Merge pull request #32 from juliaducey/support_client_assertion

Add support for authenticating with Entra (AAD) with a certificate instead of client secret

README.md

@@ -98,17 +98,22 @@ config.omniauth(
 
 All of the items listed below are optional, unless noted otherwise. They can be provided either in a static configuration Hash as shown in examples above, or via *read accessor instance methods* in a provider class (more on this later).
 
-| Option | Use |
-| ------ | --- |
-| `client_id`        | **Mandatory.** Client ID for the 'application' (integration) configured on the Azure side. Found via the Azure UI. |
-| `client_secret`    | **Mandatory.** Client secret for the 'application' (integration) configured on the Azure side. Found via the Azure UI. |
-| `base_azure_url`   | Location of Azure login page, for specialised requirements; default is `OmniAuth::Strategies::AzureActivedirectoryV2::BASE_AZURE_URL` (at the time of writing, this is `https://login.microsoftonline.com`). |
-| `tenant_id`        | _Azure_ tenant ID for multi-tenanted use. Default is `common`. Forms part of the Azure OAuth URL - `{base}/{tenant_id}/oauth2/v2.0/...` |
-| `custom_policy`    | _Azure_ custom policy. Default is nil. Forms part of the Azure Token URL - `{base}/{tenant_id}/{custom_policy}/oauth2/v2.0/...` |
-| `authorize_params` | Additional parameters passed as URL query data in the initial OAuth redirection to Microsoft. See below for more. Empty Hash default. |
-| `domain_hint`      | If defined, sets (overwriting, if already present) `domain_hint` inside `authorize_params`. Default `nil` / none. |
-| `scope`            | If defined, sets (overwriting, if already present) `scope` inside `authorize_params`. Default is `OmniAuth::Strategies::AzureActivedirectoryV2::DEFAULT_SCOPE` (at the time of writing, this is `'openid profile email'`). |
-| `adfs`             | If defined, modifies the URLs so they work with an on premise ADFS server. In order to use this you also need to set the `base_azure_url` correctly and fill the `tenant_id` with `'adfs'`. |
+To have your application authenticate with Entra (formerly known as AAD) via client secret, specify client_secret. If you instead want to use certificate-based authentication via client assertion, give the certificate_path and tenant_id instead. You should provide only client_secret or certificate_path, not both.
+
+If you're using the client assertion flow, you need to register your certificate in the Azure portal. For more information, please see [the documentation](https://learn.microsoft.com/en-us/entra/identity-platform/certificate-credentials).
+
+| Option             | Use                                                                                                                                                                                                                         |
+|--------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `client_id`        | **Mandatory.** Client ID for the 'application' (integration) configured on the Azure side. Found via the Azure UI.                                                                                                          |
+| `client_secret`    | **Mandatory for client secret flow.** Client secret for the 'application' (integration) configured on the Azure side. Found via the Azure UI. Don't give this if using client assertion flow.                               |
+| `certificate_path` | **Mandatory for client assertion flow.** Don't give this if using a client secret instead of client assertion. This should be the filepath to a PKCS#12 file.                                                               |
+| `base_azure_url`   | Location of Azure login page, for specialised requirements; default is `OmniAuth::Strategies::AzureActivedirectoryV2::BASE_AZURE_URL` (at the time of writing, this is `https://login.microsoftonline.com`).                |
+| `tenant_id`        | **Mandatory for client assertion flow.** _Azure_ tenant ID for multi-tenanted use. Default is `common`. Forms part of the Azure OAuth URL - `{base}/{tenant_id}/oauth2/v2.0/...`                                            |
+| `custom_policy`    | _Azure_ custom policy. Default is nil. Forms part of the Azure Token URL - `{base}/{tenant_id}/{custom_policy}/oauth2/v2.0/...`                                                                                             |
+| `authorize_params` | Additional parameters passed as URL query data in the initial OAuth redirection to Microsoft. See below for more. Empty Hash default.                                                                                       |
+| `domain_hint`      | If defined, sets (overwriting, if already present) `domain_hint` inside `authorize_params`. Default `nil` / none.                                                                                                           |
+| `scope`            | If defined, sets (overwriting, if already present) `scope` inside `authorize_params`. Default is `OmniAuth::Strategies::AzureActivedirectoryV2::DEFAULT_SCOPE` (at the time of writing, this is `'openid profile email'`).  |
+| `adfs`             | If defined, modifies the URLs so they work with an on premise ADFS server. In order to use this you also need to set the `base_azure_url` correctly and fill the `tenant_id` with `'adfs'`.                                 |
 
