Entra ID Microsoft Authentication Broker DRS Sign-In from Suspicious ASN

Description

Detects Microsoft Entra ID sign-in activity where the Microsoft Authentication Broker requests the Device Registration Service from a source autonomous system number (ASN) associated with VPN, residential proxy, or hosting egress commonly observed in OAuth phishing and adversary-in-the-middle device registration flows. This pattern can indicate device join or primary refresh token acquisition staged from attacker-controlled infrastructure after a user completes authentication.

Detection logic

Its licence does not clear it for publishing here

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Detection requirements

Platform: Containers ESXi IaaS Identity Provider Linux macOS Network Devices Office Suite SaaS Windows
The rule states no platform. This is derived from the 3 ATT&CK techniques it maps to.
Log source category: event_index

Known benign triggers

Users enrolling or joining devices while on corporate VPNs, consumer VPNs, or cloud egress that map to the listed ASNs may match. Legitimate mobile device management or bulk provisioning that uses the broker against Device Registration Service from the same networks can also trigger alerts. Baseline `source.as.organization.name` and successful broker-to-DRS sign-ins before tuning exclusions for approved ASNs or user groups.

References

Links the source published beside this detection. Reference is the link the source wrote, unmodified. Class is decided by this catalogue from the host and path, not stated by the source. Cited by counts published detections citing the same resource, including this one, and the sources beneath it count the repositories this catalogue ingests, which is not the same as independent research.
Reference	Class	Cited by
dirkjanm.io/phishing-for-microsoft-entra-primary-refresh-tokens/	Unclassified	10
www.volexity.com/blog/2025/04/22/phishing-for-codes-russian-threat-actors-target-microsoft-365-oauth-workflows/	Threat report	19

Detection rules

Entra ID Microsoft Authentication Broker DRS Sign-In from Suspicious ASN

Description

Detection logic

Detection requirements

Known benign triggers

MITRE ATT&CK mappings

References

From the source

Licence

Authorship

Detection rules