Securing APIs with Mutual SSL

In contrast to the usual one-way SSL authentication where a client verifies the identity of the server, in Mutual SSL the server validates the identity of the client so that both parties trust each other. This builds a system that has a very tight security and avoids any requests made to the client to provide the username/password, as long as the server is aware of the certificates that belong to the client. 

This section explains how APIs in WSO2 API Manager can be secured using Mutual SSL in addition to OAuth2.

Enable securing APIs with Mutual SSL

Follow the steps below to enable this feature in WSO2 API Manager.

1. Create the  AM_API_CLIENT_CERTIFICATE table in the APIM DB using the appropriate script given below. Note that the database name will depend on the databases present in your environment. 
   

2. Open the  <API-M_HOME>/repository/conf/axis2/axis2.xml file. Locate the transportReceiver for https as shown below.
   

   <transportReceiver name="https" class="org.apache.synapse.transport.passthru.PassThroughHttpSSLListener">

   Change the class name to  org.apache.synapse.transport.passthru.PassThroughHttpMultiSSLListener. The sample is given below.
   

   <transportReceiver name="https" class="org.apache.synapse.transport.passthru.PassThroughHttpMultiSSLListener">

   
   

3. Add the following parameters under transportReceiver similar to the sample given below.
   

   <parameter name="dynamicSSLProfilesConfig">
   	<filePath>repository/resources/security/listenerprofiles.xml</filePath>
   	<fileReadInterval>600000</fileReadInterval>
   </parameter>
   <parameter name="SSLVerifyClient">optional</parameter>

   The dynamicSSLProfilesConfig specifies the file read to load the dynamic SSL profile and the time interval in which it will be read. 
   Note that if the fileReadInterval parameter is set to 600000, it will take at least 10 minutes for the gateway to accept a newly added client certificate.

    Expand to see the transportReceiver segment after the changes mentioned above have been made...

   <transportReceiver name="https" class="org.apache.synapse.transport.passthru.PassThroughHttpMultiSSLListener">
           <parameter name="port" locked="false">8243</parameter>
           <parameter name="non-blocking" locked="false">true</parameter>
           <!--parameter name="bind-address" locked="false">hostname or IP address</parameter-->
           <!--parameter name="WSDLEPRPrefix" locked="false">https://apachehost:port/somepath</parameter-->
          <parameter name="httpGetProcessor" locked="false">org.wso2.carbon.mediation.transport.handlers.PassThroughNHttpGetProcessor</parameter>
           <parameter name="keystore" locked="false">
               <KeyStore>
                   <Location>repository/resources/security/wso2carbon.jks</Location>
                   <Type>JKS</Type>
                   <Password>wso2carbon</Password>
                   <KeyPassword>wso2carbon</KeyPassword>
               </KeyStore>
           </parameter>
           <parameter name="truststore" locked="false">
               <TrustStore>
                   <Location>repository/resources/security/client-truststore.jks</Location>
                   <Type>JKS</Type>
                   <Password>wso2carbon</Password>
               </TrustStore>
           </parameter>
          <!-- ============================================== -->
          <!-- Configuration for Listener Dynamic SSL Profile loading. -->
          <!-- Configured for 10 mins. -->
          <!-- ============================================== -->
          <parameter name="dynamicSSLProfilesConfig">
              <filePath>repository/resources/security/listenerprofiles.xml</filePath>
              <fileReadInterval>600000</fileReadInterval>
          </parameter>
          <parameter name="SSLVerifyClient">optional</parameter>
           <!--<parameter name="SSLVerifyClient">require</parameter>
               supports optional|require or defaults to none -->
   </transportReceiver>

4. Copy the code given below to create the  listenerprofiles.xml file in the  <API-M_HOME>/repository/resources/security directory.
   

   <?xml version="1.0" encoding="ISO-8859-1"?>
   <parameter name="SSLProfiles">
   <profile>
           <bindAddress>0.0.0.0</bindAddress>
            <KeyStore>
               <Location>repository/resources/security/wso2carbon.jks</Location>
               <Type>JKS</Type>
               <Password>wso2carbon</Password>
               <KeyPassword>wso2carbon</KeyPassword>
           </KeyStore>
           <TrustStore>
               <Location>repository/resources/security/client-truststore.jks</Location>
               <Type>JKS</Type>
               <Password>wso2carbon</Password>
           </TrustStore>
           <SSLVerifyClient>optional</SSLVerifyClient>
       </profile>
   </parameter>

   
   

   The  <API-M_HOME>/repository/resources/security directory can be changed according to the file path you have configured in Step 2.