 In addition, as a special case, if the request URL contains a query parameter `prompt`, then this will be written into `authorize_params` under that key, overwriting if present any other value there. Note that this comes from the current request URL at the time OAuth flow is commencing, _not_ via static options Hash data or via a custom provider class - but you _could_ just as easily set `scope` inside a custom `authorize_params` returned from a provider class, as shown in an example later; the request URL query mechanism is just another way of doing the same thing.
 

lib/omniauth/strategies/azure_activedirectory_v2.rb

@@ -23,7 +23,20 @@ def client
         end
 
         options.client_id = provider.client_id
-        options.client_secret = provider.client_secret
+
+        if provider.respond_to?(:client_secret) && provider.client_secret
+          options.client_secret = provider.client_secret
+        elsif provider.respond_to?(:certificate_path) && provider.respond_to?(:tenant_id) && provider.certificate_path && provider.tenant_id
+          options.token_params = {
+            tenant: provider.tenant_id,
+            client_id: provider.client_id,
+            client_assertion: client_assertion(provider.tenant_id, provider.client_id, provider.certificate_path),
+            client_assertion_type: client_assertion_type
+          }
+        else
+          raise ArgumentError, "You must provide either client_secret or certificate_path and tenant_id"
+        end
+
         options.tenant_id =
           provider.respond_to?(:tenant_id) ? provider.tenant_id : 'common'
         options.base_azure_url =
@@ -117,6 +130,37 @@ def raw_info
 
         @raw_info
       end
+
+      # The below methods support the flow for using certificate-based client assertion authentication.
+      # See this documentation for details:
+      # https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-auth-code-flow#request-an-access-token-with-a-certificate-credential
+      def client_assertion_type
+        'urn:ietf:params:oauth:client-assertion-type:jwt-bearer'
+      end
+
+      def client_assertion_claims(tenant_id, client_id)
+        {
+          'aud' => "https://login.microsoftonline.com/#{tenant_id}/oauth2/v2.0/token",
+          'exp' => Time.now.to_i + 300,
+          'iss' => client_id,
+          'jti' => SecureRandom.uuid,
+          'nbf' => Time.now.to_i,
+          'sub' => client_id,
+          'iat' => Time.now.to_i
+        }
+      end
+
+      def client_assertion(tenant_id, client_id, certificate_path)
+        certificate_file = OpenSSL::PKCS12.new(File.read(certificate_path))
+        certificate_thumbprint ||= Digest::SHA1.digest(certificate_file.certificate.to_der)
+        private_key = OpenSSL::PKey::RSA.new(certificate_file.key)
+
+        claims = client_assertion_claims(tenant_id, client_id)
+        x5c = Base64.strict_encode64(certificate_file.certificate.to_der)
+        x5t = Base64.strict_encode64(certificate_thumbprint)
+
+        JWT.encode(claims, private_key, 'RS256', { 'x5c': [x5c], 'x5t': x5t })
+      end
     end
   end
 end

spec/omniauth/strategies/azure_activedirectory_v2_spec.rb

@@ -55,6 +55,94 @@
         expect(subject.authorize_params[:prompt]).to eql('select_account')
       end
 
