Authentication Bypass in Defender for Endpoint Cloud API Enables Incident Response Tampering

Microsoft Defender for Endpoint’s cloud communication infrastructure contains critical vulnerabilities that allow attackers to bypass authentication mechanisms, intercept security commands, and manipulate incident response operations.

Security researchers have discovered that the endpoint detection and response (EDR) solution’s backend systems fail to properly validate authentication tokens, creating opportunities for threat actors to undermine security operations and mislead analysts during critical investigations.

Weak Authentication Enables Command Interception

Recent analysis of the Defender for Endpoint agent components, specifically MsSense.exe and SenseIR.exe, reveals fundamental flaws in the cloud communication protocol.

Once Transport Layer Security (TLS) pinning is bypassed during analysis, researchers found that the agent’s network traffic exposes inadequate server-side validation mechanisms.

The Defender agent continuously polls location-specific endpoints /edr/commands/cnc to retrieve various security commands, including isolation directives, forensics collection tasks, scanning operations, and incident response commands.

Despite including proper Authorization and Msadeviceticket headers in requests, the backend infrastructure completely ignores these authentication credentials.

This authentication bypass enables attackers who possess knowledge of machine ID and tenant ID values information accessible to low-privileged users on compromised hosts to race legitimate agents for pending commands.

Threat actors can intercept and consume commands before the authentic agent receives them, effectively creating a denial-of-service condition for security operations.

Additionally, attackers can upload fabricated telemetry data or malicious files to Azure Blob storage using returned SAS URI values, contaminating evidence and misrepresenting security outcomes.

A parallel vulnerability affects the /senseir/v1/actions/ endpoint responsible for Live Response and Automated Investigation functionalities.

The backend systems similarly fail to enforce proper authentication controls, allowing attackers to obtain valid CloudLR tokens using only machine identification data.

With these tokens, threat actors can request actions, retrieve associated Azure Blob URLs, and upload specially crafted investigation data.

The attack methodology becomes particularly sophisticated when considering that actions are encoded using the Microsoft Bond protocol.

Attackers can capture legitimate action payloads and systematically modify them to create deceptive operational impacts.

For example, they can report false isolation status, indicating “Already isolated” while leaving compromised devices fully connected to the network, or seed investigation packages with malicious files disguised as legitimate evidence that analysts might inadvertently execute.

Broader Security Implications and Mitigation Strategies

The vulnerabilities extend beyond command interception to include unauthorized access to incident response exclusions and configuration data.

Attackers can query IR-exclusions from registration endpoints using only organization identifiers accessible through system registries.

While these exclusions don’t disable detection capabilities, they provide valuable intelligence about where security responders will not take action, effectively mapping blind spots in security coverage.

Microsoft has reportedly classified these issues as low severity, though comprehensive fixes remain unclear.

Security teams should implement immediate defensive measures, including monitoring for unusual command polling patterns, validating isolation states against actual host conditions, restricting local access to critical identifiers, and establishing detection rules for suspicious Azure Blob uploads related to Defender workflows.

Network controls limiting access to Defender endpoints through trusted egress paths can reduce race condition opportunities.

Until Microsoft implements proper token validation and hardens token issuance processes, incident responders should verify cloud command results through out-of-band methods to ensure operational integrity.

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