Detect Business Email Compromise via OAuth Device Code Flow Phishing in Elastic Security
OAuth Device Code Flow phishing is a prevalent Business Email Compromise (BEC) technique actively used by Scattered Spider, Storm-2372, and nation-state actors including Midnight Blizzard. The attacker sends a phishing message containing a Microsoft device code (a short alphanumeric code from https://microsoft.com/devicelogin), social-engineered to appear as an IT helpdesk request, MFA enrollment notification, or remote support session. When the victim enters the code, the attacker receives a valid OAuth access token and refresh token for the victim's Microsoft 365 account — with no password required. The attacker then has full access to email, Teams, SharePoint, OneDrive, and any M365 service the victim is licensed for. Refresh tokens may persist for 90 days, providing long-term access even after password reset. This technique bypasses MFA entirely because the device code flow is a legitimate Microsoft authentication mechanism.
MITRE ATT&CK
- Tactic
- Credential Access Collection
Elastic Detection Query
sequence by user.name with maxspan=4h
[authentication where event.action == "user-login" and
event.dataset == "azure.signinlogs" and
azure.signinlogs.properties.authentication_protocol == "deviceCode" and
event.outcome == "success"]
[file where event.dataset == "o365.audit" and
event.action in ("New-InboxRule", "Set-InboxRule") and
o365.audit.Parameters : ("*ForwardTo*", "*DeleteMessage*", "*RedirectTo*")]EQL sequence rule correlating device code authentication success with inbox forwarding rule creation for the same user within 4 hours — high confidence BEC indicator.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate device enrollment followed by mailbox configuration
- IT admin mailbox setup workflows
Other platforms for THREAT-BEC-OAuthDeviceCode
Testing Methodology
Validate this detection against 2 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1OAuth Device Code Flow Token Request (Simulated Phishing)
Expected signal: Azure AD Sign-in logs record a DeviceCode flow initiation. When user enters the code, AADSignInLogs records a successful authentication with AuthenticationProtocol=deviceCode.
- Test 2Create BEC Inbox Forwarding Rule via Exchange Online PowerShell
Expected signal: O365 Unified Audit Log records New-InboxRule operation with ForwardTo parameter for the authenticated user.
References (6)
- https://www.microsoft.com/en-us/security/blog/2024/08/08/midnight-blizzard-conducts-targeted-social-engineering-over-microsoft-teams/
- https://www.crowdstrike.com/blog/scattered-spider-attempts-to-avoid-detection-with-bring-your-own-vulnerable-driver-tactic/
- https://www.ncsc.gov.uk/collection/phishing-attacks/phishing-techniques/oauth-phishing
- https://attack.mitre.org/techniques/T1528/
- https://learn.microsoft.com/en-us/entra/identity/conditional-access/block-legacy-authentication
- https://github.com/dafthack/TokenTactics
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