Microsoft Defender for Endpoint (DFE) and Mitigation for It

 

The recent vulnerability affecting Microsoft Defender for Endpoint (DFE) involves critical flaws in how the agent communicates with its cloud backend, potentially allowing attackers to bypass authentication and manipulate incident response processes.

Key Vulnerabilities in Defender for Endpoint (DFE)

• Authentication Bypass: Attackers can impersonate the Defender agent using only the machine ID and tenant ID, which are accessible to low-privileged users via registry reads. This allows them to spoof commands and responses.
• Command Interception: By querying endpoints like /edr/commands/cnc, attackers can intercept or fake responses (e.g., falsely reporting a device as isolated), undermining actual security status.
• Certificate Pinning Circumvention: Researchers bypassed HTTPS protections by patching memory functions (e.g., CRYPT32!CertVerifyCertificateChainPolicy) to inspect traffic in plaintext using tools like Burp Suite and WinDbg.
• Azure Blob Upload Exploits: Attackers can upload malicious files to investigation packages by exploiting ignored authentication headers and using SAS tokens that remain valid for months.
• Sensitive Configuration Dump: Unauthenticated queries to certain endpoints return up to 8 MB of configuration data, including registry monitoring rules and ASR logic—valuable for evasion tactics.
• Live Response Exploits: Similar flaws exist in /senseir/v1/actions/ endpoints, allowing manipulation of automated investigations and forensic actions without proper authentication.

 

Mitigations for Defender/DFE Vulnerabilities

1. Restrict Access to Machine Identifiers

• Lock down registry keys that expose SenseMachineId and TenantId to prevent low-privileged users from spoofing Defender agents.
• Use Group Policy or endpoint hardening tools to limit read access to these keys.

2. Monitor Defender API Traffic

• Implement network-level monitoring for suspicious traffic to Defender cloud endpoints such as:
• /edr/commands/cnc
• /senseir/v1/actions/
• Flag anomalies like unexpected command responses or spoofed isolation status.

3. Disable or Limit Live Response

• If feasible, disable Defender’s Live Response feature until authentication mechanisms are hardened.
• Alternatively, restrict its use to high-trust devices or accounts with conditional access policies.

4. Audit SAS Token Usage

• Review and rotate Shared Access Signature (SAS) tokens used for investigation package uploads.
• Monitor Azure Blob storage for unauthorized uploads or access attempts.

5. Apply Endpoint Hardening

• Use EDR layering with third-party tools to validate Defender telemetry and isolate spoofing attempts.
• Consider deploying endpoint firewall rules to restrict outbound traffic to only verified Defender cloud IPs.

6. Engage Microsoft Support

• Open a support case with Microsoft to request guidance and push for expedited patching.
• Subscribe to Microsoft Security Response Center (MSRC) alerts for updates.