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Android Network APIs

Testing Endpoint Identify Verification

Using TLS to transport sensitive information over the network is essential for security. However, encrypting communication between a mobile application and its backend API is not trivial. Developers often decide on simpler but less secure solutions (e.g., those that accept any certificate) to facilitate the development process, and sometimes these weak solutions make it into the production version, potentially exposing users to man-in-the-middle attacks.

Two key issues should be addressed:

  • Verify that a certificate comes from a trusted source (CA).
  • Determine whether the endpoint server presents the right certificate.

Make sure that the hostname and the certificate itself are verified correctly. Examples and common pitfalls are available in the official Android documentation. Search the code for examples of TrustManager and HostnameVerifier usage. In the sections below, you can find examples of the kind of insecure usage that you should look for.

Static Analysis

Verifying the Server Certificate

"TrustManager" is a means of verifying conditions necessary for establishing a trusted connection in Android. The following conditions should be checked at this point:

  • Has the certificate been signed by a "trusted" CA?
  • Has the certificate expired?
  • Is the certificate self-signed?

The following code snippet is sometimes used during development and will accept any certificate, overwriting the functions checkClientTrusted, checkServerTrusted, and getAcceptedIssuers. Such implementations should be avoided, and, if they are necessary, they should be clearly separated from production builds to avoid built-in security flaws.

TrustManager[] trustAllCerts = new TrustManager[] {
    new X509TrustManager() {
        @Override
        public X509Certificate[] getAcceptedIssuers() {
            return new java.security.cert.X509Certificate[] {};
        }

        @Override
        public void checkClientTrusted(X509Certificate[] chain, String authType)
            throws CertificateException {
        }

        @Override
        public void checkServerTrusted(X509Certificate[] chain, String authType)
            throws CertificateException {
        }
    }
 };

// SSLContext context
context.init(null, trustAllCerts, new SecureRandom());
WebView Server Certificate Verification

Sometimes applications use a WebView to render the website associated with the application. This is true of HTML/JavaScript-based frameworks such as Apache Cordova, which uses an internal WebView for application interaction. When a WebView is used, the mobile browser performs the server certificate validation. Ignoring any TLS error that occurs when the WebView tries to connect to the remote website is a bad practice.

The following code will ignore TLS issues, exactly like the WebViewClient custom implementation provided to the WebView:

WebView myWebView = (WebView) findViewById(R.id.webview);
myWebView.setWebViewClient(new WebViewClient(){
    @Override
    public void onReceivedSslError(WebView view, SslErrorHandler handler, SslError error) {
        //Ignore TLS certificate errors and instruct the WebViewClient to load the website
        handler.proceed();
    }
});
Apache Cordova Certificate Verification

Implementation of the Apache Cordova framework's internal WebView usage will ignore TLS errors in the method onReceivedSslError if the flag android:debuggable is enabled in the application manifest. Therefore, make sure that the app is not debuggable. See the test case "Testing If the App is Debuggable."

Hostname Verification

Another security flaw in client-side TLS implementations is the lack of hostname verification. Development environments usually use internal addresses instead of valid domain names, so developers often disable hostname verification (or force an application to allow any hostname) and simply forget to change it when their application goes to production. The following code disables hostname verification:

final static HostnameVerifier NO_VERIFY = new HostnameVerifier() {
    public boolean verify(String hostname, SSLSession session) {
        return true;
    }
};

With a built-in HostnameVerifier, accepting any hostname is possible:

HostnameVerifier NO_VERIFY = org.apache.http.conn.ssl.SSLSocketFactory
                             .ALLOW_ALL_HOSTNAME_VERIFIER;

Make sure that your application verifies a hostname before setting a trusted connection.

Dynamic Analysis

Dynamic analysis requires an interception proxy. To test improper certificate verification, check the following controls:

  • Self-signed certificate

In Burp, go to the Proxy -> Options tab, then go to the Proxy Listeners section, highlight your listener, and click Edit. Then go to the Certificate tab, check Use a self-signed certificate, and click Ok. Now, run your application. If you're able to see HTTPS traffic, your application is accepting self-signed certificates.

  • Accepting invalid certificates

In Burp, go to the Proxy -> Options tab, then go to the Proxy Listeners section, highlight your listener, and click Edit. Then go to the Certificate tab, check Generate a CA-signed certificate with a specific hostname, and type in the backend server's hostname. Now, run your application. If you're able to see HTTPS traffic, your application is accepting all certificates.

  • Accepting incorrect hostnames

In Burp, go to the Proxy -> Options tab, then go to the Proxy Listeners section, highlight your listener, and click Edit. Then go to the Certificate tab, check Generate a CA-signed certificate with a specific hostname, and type in an invalid hostname, e.g., example.org. Now, run your application. If you're able to see HTTPS traffic, your application is accepting all hostnames.

If you're interested in further MITM analysis or you have problems with the configuration of your interception proxy, consider using Tapioca. It's a CERT pre-configured VM appliance for MITM software analysis. All you have to do is deploy a tested application on an emulator and start capturing traffic.

Testing Custom Certificate Stores and Certificate Pinning

Overview

Certificate pinning is the process of associating the backend server with a particular X509 certificate or public key instead of accepting any certificate signed by a trusted certificate authority. After storing ("pinning") the server certificate or public key, the mobile app will subsequently connect to the known server only. Withdrawing trust from external certificate authorities reduces the attack surface (after all, there are many cases of certificate authorities that have been compromised or tricked into issuing certificates to impostors).

The certificate can be pinned and hardcoded into the app or retrieved at the time the app first connects to the backend. In the latter case, the certificate is associated with ("pinned" to) the host when the host is seen for the first time. This alternative is less secure because attackers intercepting the initial connection can inject their own certificates.

