Always Threat Model Your Applications

The Why

The motivation of a cyber attacker may fall into one or more of these categories financial gain, political motives, revenge, espionage or terrorism. Once an attacker gets in they may install malware, such as a virus or ransomware. This can disrupt operations, lock users out of their systems, or even cause physical damage to infrastructure. Understanding the motives behind cyber attacks helps to prevent and respond to them effectively.

The primary goal of threat modelling is to identify vulnerabilities and risks before they can be exploited by attackers, allowing organizations to take proactive measures to mitigate these risks.

This should include a list of critical assets and services that need to be protected. Examine the points of data entry or extraction to determine the surface attack area and check if user roles have varying levels of privileges.

Threat modelling should begin in the early stages of the SDLC, when the requirements and the design of the system are being established. This is because the earlier in the SDLC that potential security risks are identified and addressed, the easier and less expensive they are to mitigate. Also, it is advisable to engage your security operations team early in the design phase. Due to the sensitive nature of the threat modelling document, it would be advisable to label the document as confidential and not distribute freely within an organisation.

It is important to revisit the threat model to identify any new security risks that may have emerged and when new functionality is added to a system, or as the system is updated or changed.

Threat modelling is typically viewed from an attacker’s perspective instead of a defender’s viewpoint. Remember the most likely goal or motive of an attacker is information theft, espionage or sabotage. When modelling your threats, ensure you include both externally and internally initiated attacks.

To assist in determining the possible areas of vulnerabilities, a data flow diagram (DFD) would be beneficial. This will give a visual representation of how the application processes the data and highlights any persistent points. It helps to identify the potential entry points an attacker may use and the paths that they could take through the system to reach critical data or functionality.

The How

Let’s go through the process of modelling a simple CRM website which maintains a list of customers stored in a database hosted in Azure.

The hypothetical solution uses a Web Application Gateway to provide HA (High Availability) using two webservices. There are also two microservices, one manages all the SQL Database CRUD operations for customers and the other manages all customers images that are persisted to a blob store. Connection strings to the database are stored in Azure Key vault. Customers use their social enterprise identities to gain access to the website by using Azure AD B2C.

The help with the process of threat modelling the application, a DFD (Data Flow Diagram) is used to show the flow of information between the processors and stores. Once the DFD is completed, add the different trust boundaries on the DFD drawing.

Next, we will start the threat modelling process to expose any potential threats in the solution. For this we use the STRIDE Model developed by Microsoft https://learn.microsoft.com/en-us/azure/security/develop/threat-modeling-tool-threats#stride-model. There are other modelling tools available such as PASTA (Process for Attack Simulation and Threat Analysis), LINDDUN (link ability, identifiability, nonrepudiation, detectability, disclosure of information, unawareness, noncompliance), Common Vulnerability Scoring System (CVSS) and Attack Trees.

The STRIDE framework provides a structured approach to identifying and addressing potential threats during the software development lifecycle

STRIDE is an acronym for Spoofing, Tampering, Repudiation, Information disclosure, Denial of Service, and Elevation of privilege.

The method I use involves creating a table that outlines each of the STRIDE categories, documenting potential threats and the corresponding mitigation measures. Use the DFD to analyse the security implications of data flows within a system and identify potential threats for each of the STRIDE categories. The mitigation may include implementing access controls, encryption mechanisms, secure authentication methods, data validation, and monitoring systems for suspicious activities.

Spoofing (Can a malicious user impersonate a legitimate entity or system to deceive users or gain unauthorized access)

Id	Threat	Risk	Mitigation
SF01	Unauthorized user attempts to impersonate a legitimate customer or administrator account	Moderate	Users authenticate using industrial authentication mechanisms and Administers enforced to use MFA. Azure AD monitoring to detect suspicious login activities.
SF02	Forgery of authentication credentials to gain unauthorized access.	Low	Token based authentication is used to protect against forged credentials

Tampering (Can a malicious user modify data or components used by the system?)

