CVE-2020-16871
📋 TL;DR
This is a cross-site scripting (XSS) vulnerability in Microsoft Dynamics 365 on-premises that allows authenticated attackers to inject malicious scripts. When exploited, these scripts execute with the victim's permissions, potentially enabling unauthorized data access, privilege escalation, or content manipulation. Only authenticated users can exploit this vulnerability against affected Dynamics 365 on-premises deployments.
💻 Affected Systems
- Microsoft Dynamics 365 (on-premises)
📦 What is this software?
Dynamics 365 by Microsoft
Dynamics 365 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
An attacker gains full administrative control over the Dynamics 365 instance, accesses sensitive business data, modifies or deletes critical information, and uses the compromised system as a foothold for further attacks.
Likely Case
An authenticated attacker with limited privileges uses XSS to elevate their permissions, access unauthorized data, or perform actions on behalf of other users within the Dynamics 365 environment.
If Mitigated
With proper input validation and output encoding controls, the attack is prevented, though the underlying vulnerability remains until patched.
🎯 Exploit Status
Exploitation requires authenticated access and knowledge of vulnerable endpoints. XSS vulnerabilities are commonly exploited once details are known.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Security update released October 13, 2020 (specific version depends on Dynamics 365 build)
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-16871
Restart Required: Yes
Instructions:
1. Download the security update from Microsoft Update Catalog. 2. Apply the update to all affected Dynamics 365 servers. 3. Restart the Dynamics 365 services and servers as required. 4. Test functionality after patching.
🔧 Temporary Workarounds
Implement Content Security Policy (CSP)
allDeploy a strict Content Security Policy to mitigate XSS attacks by restricting script execution sources.
Input Validation and Output Encoding
allImplement server-side input validation and output encoding for all user-supplied data in Dynamics 365 customizations.
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block XSS payloads in requests to Dynamics 365 endpoints.
- Restrict user permissions to minimum required levels and monitor for unusual activity from authenticated accounts.
🔍 How to Verify
Check if Vulnerable:
Check Dynamics 365 version/build number and compare against patched versions in Microsoft advisory. Review if October 2020 security updates have been applied.Check Version:
Check Dynamics 365 version through administration console or by examining installed updates in Windows Control Panel.Verify Fix Applied:
Verify that the security update KB number from Microsoft advisory is installed on all Dynamics 365 servers. Test vulnerable endpoints with safe XSS payloads to confirm sanitization.📡 Detection & Monitoring
Log Indicators:
- Unusual POST/GET requests containing script tags or JavaScript code to Dynamics 365 endpoints
- Multiple failed authentication attempts followed by successful login and suspicious requests
Network Indicators:
- HTTP requests with encoded script payloads in parameters to Dynamics 365 servers
- Unexpected outbound connections from Dynamics 365 servers following suspicious requests
SIEM Query:
source="dynamics_logs" AND (uri="*<script*" OR uri="*javascript:*" OR params="*onload=*" OR params="*onerror=*")