+      context 'using client secret flow without client secret' do
+        subject do
+          OmniAuth::Strategies::AzureActivedirectoryV2.new(app, { client_id: 'id', tenant_id: 'tenant' }.merge(options))
+        end
+
+        it 'raises exception' do
+          expect { subject.client }.to raise_error(ArgumentError, "You must provide either client_secret or certificate_path and tenant_id")
+        end
+      end
+
+      context 'using client assertion flow' do
+        subject do
+          OmniAuth::Strategies::AzureActivedirectoryV2.new(app, options)
+        end
+
+        it 'raises exception when tenant id is not given' do
+          @options = { client_id: 'id', certificate_path: 'path/to/cert.p12' }
+          expect { subject.client }.to raise_error(ArgumentError, "You must provide either client_secret or certificate_path and tenant_id")
+        end
+
+        it 'raises exception when certificate_path is not given' do
+          @options = { client_id: 'id', tenant_id: 'tenant' }
+          expect { subject.client }.to raise_error(ArgumentError, "You must provide either client_secret or certificate_path and tenant_id")
+        end
+
+        context '#token_params with correctly formatted request' do
+          let(:key) { OpenSSL::PKey::RSA.new(2048) }
+          let(:cert) { OpenSSL::X509::Certificate.new.tap { |cert|
+            cert.subject = cert.issuer = OpenSSL::X509::Name.parse("/CN=test")
+            cert.not_before = Time.now
+            cert.not_after = Time.now + 365 * 24 * 60 * 60
+            cert.public_key = key.public_key
+            cert.serial = 0x0
+            cert.version = 2
+            cert.sign(key, OpenSSL::Digest::SHA256.new)
+          } }
+
+          before do
+            @options = {
+              client_id: 'id',
+              tenant_id: 'tenant',
+              certificate_path: 'path/to/cert.p12'
+            }
+
+            allow(File).to receive(:read)
+            allow(OpenSSL::PKCS12).to receive(:new).and_return(OpenSSL::PKCS12.create('pass', 'name', key, cert))
+            allow(SecureRandom).to receive(:uuid).and_return('unique-jti')
+
+            allow(subject).to receive(:request) { request }
+            subject.client
+          end
+
+          it 'has correct tenant id' do
+            expect(subject.options.token_params[:tenant]).to eql('tenant')
+          end
+
+          it 'has correct client id' do
+            expect(subject.options.token_params[:client_id]).to eql('id')
+          end
+
+          it 'has correct client_assertion_type' do
+            expect(subject.options.token_params[:client_assertion_type]).to eql('urn:ietf:params:oauth:client-assertion-type:jwt-bearer')
+          end
+
+          context 'client assertion' do
+            it 'has correct claims' do
+              jwt = subject.options.token_params[:client_assertion]
+              decoded_jwt = JWT.decode(jwt, nil, false).first
+
+              expect(decoded_jwt['aud']).to eql('https://login.microsoftonline.com/tenant/oauth2/v2.0/token')
+              expect(decoded_jwt['exp']).to be_within(5).of(Time.now.to_i + 300)
+              expect(decoded_jwt['iss']).to eql('id')
+              expect(decoded_jwt['jti']).to eql('unique-jti')
+              expect(decoded_jwt['nbf']).to be_within(5).of(Time.now.to_i)
+              expect(decoded_jwt['sub']).to eql('id')
+            end
+
+            it 'contains x5c and x5t headers' do
+              jwt = subject.options.token_params[:client_assertion]
+              headers = JWT.decode(jwt, nil, false).last
+
+              expect(headers['x5c']).to be_an_instance_of(Array)
+              expect(headers['x5t']).to be_a(String)
+            end
+          end
+        end
+      end
+
       describe "overrides" do
         it 'should override domain_hint' do
           @options = {domain_hint: 'hint'}
@@ -72,7 +160,6 @@
         end
       end
     end
-
   end
 
   describe 'static configuration - german' do