Id	Threat	Risk	Mitigation
TP01	Unauthorized modification of CRM data in transit or at rest.	Moderate	TLS transport is used between all the components. SQL Database and Blob store are encrypted at rest by default.
TP02	Manipulation of form input fields to submit malicious data.	High	Data validation and sanitation is performed at every system process. Configure the Web Application Firewall (WAF) rules to mitigate potential attacks.
TP03	SQL injection attacks targeting the SQL Database.	Moderate	Secure coding practices to prevent SQL injection vulnerabilities. Configure the Web Application Firewall (WAF) rules to mitigate potential attacks.
TP04	Unauthorized changes to the CRM website’s code or configuration.	Low	Source code is maintained in Azure Devops and deployed using pipelines which incorporate code analyses and testing.

Repudiation (Can a malicious user deny that they performed an action to change system state?)

Id	Threat	Risk	Mitigation
RP01	Users deny actions performed on the CRM website.	Low	Logging and audit mechanisms implemented to capture user actions and system events
RP02	Attackers manipulate logs or forge identities to repudiate their actions.	Moderate	Access to log files are protected by RBAC roles.

Information Leakage (Can a malicious user extract information that should be kept secret?)

Id	Threat	Risk	Mitigation
IL01	Customer data exposed due to misconfigured access controls.	High	Follow the principle of least privilege and enforce proper access controls for CRM data.
IL02	Insecure handling of sensitive information during transit or storage.	High	Encryption for sensitive data at rest and TLS is used when in transit.
IL03	Improperly configured Blob Store permissions leading to unauthorized access to customer images.	Moderate	Regularly assess and update Blob Store permissions to ensure proper access restrictions.

Denial of Service (Can a malicious user disrupt or degrade system functionality or availability?)

Id	Threat	Risk	Mitigation
DS01	Application Gateway targeted with a high volume of requests, overwhelming its capacity.	High	Configure appropriate rate limiting and throttling rules on the Application Gateway.
DS02	SQL Database subjected to resource-intensive queries, causing service disruption.	Low	Implemented SQL Database performance tuning and query optimization techniques.
DS03	Azure Blob Store overwhelmed with excessive image upload requests, impacting availability.	Moderate	Exponential back-off is implemented to decrease and to ease out spikes in traffic the store.

Elevation of Privilege (Can a malicious user escalate their privileges to gain unauthorized access to restricted resources or perform unauthorized actions?)

Id	Threat	Risk	Mitigation
EP01	Unauthorized users gaining administrative privileges and accessing sensitive functionalities.	Moderate	Sensitive resource connection strings stored in Key vault. Implement network segmentation and access controls to limit lateral movement within Azure resources. SQL Database hosted inside VNet and access controlled by NSG
EP02	Exploiting vulnerabilities to escalate user privileges and gain unauthorized access to restricted data or features.	Low	Regularly apply security patches and updates to the CRM website and underlying Azure components. Conduct regular security assessments and penetration testing to identify and address vulnerabilities.

Alternative Approach

Instead of going through this process yourself, Microsoft offer a threat modeling tool which allows you to draw your Data Flow Diagram that represents your solution. Then the tool allows you to generate an HTML report of all the potential security issues and suggested mitigations. The tool can be downloaded from here https://learn.microsoft.com/en-us/azure/security/develop/threat-modeling-tool

Conclustion

With the rising sophistication of attacks and the targeting of critical infrastructure, cyber threats have become increasingly imminent and perilous. The vulnerabilities present in the Internet of Things (IoT) further contribute to this escalating threat landscape. Additionally, insider threats, risks associated with cloud and remote work, and the interconnected nature of the global network intensify the dangers posed by cyber threats.

To combat these threats effectively, it is crucial for organisations and individuals to priorities cybersecurity measures, including robust defenses, regular updates, employee training, and strong encryption techniques.

By reading this article, it is hoped that the significance of incorporating threat modeling into your application development process is emphasized.

18 August 2020

Extracting Claims from an OAuth Token in an Azure Function

This is a follow up from my previous blog on adding custom application claims to an App registered in AAD (https://connectedcircuits.blog/2020/08/13/setting-up-custom-claims-for-an-api-registered-in-azure-ad/). In this article I will be developing an Azure Function that accepts an OAuth token which has custom claims. The function will validate the token and return all the claims found in the bearer token as the response message.

Instead of using the built-in Azure AD authentication and authorization support in Azure Functions, I will be using the NuGet packages Microsoft.IdentityModel.Protocols and System.IdentityModel.Tokens.Jwt to validate the JWT token. This will allow decoding of bearer tokens from other authorisation providers.

