Attackers Exploit Defender for Endpoint Cloud API to Bypass Authentication and Disrupt Incident Response

Microsoft Defender for Endpoint’s cloud communication can be abused to bypass authentication, intercept commands, and spoof results, allowing attackers to derail incident response and mislead analysts.

Recent research shows that multiple backend endpoints accept requests without effectively validating tokens, enabling unauthenticated manipulation if a machine ID and tenant ID are known.

Microsoft reportedly classified the issues as low severity, and fixes remain unclear.

Defender Cloud Traffic Weaknesses Enable Command Interception

Research into the Defender for Endpoint agent (MsSense.exe and SenseIR.exe) reveals that once TLS pinning is bypassed for analysis, the agent’s network traffic exposes weak server-side validation.

The agent regularly polls a location-specific /edr/commands/cnc endpoint for commands like isolationcommand, forensicscollectioncommand, scancommand, and incidentresponsecommand.

Although requests include Authorization and Msadeviceticket headers, the backend ignores them.

An attacker who knows a machine ID and tenant ID values readable by low-privileged users on the host can race the agent to retrieve pending commands and consume them first.

The legitimate agent then receives nothing. The attacker can also upload spoofed telemetry or files to Azure Blob storage via returned sasuri values, pollute evidence, or misreport outcomes.

A parallel flaw affects /senseir/v1/actions/, which handles Live Response and Automated Investigation.

The backend again fails to enforce authentication properly. Attackers can obtain a valid CloudLR token using only the machine ID, then request actions, fetch associated Azure Blob URLs, and upload crafted data.

Because actions are encoded using Microsoft Bond, attackers can capture legitimate action payloads and modify them.

This enables deceptive operational impacts, such as reporting “Already isolated” while leaving a device online, or seeding investigation packages with lookalike malicious files that analysts might open.

Throttling behavior observed in October suggests some backend adjustments, but the core weaknesses persist.

Attackers can query IR-exclusions from the registration endpoint using only the organization ID, which is accessible from the registry to any user.

These exclusions do not disable detection, but they shape automated and manual IR behavior, revealing where responders will not act.

Additionally, an unauthenticated call to /edr/commands/cnc can return an approximately 8MB configuration bundle containing RegistryMonitoringConfiguration, driver access lists, and attack surface reduction data.

While not clearly tenant-specific, it offers valuable insight into rules and blind spots.

If an investigation package was previously generated on a compromised host, it may remain readable to any user, exposing autoruns, installed programs, network connections, and more for adversary reconnaissance.

The attacks primarily apply post-compromise, when an adversary can harvest machine and tenant identifiers from the host and pivot into the cloud control plane.

The most damaging outcomes include silently defeating isolation, poisoning evidence, and tricking analysts with booby-trapped investigation files.

Defenders should monitor for unexpected CNC/action polling patterns, validate that isolation states match host reality, restrict local access to identifiers, and set detections for suspicious Azure Blob uploads tied to Defender workflows.

Network controls that restrict access to Defender endpoints to trusted egress paths can reduce race conditions.

Until Microsoft enforces token validation on the backend and hardens token issuance, incident responders should assume cloud command channels can be contested and verify results out-of-band.

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