CVE-2025-53770: Critical Deserialization Vulnerability in Microsoft SharePoint Server

Overview

The CVE-2025-53770 is a severe vulnerability discovered in Microsoft SharePoint Server that allows unauthorized attackers to execute harmful code over a network by deserializing untrusted data. This vulnerability primarily affects organizations using on-premises Microsoft SharePoint Server, posing a substantial threat to their system security. Remediation of this vulnerability is of paramount importance as an exploit for this flaw already exists in the wild, increasing the risk of system compromise and data leakage.

Vulnerability Summary

CVE ID: CVE-2025-53770
Severity: Critical (9.8 CVSS Score)
Attack Vector: Network
Privileges Required: Low
User Interaction: None
Impact: Potential system compromise or data leakage

Affected Products

Product | Affected Versions

Microsoft SharePoint Server | All on-premises versions

How the Exploit Works

The vulnerability stems from the deserialization of untrusted data in Microsoft SharePoint Server. Deserialization is a process that converts a byte stream into an object. In this case, an attacker sends a serialized object containing malicious code to the server. The server, not properly validating or checking the data it’s deserializing, executes the harmful code, thereby granting the attacker the ability to perform actions on the system such as installing programs, deleting data, or creating new accounts with full user rights.

Conceptual Example Code

Here is a
conceptual
example of how the vulnerability might be exploited. This is a simplified example and actual exploit code may be more complex.

POST /SharePoint/endpoint HTTP/1.1
Host: target.example.com
Content-Type: application/octet-stream
{ "serialized_object": "rO0ABXNyABdqYXZhLnV0aWwuSGFzaE1hcAUH2sHDFmDRAwACRgAKbG9hZEZhY3RvckkACXRocmVzaG9sZHhwP0AAAAAAAAB3CAAAACAAAAAQAAAATlQAFm1hbGljaW91c19jb2RlX3RvX2JlX2V4ZWN1dGVk" }

In this example, the attacker sends a POST request to a SharePoint endpoint with a serialized object in the body of the request. This serialized object (encoded in Base64 for simplicity) contains malicious code that would be executed upon deserialization on the server side.
Remember, this vulnerability can result in serious implications for system security, and it’s critical to apply the vendor patch or use WAF/IDS as a temporary mitigation until Microsoft releases a comprehensive update.