These packages uses the JSON Web Key Set (JWKS) endpoint from the authorisation server to obtain the public key. The key is then used to validate the token to ensure it has not been tampered with. The main advantage of this option is you don’t need to worry about storing the issuer’s public key and remembering to update the certs before they expire.

Function Code

The full code for this solution can found on my github repository https://github.com/connectedcircuits/azOAuthClaimsFunct.

Below is a function which returns the list of signing keys from the jwks_uri endpoint. Ideally the response should be cached, as downloading the keys from the endpoint can take some time.

// Get the public keys from the jwks endpoint      
      private static async Task<ICollection<SecurityKey>> GetSecurityKeysAsync(string idpEndpoint )
      {
          var openIdConfigurationEndpoint = $"{idpEndpoint}.well-known/openid-configuration";
          var configurationManager = new ConfigurationManager<OpenIdConnectConfiguration>(openIdConfigurationEndpoint, new OpenIdConnectConfigurationRetriever());
          var openIdConfig = await configurationManager.GetConfigurationAsync(CancellationToken.None);
          return openIdConfig.SigningKeys;
      }

The next part of the code is to configure the TokenValidationParameters properties with the authorisation server address, the audiences and the signing keys obtained from the GetSecurityKeysAsync function mentioned above.

TokenValidationParameters validationParameters = new TokenValidationParameters
          {
              ValidIssuer = issuer,
              ValidAudiences = new[] { audiences },
              IssuerSigningKeys = keys 
          };

Next is to validate the token and acquire the claims found in the token which is assigned to the Claims Principal object.

//Grab the claims from the token.
           JwtSecurityTokenHandler handler = new JwtSecurityTokenHandler();
           SecurityToken validatedToken;
           ClaimsPrincipal principal;
           try
           {
               principal = handler.ValidateToken(token, validationParameters, out  validatedToken);
           }
           catch(SecurityTokenExpiredException ex)
           {
               log.LogError(ex.Message);
               req.HttpContext.Response.Headers.Add("X-Error-Message", $"Token expired at {ex.Expires}");
               return new UnauthorizedResult();
           }
           catch(Exception ex)
           {
               log.LogError(ex.Message);
               return new UnauthorizedResult();
           }

Once you have the principle object instantiated, you can use the IsInRole(“<role_name>”) method to check if the token contains the role. This method will return a boolean true value if the role is found.

Runtime results

This is the token request for an app registered in Azure AD that has the crm.read and crm.write claims assigned.

This is the response from the Azure Function using the bearer token attained from AAD. Here you can see the two custom application claims crm.read and crm.write listed amongst the default claims.

This is another example of using Auth0 (https://auth0.com/) as an alternative authorisation server. The API was registered with full crud permissions added whilst the client was only given access to read & update roles. Below is the request sent to Auth0 for a token.

This is the response from calling the Azure Function using the Bearer token from Auth0 with the two custom claims crm:read and crm:update returned with the other default claims.

Conclusion

As you can see, you can use any authorisation server that supports a jwks_uri endpoint to acquire the public signing keys and when the generated token uses RS256 as the algorithm for signing and verifying the token signatures.

Enjoy…

13 August 2020

Setting up custom Claims for an API registered in Azure AD

One way to further reduce the surface area attack on an API when using the Client Credential OAuth flow is to pass claims in the token. This adds another layer of security and provides more granular control over your API. By adding custom claims within the token, the API can inspect these and restrict what operations the client may perform within the API service. This may be just to limit basic crud type operations or some other type of grant requirement. More information about optional claims can be found here: https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-optional-claims

Just to recall, Client Credentials flow is normally Client-to-Service requests and has no users involved in the flow. A Service Principal (SP) object is associated with the Client App when making calls to a backend service API. The value of this Id can be found under AAD Enterprise Applications and is the ObjectId of the application name. To ensure the SP used to call your API is the one you are expecting, use the oid or sub claims to validate the SP as another security check.

Also the Client Credentials flow is meant to be used with confidential clients. These are clients that are running on the server as opposed to those running on user devices (which are often referred to as ‘public clients’) as they are able to store the secret securely. Confidential clients provide their client ID and client secret in the requests for access tokens.

