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Reply URL Brute

Tool that looks for unregistered reply URLs of service principals and checks if they can be used in a reply URL hijack attack. Works for local and multitenant applications.

More information about this tool in its blog post here.

Tool main steps:

  1. Initialize variables and request access tokens for Microsoft Graph (graph.microsoft.com), Azure Active Directory Graph (graph.windows.net) and several others.
  2. Request service principals in the tenant using the Microsoft Graph API.
  3. Enrich service principals' reply URLs using the undocumented Azure Active Directory Graph.
  4. Start DNS resolution of all reply URLs; keep the ones that aren't found.
  5. Check in Azure Resource Manager if the resource to take over the domain is available. More often than not, the resource name that would allow to take over a certain domain is unavailable. I imagine this is due to the fact a developer created the resource, but then left it in a disabled step. Even though writing checks for most of the resources was an awfully time-intensive endeavor, it really helped in bringing down the number of false positives. Shout out to the guys at Stratus Security for collecting a very comprehensive list of Azure domains take over.
  6. Determine if a given reply URL can be taken advantage of. Check the blog post linked in the beginning for more details. Optional: consent user scope automatically.
  7. Reapply the same process in point 6 but for a set of different scopes.
  8. Take all the information gathered in the previous points and present it in a human-friendly way.
  9. Every time a request to the authorize endpoint is made, Azure returns a new ESTSAUTHPERSISTENT cookie. At the end of the enumeration, the script will overwrite the values in the config.yaml file, so the cookies are always up to date.

How to use it

Clone the project and open the config_doc.yaml:

git clone https://github.com/FalconForceTeam/reply-url-brute

You need to provide several pieces of information before running the script for the first time. Set the tenant ID and the subscription ID in the appropriate fields:

subscription_id: 4493ad17-...
tenant_id: 2a0002d5-2368-...
...

Now, we need to set the ESTSAUTHPERSISTENT cookie. To get it, I recommend to open Microsoft Edge, navigate to https://portal.azure.com and log in. When it asks you to stay signed in, click "Yes", so the ESTSAUTHPERSISTENT is properly set. Then go to "Settings" > "Cookies and site permissions" > "Manage and delete cookies and site data" > "See all coolies and site data". Expand the domain "microsoftonline.com" and then "login.microsoftonline.com". Then copy the value for the ESTSAUTHPERSISTENT cookie.

Some considerations to keep in mind:

  • No special rights in Entra ID are required for the user.
  • To check for Azure resource availability, the user must have access to a subscription. The easiest way is to provide read-only access to the subscription, but access to any resource should work anyway.
  • You could also use the ESTSAUTH, but then it's possible you'd get some issues in the long term, since it doesn't last as long.
subscription_id: 4493ad17-...
tenant_id: 2a0002d5-2368-...
token_user_ro: 0.AV8AzUIqqYy...

Finally, save the file as config.yaml.

To run the script against all the service principals available in the tenant, just run it without arguments. Alternatively, you can also use some of the available CLI arguments:

  • --appId: only checks one specific application. Thanks to the undocumented internal Azure Active Directory API, the application doesn't need to be present in the tenant.
  • --auto-consent-user: Automatically accept consent for https://graph.microsoft.com/user.read scope to improve results fidelity.
  • --scope: by default, there are 11 scopes hardcoded in the script. Pass this option to overwrite them.
  • --scope-list: you can also pass the path to a file with a list of scopes to check. Every scope must be in a new line.

Output:

  • The output is similar, regardless if a single application is tested or all the ones present in the tenant. The same is true when it comes to the number of scopes tested.
  • Depending on the number of service principals, the script can take a lot of time, but it greatly depends on the number of reply URLs per application and the scopes to test. For example, for a tenant with 1000 service principals and the default scope list, it can take more than 30 minutes.
  • The script output is very verbose, skip to the reporting step to have a quick summary of the findings.
  • DNS resolution step:
    • No: an unregistered reply URL has been found, but the domain can't be taken over.
    • Maybe: an unregistered reply URL has been found, but there's no method to check whether the resource that would register the domain is available.
    • Yes: an unregistered reply URL has been found and the resource is available.
  • Flow and scope brute force step:
    • Maybe: the application asked for consent and therefore it's not sure if the reply URL is vulnerable or not. Consider using --auto-consent-user.
    • YES!: the application returned access tokens for the given scope.
  • Reporting step:
    • Summary of all the applications vulnerable to the reply URL hijack that return tokens without asking for consent.
    • If you don't care whether the consent is shown or not, make sure you check the output during the flow brute force step.
python reply-url-brute.py --appId 89efd99e-0ec9-4bcf-9e5c-74c31b45f181

