Microsoft SharePoint is one of the world’s most popular business collaboration platforms. If you’re running a company intranet or managing document workflows, there’s a good chance you’re using SharePoint. However, popularity also means cybercriminals are watching, and with CVE-2025-21348, SharePoint users have a new, critical reason to worry.

In this post, we’ll simplify what CVE-2025-21348 is, how attackers can exploit it, and what you can do to protect your SharePoint servers. Plus, we’ll look at a code snippet that demonstrates how this vulnerability might be exploited, and share all the links you need to learn more.

What is CVE-2025-21348?

CVE-2025-21348 is a remote code execution (RCE) vulnerability in Microsoft SharePoint Server. In plain English, it means someone can run malicious code on your SharePoint server over the network, without needing to log in or have special access. If exploited, hackers could steal data, deface your intranet, or move deeper into your corporate network.

How Bad Is It?

Microsoft and many security experts have given this a critical rating. With the right exploit, an attacker can compromise the entire SharePoint environment. It affects multiple versions of SharePoint Server (see the links below for a complete list).

How Does the Exploit Work?

At its core, an RCE like CVE-2025-21348 lets an attacker send crafted requests to a vulnerable SharePoint endpoint. Usually, the vulnerability exists in the way SharePoint handles deserialization of user-supplied data, a common weak spot in many web apps.

Here’s a simplified flow of the exploit

1. Attacker Identifies a Vulnerable SharePoint Server: Using tools like Shodan, attackers can find exposed SharePoint instances.
2. Crafts a Malicious Payload: The attacker builds a specially crafted request—often a SOAP or HTTP POST request containing dangerous serialized .NET objects.
3. Triggers Code Execution: The server processes the payload and, because of improper validation/deserialization, executes the supplied malicious code.
4. Attacker Gains Control: The server runs the attacker’s code with the privileges of the SharePoint service, giving control over files, commands, or further movement in the network.

Proof-of-Concept Exploit (Simplified)

The exact exploitation process can vary. Here’s a pseudo-code snippet (Python style) that mimics what an attacker’s request might look like:

import requests

# Target SharePoint site
target = "https://vulnerable-sharepoint.com/_layouts/15/SomeAPIEndpoint.aspx";

# Malicious payload: This is just a placeholder, actual payloads use serialized .NET objects
attacker_payload = '''
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
               xmlns:xsd="http://www.w3.org/2001/XMLSchema";
               xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">;
  <soap:Body>
    <SomeMethod>
      <serializedObject>
        <!-- Malicious .NET payload here -->
      </serializedObject>
    </SomeMethod>
  </soap:Body>
</soap:Envelope>
'''

headers = {
    "Content-Type": "text/xml; charset=utf-8",
    "User-Agent": "Mozilla/5."
}

# Send the malicious request to SharePoint
response = requests.post(target, data=attacker_payload, headers=headers)

print("Server responded with status:", response.status_code)

Note: This code is for educational purposes—actual in-the-wild exploits use complex and obfuscated payloads, sometimes created with tools like ysoserial.net.

Mitigation and Patching

Microsoft has released a security update to address this vulnerability.

If you haven’t already, patch immediately

- Check Microsoft’s official guidance and download the appropriate patch for your version of SharePoint Server.

References and Further Reading

- Microsoft Security Update Guide: CVE-2025-21348
- Microsoft SharePoint Security Updates
- ysoserial.net: A Deserialization Payload Generator
- How to secure SharePoint Server

Conclusion

CVE-2025-21348 serves as a critical reminder: if you’re running SharePoint Server, patching and restricting exposure are not just best practices—they’re essential.

Left unpatched, this one vulnerability could let an attacker control your collaboration hub. Keep your systems updated, monitor for unusual behavior, and stay ahead of the latest security advisories.

Timeline

Published on: 01/14/2025 18:16:00 UTC
Last modified on: 02/21/2025 20:28:10 UTC