Exploiting Graph API and Tokens: A Red Team Perspective

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Introduction

The Microsoft Graph API provides a unified endpoint to access Microsoft 365 data, including calendars, emails, and attachments. While powerful for legitimate use, it can be exploited by red teams to simulate adversary tactics. This article explores how attackers leverage Graph API tokens to access sensitive data and provides mitigation strategies.

Learning Objectives

  • Understand how Graph API tokens can be abused for unauthorized access.
  • Learn defensive techniques to secure OAuth tokens and API permissions.
  • Explore detection methods for anomalous Graph API activity.

1. Extracting Calendar Events via Graph API

Command:

 Fetch calendar events using Graph API 
$token = "Bearer <ACCESS_TOKEN>" 
$headers = @{ Authorization = $token } 
$response = Invoke-RestMethod -Uri "https://graph.microsoft.com/v1.0/me/events" -Headers $headers 
$response.value | Select-Object subject, start, end

Step-by-Step Guide:

  1. Obtain an OAuth token (e.g., via phishing or token theft).
  2. Use the token to query `/me/events` endpoint to extract calendar data.

3. Filter events for attachments using `$response.value.hasAttachments`.

Mitigation:

  • Enforce Conditional Access policies requiring MFA for Graph API access.
  • Monitor for unusual `GetEvents` API calls.

2. Downloading Attachments from Events

Command:

 Download attachments via Graph API (Linux/macOS) 
curl -H "Authorization: Bearer <ACCESS_TOKEN>" \ 
"https://graph.microsoft.com/v1.0/me/events/<EVENT_ID>/attachments" \ 
-o attachment.zip

Step-by-Step Guide:

  1. Identify an event with attachments using the `hasAttachments` property.

2. Query `/events/{id}/attachments` to download files.

  1. Extract sensitive data (e.g., meeting notes, shared documents).

Mitigation:

  • Restrict attachment access via Azure AD “Least Privilege” roles.
  • Enable Defender for O365 to scan malicious attachments.
    1. Detecting Token Abuse with Azure AD Logs

KQL Query (Azure Sentinel):

SigninLogs 
| where AppId == "00000003-0000-0000-c000-000000000000" // Graph API App ID 
| where ResultType == 0 // Successful logins 
| summarize count() by UserPrincipalName, IPAddress 
| where count_ > 5 // Threshold for anomalous activity

Step-by-Step Guide:

  1. Monitor `SigninLogs` for Graph API (AppId: 00000003-...) access.
  2. Alert on token reuse from unusual IPs or high-frequency calls.

Mitigation:

  • Enable Azure AD Identity Protection for token anomaly detection.

4. Hardening OAuth Permissions

PowerShell (Azure AD Module):

 Review OAuth permissions granted to apps 
Get-AzureADServicePrincipal -All $true | 
Where-Object { $_.Oauth2Permissions -ne $null } | 
Select-Object DisplayName, AppId

Step-by-Step Guide:

  1. Audit apps with Calendars.Read, Mail.Read, or other high-risk scopes.

2. Revoke unnecessary permissions via Azure AD Portal.

5. Exploiting Misconfigured App Registrations

Command:

 Enumerate apps with excessive permissions (Azure CLI) 
az ad app list --query "[].{name:displayName, permissions:requiredResourceAccess}"

Step-by-Step Guide:

  1. Identify apps with overly permissive scopes (e.g., Calendars.ReadWrite).

2. Exploit weak consent policies to escalate access.

Mitigation:

  • Enforce admin consent for sensitive permissions.

What Undercode Say

  • Key Takeaway 1: Graph API tokens are a prime target for lateral movement and data exfiltration.
  • Key Takeaway 2: Defenders must audit OAuth scopes and monitor token usage patterns.

Analysis:

The abuse of Graph API tokens highlights the blurred line between legitimate functionality and exploitation. Red teams mimic real-world attackers who leverage OAuth tokens for stealthy access. Organizations should adopt a zero-trust approach, treating all token-based access as potentially malicious until verified. Future attacks may leverage AI-generated phishing lures to harvest tokens more effectively, making proactive hardening critical.

Prediction:

As Microsoft 365 adoption grows, Graph API attacks will surge, with attackers automating token theft via malicious apps. Defenders will counter with AI-driven anomaly detection, but token hygiene remains the first line of defense.

IT/Security Reporter URL:

Reported By: Dmcxblue Redteam – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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