Most Azure environments that we test contain multiple kinds of application hosting services (App Services, AKS, etc.). As these applications grow and scale, we often find that the application configuration parameters will be shared between the multiple apps. To help with this scaling challenge, Microsoft offers the Azure App Configuration service. The service allows Azure users to create key-value pairs that can be shared across multiple application resources. In theory, this is a great way to share non-sensitive configuration values across resources. In practice, we see these configurations expose sensitive information to users with permission to read the values.
The Azure App Configuration service can often hold sensitive data values. This blog post outlines gathering and using access keys for the service to retrieve the configuration values.
What are App Configurations?
The App Configuration service is a very simple service. Provide an Id and Secret to an “azconfig.io” endpoint and get back a list of key-value pairs that integrate into your application environment. This is a really simple way to share configuration information across multiple applications, but we have frequently found sensitive information (keys, passwords, connection strings) in these configuration values. This is a known problem, as Microsoft specifically calls out secret storage in their documentation, noting Key Vaults as the recommended secure solution.
Gathering Access Keys
Within the App Configuration service, two kinds of access keys (Read-write and Read-only) can be used for accessing the service and the configuration values. Additionally, Read-write keys allow you to change the stored values, so access to these keys could allow for additional attacks on applications that take action on these values. For example, by modifying a stored value for an “SMBSHAREHOST” parameter, we might be able to force an application to initiate an SMB connection to a host that we control. This is just one example, but depending on how these values are utilized, there is potential for further attacks.
Regardless of the type of key that an attacker acquires, this can lead to access the configuration values. Much like the other key-based authentication services in Azure, you are also able to regenerate these keys. This is particularly useful if your keys are ever unintentionally exposed.
To read these keys, you will need Contributor role access to the resource or access to a role with the “Microsoft.AppConfiguration/configurationStores/ListKeys/” action.
From the portal, you can copy out the connection string directly from the “Access keys” menu.
This connection string will contain the Endpoint, Id, and Secret, which can all be used together to access the service.
Alternatively, using the Az PowerShell cmdlets, we can list out the available App Configurations (Get-AzAppConfigurationStore) and for each configuration store, we can get the keys (Get-AzAppConfigurationStoreKey). This process is also automated by the Get-AzPasswords function in MicroBurst with the “AppConfiguration” flag.
Finally, if you don’t have initial access to an Azure subscription to collect these access keys, we have found App Configuration connection strings in web applications (via directory traversal/local file include attacks) and in public GitHub repositories. A cursory search of public data sources results in a fair number of hits, so there are a few access keys floating around out there.
Using the Keys
Typically, these connection strings are tied to an application environment, so the code environment makes the calls out to Azure to gather the configurations. When initially looking into this service, we used a Microsoft Learn example application with our connection string and proxied the application traffic to look at the request out to azconfig.io.
This initial look into the azconfig.io API calls showed that we needed to use the Id and Secret to sign the requests with a SHA256-HMAC signature. Conveniently, Microsoft provides documentation on how we can do this. Using this sample code, we added a new function to MicroBurst to make it easier to request these configurations.
In our example, I just have “test” values for the keys. As noted above, if you have the Read-write key for the App Configuration, you will be able to modify the values of any of the keys that are not set to “locked”. Depending on how these configuration values are interpreted by the application, this could lead to some pivoting opportunities.
Since we just provided some potential attack options, we also wanted to call out any IoCs that you can use to detect an attacker going after your App Configurations:
We showed you how to gather access keys for App Configuration resources and how to use those keys to access the configuration key-value pairs. This will hopefully give Azure pentesters something to work with if they run into an App Configuration connection string and defenders areas to look at to help secure their configuration environments.
For those using Azure App Configurations, make sure that you are not storing any sensitive information within your configuration values. Key Vaults are a much better solution for this and will give you additional protections (Access Policies and logging) that you don’t have with App Configurations. Finally, you can also disable access key authentication for the service and rely on Azure Active Directory (AAD) for authentication. Depending on the configuration of your environment, this may be a more secure configuration option.
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