[*] Loading tenant informacion and cookies from config.yaml file: config.yaml
[*] Tenand ID: a92a42cd-bf8c-46ba-aa4e-64cbc9e030d9
[*] Subscription ID: 80110e3c-3ec4-4567-b06d-7d47a72562f5

STEP: Get tokens for MS Graph, APP MGMT and AAD
[*] Send Authorize for scope: https://graph.microsoft.com/Directory.AccessAsUser.All
[*] Send Token request for scope: https://graph.microsoft.com/Directory.AccessAsUser.All
[*] Send Authorize for scope: https://graph.windows.net/user_impersonation
[*] Send Token request for scope: https://graph.windows.net/user_impersonation
[*] Send Authorize for scope: https://appmanagement.activedirectory.microsoft.com/user_impersonation
[*] Send Token request for scope: https://appmanagement.activedirectory.microsoft.com/user_impersonation

STEP: Single AppId mode
[*] Quering AppId
[+] Response: 200, Service Principals: 1

STEP: Query reply URLs via Windows Graph
[*] Reply URLs from Windows Graph, SP: app-demo: +0

RECAP: The appId could be found:
[*] app-demo, 89efd99e-0ec9-4bcf-9e5c-74c31b45f181, replyUrls:
    https://localhost
    https://idontexist23843.azurewebsites.net

STEP: Get tokens for ARM
[*] Send Authorize for scope: https://management.azure.com/.default
[*] Send Token request for scope: https://management.azure.com/.default

STEP: Starting DNS resolution of reply URLs
[*] Checking availability for idontexist23843.azurewebsites.net: Available!
[+] Domain not found, take over: Yes,   SP: app-demo, https://idontexist23843.azurewebsites.net
[*] SP with missing reply URLs + take over (Yes, Maybe): 1
[*] SP with missing reply URLs but no take over: 0, this won't be checked

STEP: Starting flow bruteforce
[*] app-demo, https://idontexist23843.azurewebsites.net, scope: https://graph.microsoft.com/User.Read
[*] Authorize request, Web: NO, response_type 'token' not enabled, implicing grant disabled
[*] Authorize request, SPA: Maybe, needs user consent
...
[*] Total vulnerable Apps: 1

STEP: Starting scope bruteforcing
[*] Scopes to bruteforce: 17
[*] SP: app-demo, https://idontexist23843.azurewebsites.net (SPA/Desktop), scope: https://graph.microsoft.com/.default
[*] Authorize request, SPA: Auth code found
[*] Token request, SPA (origin): YES!
[+] Found vulnerable reply URL:
      Type: SPA
      Display name: app-demo
      Appid: 89efd99e-0ec9-4bcf-9e5c-74c31b45f181
      Reply URL: https://idontexist23843.azurewebsites.net
      Token resource: https://graph.microsoft.com
      Token scope: Directory.AccessAsUser.All

[*] SP: app-demo, https://idontexist23843.azurewebsites.net (SPA/Desktop), scope: https://graph.microsoft.com/User.Read
...
[*] Final total vulnerable Apps: 1


REPORTING: Reporting SP - reply - scopes
[*] SP: app-demo, scope: https://graph.microsoft.com/user.read, consent: Granted!
[*] SP: app-demo, scope: https://graph.windows.net/Directory.AccessAsUser.All, consent: Granted!
[*] SP: app-demo, Reply Url: https://idontexist23843.azurewebsites.net

[+] Saving cookies to config.yaml

In this example, we see how the app-demo has the reply URL https://idontexist23843.azurewebsites.net set as a SPA. We also see how the application has defined several OAuth permissions, but most of them require a consent by the user or an admin. However, the scope https://graph.windows.net/Directory.AccessAsUser.All has been granted and the application returned tokens for that. Therefore, if we register the idontexist23843 as a new Azure website, we could impersonate the user in the tenant, just by luring him to follow a link.