Exploiting OAuth in Entra ID: How Malicious Apps Mimic ChatGPT to Access Emails
Cybercriminals are continually devising innovative methods to exploit trusted platforms, with Microsoft Entra ID emerging as a prime target through a technique known as OAuth consent abuse. A recent attack scenario reveals how malicious or overly permissive third-party applications, masquerading as reputable tools like ChatGPT, can clandestinely access corporate users’ email accounts without requiring their passwords.
Understanding OAuth and Its Exploitation
OAuth, or Open Authorization, is a standard protocol that enables applications to access user data with explicit permission. Within Entra ID, when users connect third-party apps to their Microsoft accounts, they encounter consent prompts detailing the permissions requested by the app. Attackers exploit this process by creating or disguising applications that request sensitive permissions, such as Mail.Read. Once a user grants consent, the application gains full access to read all emails in the user’s account.
Case Study: The ChatGPT Impersonation
Analysts at Red Canary identified a case where a corporate user, [email protected], added ChatGPT as a third-party service principal within an Entra ID tenant. The user, operating as a non-administrator, consented to OAuth permissions including Mail.Read, offline_access, profile, and openid. Although this instance involved the legitimate OpenAI-owned ChatGPT, the investigative steps mirrored those of real-world incidents previously observed. The activity was traced to IP address 3.89.177.26 on December 2, 2025, at 20:22:16 UTC.
The Broader Threat Landscape
The primary concern extends beyond ChatGPT to the attack pattern itself. Any third-party application, whether legitimate or malicious, that obtains the Mail.Read permission through user consent can silently access every message in the targeted inbox. In a genuine attack, a threat actor could design an application with a convincing name, distribute it via phishing links, and harvest sensitive emails, internal communications, or credentials without the victim realizing their account has been compromised.
Default Permissions and Organizational Vulnerabilities
A significant risk factor is that Entra ID, by default, allows standard, non-administrative users to consent to applications requesting permissions that do not require administrator-level approval. This means that a single employee, without elevated access, can inadvertently expose sensitive organizational data by accepting what appears to be a routine app connection request.
Mechanics of the Consent Attack in Entra ID
When a user is prompted to connect an application—whether through phishing emails, social engineering tactics, or legitimate browsing—Entra ID records two specific audit log events: Add service principal and Consent to application. Both events share a CorrelationId, allowing security teams to link them and trace the full consent chain back to a single user action.
Red Canary’s detection strategy focuses on identifying non-admin consent grants associated with newly introduced third-party applications that request commonly abused OAuth scopes. A critical indicator is the AppOwnerOrganizationId field in the audit log. If this value does not match the tenant’s own ID or known Microsoft first-party identifiers, the application is third-party and warrants immediate scrutiny. Frequently abused scopes in these attacks include Mail.Read, Files.Read.All, Chat.Read, and Sites.Read.All.
Immediate Remediation Steps
Upon confirming a malicious or unsanctioned consent grant, two immediate actions are recommended:
1. Revoke the OAuth Permission Grant: Utilize the grant ID from the Consent to application audit event to revoke the permission.
2. Remove the Service Principal: Use the object’s ID to remove the service principal from the tenant.
Both tasks can be executed using Microsoft Graph PowerShell commands.
Preventative Measures and Policy Configurations
To mitigate such risks, Microsoft offers three configurable consent policy options:
1. Administrator Approval for All Consent Requests: The most secure approach, requiring an administrator to approve all consent requests, thereby preventing non-admin users from authorizing any applications.
2. Restrict Consent to Verified Publishers: Allows consent only to applications from verified publishers with pre-approved, low-risk permissions.
3. Microsoft’s Recommended Configuration: Automatically applies Microsoft’s current user consent guidelines to the organization, balancing security and operational convenience.
Conclusion
The exploitation of OAuth consent in Entra ID underscores the critical need for organizations to scrutinize third-party applications and their requested permissions. By implementing stringent consent policies and maintaining vigilant monitoring, organizations can safeguard sensitive data from unauthorized access facilitated by deceptive applications.