5. Open the <API-M_HOME>/repository/conf/api-manager.xml file. Set the  EnableMTLSForAPIs parameter to true.
   
   

   <APIManager>
   	...
     <EnableMTLSForAPIs>true</EnableMTLSForAPIs>

6. Restart the server.

Create an API secured with Mutual SSL

1. Create an API.
2. Edit the API and navigate to the Manage tab.

3. Select Mutual SSL under API Security.
   

   You can select both OAuth2 and Mutual SSL options. This means that the user can access the API using a valid OAuth2 token or using a valid client certificate. When OAuth2 and Mutual SSL are both enabled, Mutual SSL authentication will have a higher priority than OAuth2 as explained as follows:

   API Security		User's Response		Outcome
   OAuth2	Mutual SSL	OAuth2 token	Mutual SSL Certificate
   Enabled	Enabled	Sent	Sent	Initially, the Mutual SSL certificate is validated. If the validation is successful, the OAuth2 token is ignored. If the Mutual SSL certificate fails, the OAuth2 token is validated, but if the OAuth2 token fails as well, the API call will fail.
   Enabled	Enabled	Not sent	Sent	The Mutual SSL certificate is validated. If the validation fails, the API call will fail.
   Enabled	Enabled	Sent	Not sent	The OAuth2 token is validated. If the validation fails, the API call will fail.

4. Click Manage Certificates to upload a new client certificate. Select Add New Certificate.
   
   
   

   This feature currently supports only the following formats for keystores and certificates.

   • Keystore : .jks
   • Certificate : .crt

   If you need to use a certificate in any other format, you can convert it using a standard tool before uploading.

   After configuring, the certificate will be added to the Gateway nodes which are defined under the Environments in api-manager.xml. In a clustered setup, as gateway configurations are identical, sync the <API-M_HOME>/repository/resources/security/listenerprofiles.xml and <API-M_HOME>/repository/resources/security/client-truststore.jks among the gateway nodes. After the configured interval, the synapse transport will be reloaded in all the gateway nodes.

5. Provide an alias and public certificate. Select the tier that should be used to throttle out the calls using this particular client certificate and click Upload.
   

   If you are marking the API as the Default Version

   This is available only as a  WUM update and is effective from 15th July 2020 (2020-07-15). For more information on updating WSO2 API Manager, see Updating WSO2 API Manager.

   To make Mutual SSL work with Default APIs follow the below steps before following step 6.

   1. Navigate to the <API-M_HOME>/repository/resources/api_templates/default_api_template.xml file in the Publisher node.

   2. Add the following handler to the top of the set of handlers defined inside <handlers> config.

      <handler class="org.wso2.carbon.apimgt.gateway.handlers.common.MutualSSLCertificateHandler"/>

      
      A sample default_api_template.xml file with the above would look like below.

      <api xmlns="http://ws.apache.org/ns/synapse" name="$!apiName" context="$!apiContext" transports="$!transport">
        ...
        <handlers>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.common.MutualSSLCertificateHandler"/>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.common.SynapsePropertiesHandler"/>
            <handler class="org.wso2.carbon.apimgt.gateway.handlers.security.CORSRequestHandler">
               <property name="apiImplementationType" value="ENDPOINT"/>
            </handler>
        </handlers>

6. Save and Publish the API

Invoke an API secured with Mutual SSL from the API Store

1. Invoke an API from the API Store.
2. When you click Execute the browser will send a prompt similar to the one shown below. Select the corresponding certificate for the API.
   

Differences compared to APIs secured over OAuth 2.0

Listed below are the known differences in features when APIs are secured using Mutual SSL compared to OAuth 2.0.

• Comparative to subscription level rate limiting policies, with Mutual SSL what you get is rate limiting policies by certificate. API developers can define the rate limit to be applied by each accepted certificate.

• In the case of Mutual SSL since there are no access tokens involved, the concept of resource security "type" for API resources doesn't apply. Security "type" in API resources are based on whether the access token belongs to an application or an end-user. In the case of Mutual SSL the certificate is issued to the client application.

• Fine grained authorization has to be done using message mediation policies. Since mutual TLS does not have a concept similar to that of OAuth 2.0 scopes, features such as role-based-access-control for example would need to be achieve using alternatives similar to message mediation policies.

Handling MTLS when SSL is terminated by the load balancer/reverse proxy

When SSL Termination of API requests happens at the load balancer/reverse proxy, the following prerequisites need to be met from the load balancer.

• Terminate the mutual SSL connection from client
• Pass the client SSL certificate to the Gateway in an HTTP Header. ( eg: https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_client_certificate )

The below diagram shows how MutualSSL works in such an environment.

By default, the WSO2 API Manager retrieves the client certificate from the X-WSO2-CLIENT-CERTIFICATE HTTP header. In order to change the header,

1. Navigate to the <API-M_HOME>/repository/conf/api-manager.xml file in Gateway node.

2. Put the following under <APIManager> section.

   <APIManager>
   	...
      <MutualSSL> 
          <ClientCertificateHeader>X-WSO2-CLIENT-CERTIFICATE</ClientCertificateHeader> 
          <!-- This property need to be true if MutualSSL connection established between load balancer and gateway.--> 
         <EnableClientCertificateValidation>false</EnableClientCertificateValidation> 
      </MutualSSL>

3. Restart the API Manager.