- HTTP POST requests to /layouts/15/ToolPane.aspx with a spoofed Referer: /_layouts/SignOut.aspx header—this is the key exploitation pattern.
- Follow-up GET requests for spinstall0.aspx in the SharePoint LAYOUTS directory (...\TEMPLATE\LAYOUTS\spinstall0.aspx), often indicating web shell activity.
MITRE ATT&CK Mapping:
- T1190[5] – Exploit Public-Facing Application
- T1505.003[6] – Server Software Component: Web Shell
- T1059.001[7] / T1059.003[8] – PowerShell or CMD execution via w3wp.exe
- T1112[9] – Modify Registry / disable Defender
- T1090[10] – Proxy Technique for C2
- T1003.001[11] – LSASS credential dumping via Mimikatz
Indicators of Compromise (IOCs/IOAs):
- File presence:
spinstall0.aspx or similar .aspx shells in SharePoint’s LAYOUTS directory. - SHA256 hash:
92bb4ddb98eeaf11fc15bb32e71d0a63256a0ed826a03ba293ce3a8bf057a514 matched to malicious spinstall0.aspx. - IIS logs showing a POST to /ToolPane.aspx?DisplayMode=Edit with Referer: /layouts/SignOut.aspx, often followed by GET to the dropped web shell.
Behavioral Indicators:
- Creation of spinstall0.aspx by w3wp.exe or PowerShell processes outside of normal admin workflows.
- Encoded PowerShell commands spawned by IIS worker process referencing spinstall or known layout paths.
- Suspicious network connections to C2 IPs such as 65.38.121.198 or domains like update.updatemicfosoft.com.
Alerting Strategy:
- Priority Level: High
- Trigger alerts for:
- HTTP POST requests to ToolPane.aspx using Referer: /_layouts/SignOut.aspx with no authenticated session.
- Unexpected file creation events for spinstall0.aspx in SharePoint’s LAYOUTS path.
- Detection of w3wp.exe spawning PowerShell with encoded payloads referencing spinstall paths.
- Network traffic or DNS queries to Storm2603 C2 indicators (e.g. 65.38.121.198, update.updatemicfosoft.com).
Remediation & Response
Patch/Upgrade Instructions:
- Apply the July 2025 security updates containing patches that address CVE‑2025‑53771 (e.g., KB5002760 for SharePoint 2016, KB5002754 for SharePoint 2019, KB5002768 for Subscription Edition).
Remediation Timeline:
- Immediate (0–2 hours): Deploy proxy or WAF rule to block spoofed Referer requests.
- Within 8 hours: Patch all designated SharePoint servers.
- Within 24 hours: Confirm all production and CI/CD environments are running patched builds and no legacy vulnerable versions are deployed.
Rollback Plan:
- If patching causes issues, revert to the last stable build and maintain access restrictions via proxy or firewall.
- Document rollback procedures clearly in your change-management system, including timestamps, engineers involved, and affected SharePoint version.
Incident Response Considerations:
- Isolate potentially compromised servers until remediation is complete.
- Collect forensic logs from IIS, network devices, proxies, and endpoint detection tools to trace unauthorized activity.
- Conduct a root cause analysis to determine if web shells (e.g. spinstall0.aspx) were deployed or machine keys stolen.
- Post-remediation, enhance monitoring for suspicious requests to ToolPane.aspx and validate that authentication bypass patterns no longer succeed.
Where Can I Find More Information on CVE-2025-53771?
CVE References:
- ^NVD – CVE-2025-53771
- ^Microsoft Security Update Guide – CVE-2025-53771
- ^Microsoft Blog – Customer Guidance for SharePoint Vulnerability CVE-2025-53770
- ^CVE Details – CVE-2025-53771
- ^T1190 – Exploit Public-Facing Application
- ^T1505.003 – Server Software Component: Web Shell
- ^T1059.001 – Command and Scripting Interpreter: PowerShell
- ^T1059.003 – Command and Scripting Interpreter: Windows Command Shell
- ^T1112 – Modify Registry
- ^T1090 – Proxy
- ^T1003.001 – OS Credential Dumping: LSASS Memory
CVSS Breakdown Table
| Metric | Value | Description |
|---|---|---|
| Base Score | 7.1 | Medium-severity vulnerability due to spoofing via header manipulation |
| Attack Vector | Network | Remotely exploitable over HTTP(S) without local access |
| Attack Complexity | Low | Requires no special conditions; straightforward spoofing request |
| Privileges Required | None | No authentication needed to exploit the vulnerability |
| User Interaction | None | Exploitation happens with no user input or click necessary |
| Scope | Unchanged | Attack impacts only SharePoint component; no wider system compromise implied |
| Confidentiality Impact | Low | Spoofing allows privileged actions but not direct data disclosure |
| Integrity Impact | Low | Attack enables spoofed requests but not data modification in itself |
| Availability Impact | None | No system disruption or denial of service observed from this specific vulnerability |
