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M365 Identity Device Code Grant with Unusual User and ASN

Identifies a Microsoft 365 OAuth device code grant ("Cmsi:Cmsi") with application Microsoft Authentication Broker ("29d9ed98-a469-4536-ade2-f981bc1d605e") for Microsoft Graph from a source ASN not previously observed for that user in a historical window. Phishing kits leveraging device code phishing complete the full login (password and MFA) at the genuine Microsoft endpoint and harvest the resulting token by polling, so MFA does not stop them and the authorization commonly originates from attacker-controlled residential proxy or hosting infrastructure rather than the user's normal network.

Rule type: new_terms
Rule indices:

  • logs-o365.audit-*

Rule Severity: high
Risk Score: 73
Runs every:
Searches indices from: now-9m
Maximum alerts per execution: 100
References:

Tags:

  • Domain: Cloud
  • Domain: SaaS
  • Domain: Identity
  • Data Source: Microsoft 365
  • Data Source: Microsoft 365 Audit Logs
  • Use Case: Identity and Access Audit
  • Use Case: Threat Detection
  • Resources: Investigation Guide
  • Tactic: Initial Access

Version: 1
Rule authors:

  • Elastic

Rule license: Elastic License v2

This rule requires the Microsoft 365 integration with unified audit logs (Azure AD / Entra sign-in events surfaced in the Microsoft 365 audit log) enabled and shipping to Elastic.

This rule detects a user completing an OAuth device code grant (Cmsi:Cmsi) to the Microsoft Authentication Broker for Microsoft Graph from a source ASN not seen for that user within the rule's historical window (defined by the new terms history setting). A match means the user has not authenticated via this flow from that ASN during the lookback window, not necessarily that it has never happened. Device code phishing kits (for example Kali365, Storm-2372 tradecraft) drive the device code flow against the genuine Microsoft endpoint and poll the token endpoint in the background, so the victim satisfies MFA while the attacker harvests a fully MFA-satisfied token. The grant therefore frequently appears from residential proxy or hosting/datacenter infrastructure the user has not authenticated from during the window.

  • Review o365.audit.UserId to identify the impacted account and confirm whether the user expected to perform a device code sign-in.
  • Inspect source.as.number and source.as.organization.name for the source ASN. Hosting, VPN, or datacenter providers (for example Tencent, Alibaba, DigitalOcean) are unusual for interactive user authentication.
  • Review source.ip, source.geo.country_name, and source.geo.city_name and compare with the user's normal sign-in locations.
  • Examine o365.audit.DeviceProperties and user_agent.original for non-managed devices and automation or headless-browser patterns.
  • Confirm o365.audit.ApplicationId is the Microsoft Authentication Broker (29d9ed98-a469-4536-ade2-f981bc1d605e) and o365.audit.Target.ID is Microsoft Graph (00000003-0000-0000-c000-000000000000).
  • Pivot to azure.signinlogs for the corresponding deviceCode sign-in, including is_interactive, conditional access decisions, and any concurrent non-interactive token-issuance legs from a different ASN (the kit's polling backend).
  • Pivot to azure.graphactivitylogs for follow-up Graph activity (/me recon, mailbox or file enumeration) from the same or related ASNs.
  • Check azure.auditlogs for subsequent device registration events on the user, which device code phishing kits use to establish Primary Refresh Token persistence.
  • A legitimate first-time device code sign-in from a new ISP, mobile carrier, corporate VPN, or while traveling.
  • Provisioning of input-constrained devices (smart TVs, kiosks, IoT, conference room devices).
  • CLI or headless developer workflows that use the device code flow against the Authentication Broker.
  • If a source ASN is confirmed benign and recurring for the environment, suppress it via a rule exception rather than broadening the query.
  • Contact the user to confirm whether they initiated the device code sign-in or may have entered a code presented on a phishing page.
  • If unauthorized, revoke all refresh tokens for the user and reset credentials to invalidate the harvested token.
  • Review and remove any unauthorized device registrations to cut off Primary Refresh Token persistence.
  • Review recent Microsoft Graph, Exchange, SharePoint, and Teams activity for the user for signs of recon or exfiltration.
  • Restrict device code authentication to only the users and applications that require it via Conditional Access authentication flow policies.
  • Educate users on device code phishing and the risk of entering codes presented by unsolicited documents or messages.
event.dataset: "o365.audit"
    and o365.audit.ExtendedProperties.RequestType: "Cmsi:Cmsi"
    and o365.audit.Actor.Type: (0 or 2 or 3 or 5 or 10)
    and o365.audit.ApplicationId: "29d9ed98-a469-4536-ade2-f981bc1d605e"
    and o365.audit.Target.ID: "00000003-0000-0000-c000-000000000000"
    and o365.audit.DeviceProperties.Value: "False"

Framework: MITRE ATT&CK

Framework: MITRE ATT&CK