Scenario

A typical scenario is where a Client App needs to call a backend API Service. Here the Client App is a CRM Website which is calling a CRM Service which is the backend API Service to either get a users profile or update their details using the Client Credentials flow. Also to limit operational functionality, two roles will be sent with the bearer token to allow only reading or updating the CRM contact information.

In this article I will go through the process of adding a list of Application claims to an App registered as an API service (CRM Service) in Azure Active Directory (AAD). Another App registration is also required for the client (CRM Website) which has a set of restricted claims. These restricted claims will be inserted into the Access token where the API service (CRM Service) would interrogate the claims and perform the required operations if the claims are valid.

Configuration Walkthrough

Below are the steps required to create an AAD App Registration for the CRM Service API and to add the required application claims (roles). The claims are added by modifying the manifest file after the App has been registered as there is no UI available to manage this.

Registering the Service API

1. Register the CRM Service API – Under the Azure Active Directory page, click on App registrations and New registration

2. Add a name (CRM Service) for the application and then click the Register button at the bottom of the page.

3. Adding Claims – Once the App registration has been saved, click on the Manifest menu option to view and update the manifest appRoles via the portal.

Now we can add a of collection of custom roles under the “appRoles” attribute using the following JSON structure for the claims. This is where you define the complete list of custom roles that your API service will support. The claims I will be adding for the CRM service are basic crud type operations. Note the value for the id for each role is just a Guid to keep it unique within the tenant.

Remember to click the Save button to on the top to update the manifest.

4. Set Application ID URL – Under the Expose an API menu option, click the Set for Application ID URI. This value will be used as the Scope parameter when requesting an OAuth token for the Client App. You can also view this value from the app registration Overview page.

That is all required to register the API Service and to add custom application claims. Next is registering the client.

Registering the Client App

1. Create a new App registration as before, but for the client (CRM Website)

2. Add the required permissions – This section is where you define the roles required for the client app. To add the permissions, click on API Permissions and then Add a permission.

Then click on My API’s on the top menu to list all your registered apps. Select the CRM Service app which was added in the first section to which you want access to.

Select Application permissions and then expand the permissions. This will list all the available roles available that was added when registering the CRM Service API. For the CRM Website, I only require the read and update roles. Once checked, then click on the Add permissions button.

Once the permissions have been added, Click on Grant admin consent for the cloud and then press Yes on the dialogue box.

The status of the permissions should then be all green as highlighted below.

5. Add a client secret – Click on the Certificates & secrets menu option to add a new client secret. Remember to take note of this value as it is required when obtaining an OAuth token later.

Requesting an OAuth Token

Before the Client App (CRM Website) can call the API Service (CRM Service), a bearer token needs to be obtained from the OAuth 2.0 token endpoint of AAD. For this, I am going to use Postman to act as the Client App by sending the following parameters. You should be able to get all these values from the Overview of the Client App (CRM Website) and the Scope value from the API Service (CRM Service)

grant_type – Must be set to client_credentials
client_id – The application found in the Client App (CRM Website)
client_secret – The secret that was generated in the Client App (CRM Website) which must be URL encoded.
scope – This the application ID URL of the API Service (CRM Service) which must be URL encoded. Also you need to append /.default to the end of the URL eg (scope=api://189f0961-ba0f-4a5e-93c1-7f71a10b1a13/.default)

Here is an example of the PostMan request to obtain the token. Remember to replace {your-tenant-id} with your one.

When you send it and all the parameters are correct, you should receive a JWT token as per below.

Now if you take the value of the “access_token” and use a tool like https://jwt.ms/ to decode it, you should see the custom application claims.

In conclusion, use custom claims to provide granular control of operations in your backend API’s and any security breaches from hijacked client applications.

Keep a watch out for my next blog where I will show you how to access these claims from within an Azure Function.

Enjoy…

Category: Security

Always Threat Model Your Applications

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Extracting Claims from an OAuth Token in an Azure Function

Function Code

Runtime results

Conclusion

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Setting up custom Claims for an API registered in Azure AD

Scenario

Configuration Walkthrough

Registering the Service API

Registering the Client App

Requesting an OAuth Token

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Like this:

Function Code

Runtime results

Conclusion

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Scenario

Configuration Walkthrough

Registering the Service API

Registering the Client App

Requesting an OAuth Token

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