Microsoft SharePoint is a staple in many organizations for collaboration and document management. That’s why security flaws in SharePoint are a big deal, especially when they could let attackers trick users. One such flaw is CVE-2022-21987, a SharePoint Server spoofing vulnerability that caught widespread attention in early 2022. In this exclusive long read, we break down what this bug means, how it works, and what you can do to stay safe.

What Is CVE-2022-21987?

CVE-2022-21987 is a spoofing vulnerability found in Microsoft SharePoint Server. If exploited, it could trick users into believing content or links come from a trusted SharePoint source, when in fact, they originate from an attacker. This can open the door for phishing, credential theft, or just spreading misinformation inside an organization.

Microsoft assigned this a CVSS score of 7.1 (High). You can check the official Microsoft Security Update Guide here:
Microsoft Security Update Guide - CVE-2022-21987

How Does It Work?

A spoofing vulnerability lets an attacker pose as something or someone they're not. In SharePoint Server, improper validation of user-supplied input in certain URLs and web parts made it possible for an attacker to:

If a victim clicks such a link, they may be led to enter their credentials or interact with fake, but realistic, SharePoint content.

Technical Details & Exploit Example

Microsoft did not publicly release Proof-of-Concept (PoC) code, but security researchers have reconstructed potential exploitation paths based on patch analysis.

What Was Vulnerable?

The main problem was that SharePoint wasn’t validating some HTTP request parameters correctly. By tampering with these, attackers could insert their own content or references.

Example Exploit Path

Let’s imagine the vulnerable SharePoint component accepts a query parameter called returnUrl on a login-related resource:

https://sharepoint.example.com/_layouts/15/Authenticate.aspx?Source=https://example.com/

If SharePoint fails to validate the Source or returnUrl parameter, an attacker could craft a URL such as:

https://sharepoint.example.com/_layouts/15/Authenticate.aspx?Source=https://malicious-website.com/phish

Now, when a user follows this link, after authentication, they’ll be redirected to malicious-website.com instead of a genuine SharePoint page.

Below is a simplified Python snippet, showing how such a URL might be crafted and sent to targets

# Spoofed SharePoint SSO URL
base_url = "https://sharepoint.example.com/_layouts/15/Authenticate.aspx";
malicious_redirect = "https://malicious-site.attacker.com/login";

exploit_url = f"{base_url}?Source={malicious_redirect}"

print("[*] Send this URL to the victim:")
print(exploit_url)

Once the victim logs in, they're redirected to the attacker's own site.

References

* Microsoft Advisory - CVE-2022-21987
* NVD - CVE-2022-21987 Detail
* Microsoft Patch Tuesday (January 2022)

Protecting Your SharePoint

Patch Immediately:
Microsoft released a fix for all the supported SharePoint versions as part of their January 2022 Patch Tuesday. The most effective protection is to make sure your SharePoint servers are fully updated.

Educate Users:
Let your users know to watch out for odd SharePoint links, especially those coming from email or chat.

Check Logs:
If you suspect you may have been targeted, review your SharePoint logs for unusual redirect or login patterns.

Conclusion

Spoofing vulnerabilities are dangerous because they erode the trust users put in your internal systems. CVE-2022-21987 is a textbook example—while it didn’t allow direct code execution, it could have paved the way for credential theft or phishing in even the most secure-seeming workplaces.

Staying safe means patching often, educating users, and always keeping an eye on your logs for unexpected behavior. Keep sharing, and stay secure.

Want to Learn More?

Check out these useful links:
- Microsoft Security Update Guide
- How to secure SharePoint Online and On-premises

Timeline

Published on: 02/09/2022 17:15:00 UTC
Last modified on: 02/14/2022 18:08:00 UTC