CVE-2020-16858
📋 TL;DR
This is a cross-site scripting (XSS) vulnerability in Microsoft Dynamics 365 (on-premises) that allows authenticated attackers to inject malicious scripts into web requests. When exploited, these scripts execute in the security context of the current authenticated user, potentially allowing unauthorized data access, privilege escalation, or content manipulation. Only organizations running affected on-premises Dynamics 365 installations are impacted.
💻 Affected Systems
- Microsoft Dynamics 365 (on-premises)
📦 What is this software?
Dynamics 365 by Microsoft
⚠️ Risk & Real-World Impact
Worst Case
An authenticated attacker could gain administrative privileges, exfiltrate sensitive business data, delete critical content, or perform actions on behalf of any user in the system.
Likely Case
An authenticated user with malicious intent could steal session cookies, perform unauthorized actions within their permission scope, or deface/modify content visible to other users.
If Mitigated
With proper input validation and output encoding controls, the attack would fail to execute malicious scripts, limiting impact to failed exploitation attempts.
🎯 Exploit Status
Exploitation requires authenticated access to the Dynamics server. The vulnerability involves improper input sanitization in web requests.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Apply security update from Microsoft October 2020 Patch Tuesday or later
Vendor Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2020-16858
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 servers as required. 4. Test functionality after patching.
🔧 Temporary Workarounds
Implement Input Validation
allAdd server-side input validation to sanitize all user-supplied data before processing.
Enable Content Security Policy
allImplement CSP headers to restrict script execution sources and reduce XSS impact.
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block XSS payloads in requests.
- Restrict user permissions to minimum necessary and implement strong authentication controls.
🔍 How to Verify
Check if Vulnerable:
Check Dynamics 365 version against patched versions in Microsoft advisory. Test with safe XSS payloads in controlled environment.Check Version:
Check Dynamics 365 version through administrative interface or server configuration files.Verify Fix Applied:
Verify patch installation via Windows Update history or version check. Test that previously vulnerable endpoints now properly sanitize input.📡 Detection & Monitoring
Log Indicators:
- Unusual POST/GET requests with script tags or JavaScript payloads in Dynamics logs
- Multiple failed authentication attempts followed by successful login and suspicious requests
Network Indicators:
- HTTP requests containing <script>, javascript:, or encoded XSS payloads to Dynamics endpoints
SIEM Query:
source="dynamics_logs" AND ("<script>" OR "javascript:" OR "onerror=" OR "onload=")