How to Hack OAuth: Understanding and Mitigating UNC6040’s Attack Vector

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Introduction:

OAuth-based attacks are on the rise, with threat actors like UNC6040 exploiting social engineering and misconfigured permissions to bypass multi-factor authentication (MFA). By impersonating IT support and tricking users into authorizing malicious OAuth apps, attackers gain persistent access to APIs, leading to data breaches. This article breaks down UNC6040’s tactics and provides actionable defenses.

Learning Objectives:

  • Understand how OAuth token hijacking works.
  • Learn how to detect and restrict malicious OAuth apps.
  • Implement phishing-resistant MFA and secure API access.

You Should Know:

1. How Attackers Exploit OAuth Tokens

Command (PowerShell – Check OAuth Apps in Azure AD): 

Get-AzureADServicePrincipal | Where-Object { $_.Tags -contains "WindowsAzureActiveDirectoryIntegratedApp" } | Select-Object DisplayName, AppId

Step-by-Step Guide:

  1. Attackers trick users into granting OAuth access via fake IT calls.
  2. A malicious app (e.g., fake “Salesforce Data Loader”) generates an access token.
  3. The token grants API access without MFA, persisting until revoked.
  4. Use the PowerShell command above to audit registered OAuth apps in Azure AD.

2. Restricting Unauthorized OAuth Apps

Command (Microsoft Graph API – List OAuth Permissions):

curl -H "Authorization: Bearer <access_token>" https://graph.microsoft.com/v1.0/oauth2PermissionGrants

Step-by-Step Guide:

  1. Regularly review OAuth permissions via Microsoft Graph API.

2. Restrict app registrations to pre-approved vendors only.

3. Revoke suspicious tokens using:

Revoke-AzureADUserAllRefreshToken -ObjectId <user_id>

3. Enforcing Phishing-Resistant MFA

Command (Linux – Configure FIDO2 U2F for SSH):

sudo nano /etc/ssh/sshd_config 
 Add: 
AuthenticationMethods publickey,password publickey,keyboard-interactive

Step-by-Step Guide:

1. Replace SMS/email MFA with FIDO2 security keys.

  1. Enforce conditional access policies in Azure AD/Microsoft 365.

3. Use hardware tokens (YubiKey) for critical systems.

4. Detecting Malicious API Activity

Command (SIEM Query – Suspicious OAuth Token Usage):

SELECT  FROM oauth_logs WHERE app_name NOT IN ("Salesforce", "Okta") AND token_scope = "full_access"

Step-by-Step Guide:

  1. Monitor OAuth token usage in SIEM tools (Splunk, ELK).
  2. Alert on unusual API calls (e.g., mass data exports).

3. Automate revocation of tokens from unknown IPs.

5. Preventing Lateral Movement via OAuth

Command (AWS CLI – Restrict IAM Roles):

aws iam put-role-policy --role-name <role> --policy-document file://restrictive_policy.json

Step-by-Step Guide:

  1. Limit OAuth apps’ access to only necessary APIs.

2. Use network segmentation to block pivoting attempts.

3. Disable unused OAuth scopes (e.g., `offline_access`).

What Undercode Say:

  • Key Takeaway 1: OAuth tokens are a goldmine for attackers—ensure strict app review policies.
  • Key Takeaway 2: Phishing-resistant MFA (FIDO2) is critical to stopping token theft.

Analysis:

UNC6040’s attack highlights the risks of overprivileged OAuth apps. Unlike MFA codes, OAuth tokens don’t expire automatically, allowing long-term access. Organizations must adopt zero-trust principles, automate token audits, and train employees to recognize social engineering.

Prediction:

As API-driven workflows grow, OAuth attacks will surge. Future tactics may include AI-generated voice phishing (“vishing”) to bypass human verification. Proactive monitoring and hardware-based authentication will become mandatory.

Reference: Google Threat Intelligence Report

IT/Security Reporter URL:

Reported By: Kondah Bonjour – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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