Static Analysis

Network Security Configuration

To customize their network security settings in a safe, declarative configuration file without modifying app code, applications can use the Network Security Configuration (NSC) that Android provides for versions 7.0 and above.

The Network Security Configuration feature can also be used to pin declarative certificates to specific domains. If an application uses the NSC feature, two things should be checked to identify the defined configuration:

  1. Specification of the NSC file reference in the Android application manifest via the "android:networkSecurityConfig" attribute on the application tag:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="owasp.com.app">
    <application android:networkSecurityConfig="@xml/network_security_config">
        ...
    </application>
</manifest>
  1. Contents of the NSC file stored in "res/xml/network_security_config.xml":
<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config>
        <!-- Use certificate pinning for OWASP website access including sub domains -->
        <domain includeSubdomains="true">owasp.org</domain>
        <pin-set expiration="2018/8/10">
            <!-- Hash of the public key (SubjectPublicKeyInfo of the X.509 certificate) of
            the Intermediate CA of the OWASP website server certificate -->
            <pin digest="SHA-256">YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=</pin>
            <!-- Hash of the public key (SubjectPublicKeyInfo of the X.509 certificate) of
            the Root CA of the OWASP website server certificate -->
            <pin digest="SHA-256">Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=</pin>
        </pin-set>
    </domain-config>
</network-security-config>

If an NSC configuration exists, the following event may be visible in the log:

D/NetworkSecurityConfig: Using Network Security Config from resource network_security_config

If a certificate pinning validation check has failed, the following event will be logged:

I/X509Util: Failed to validate the certificate chain, error: Pin verification failed

Static Analysis

  • Use a decompiler (Ex. Jadx) or apktool to confirm if the <pin> entry is present in the network_security_config.xml file located in the /res/xml/ folder.
TrustManager

Implementing certificate pinning involves three main steps:

  • Obtain the certificate of the desired host(s).
  • Make sure the certificate is in .bks format.
  • Pin the certificate to an instance of the default Apache Httpclient.

To analyze the correct implementation of certificate pinning, the HTTP client should load the Keystore:

InputStream in = resources.openRawResource(certificateRawResource);
keyStore = KeyStore.getInstance("BKS");
keyStore.load(resourceStream, password);

Once the Keystore has been loaded, we can use the TrustManager that trusts the CAs in our KeyStore:

String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
tmf.init(keyStore);
Create an SSLContext that uses the TrustManager
// SSLContext context = SSLContext.getInstance("TLS");
sslContext.init(null, tmf.getTrustManagers(), null);

The app's implementation may be different, pinning against the certificate's public key only, the whole certificate, or a whole certificate chain.

Network Libraries and WebViews

Applications that use third-party networking libraries may utilize the libraries' certificate pinning functionality. For example, okhttp can be set up with the CertificatePinner as follows:

OkHttpClient client = new OkHttpClient.Builder()
        .certificatePinner(new CertificatePinner.Builder()
            .add("example.com", "sha256/UwQAapahrjCOjYI3oLUx5AQxPBR02Jz6/E2pt0IeLXA=")
            .build())
        .build();

Applications that use a WebView component may utilize the WebViewClient's event handler for some kind of "certificate pinning" of each request before the target resource is loaded. The following code shows an example verification of the Issuer DN of the certificate sent by the server:

WebView myWebView = (WebView) findViewById(R.id.webview);
myWebView.setWebViewClient(new WebViewClient(){
    private String expectedIssuerDN = "CN=Let's Encrypt Authority X3,O=Let's Encrypt,C=US;";

    @Override
    public void onLoadResource(WebView view, String url)  {
        //From Android API documentation about "WebView.getCertificate()":
        //Gets the SSL certificate for the main top-level page
        //or null if there is no certificate (the site is not secure).
        //
        //Available information on SslCertificate class are "Issuer DN", "Subject DN" and validity date helpers
        SslCertificate serverCert = view.getCertificate();
        if(serverCert != null){
            //Apply check on Issuer DN against expected one
            SslCertificate.DName issuerDN = serverCert.getIssuedBy();
            if(!this.expectedIssuerDN.equals(issuerDN.toString())){
                //Throw exception to cancel resource loading...
            }
        }
    }
});
Xamarin Applications

Applications developed in Xamarin will typically use ServicePointManager to implement pinning.

Normally a function is created to check the certificate(s) and return the boolean value to the method ServerCertificateValidationCallback:

[Activity(Label = "XamarinPinning", MainLauncher = true)]
    public class MainActivity : Activity
    {
        // SupportedPublicKey - Hexadecimal value of the public key.
        // Use GetPublicKeyString() method to determine the public key of the certificate we want to pin. Uncomment the debug code in the ValidateServerCertificate function a first time to determine the value to pin. 
        private const string SupportedPublicKey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

        private static bool ValidateServerCertificate(
                object sender,
                X509Certificate certificate,
                X509Chain chain,
                SslPolicyErrors sslPolicyErrors
            )
        {
            //Log.Debug("Xamarin Pinning",chain.ChainElements[X].Certificate.GetPublicKeyString());
            //return true;
            return SupportedPublicKey == chain.ChainElements[1].Certificate.GetPublicKeyString();
        }

        protected override void OnCreate(Bundle savedInstanceState)
        {
            System.Net.ServicePointManager.ServerCertificateValidationCallback += ValidateServerCertificate;
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.Main);
            TesteAsync("https://security.claudio.pt");
  
        }

In this particular example we are pinning the intermediate CA of the certificate chain. The output of the HTTP response will be available in the system logs.

Sample Xamarin app with the previous example can be obtained at https://github.com/owasp-mstg/blob/master/Samples/Android/02_CertificatePinning/certificatePinningXamarin.apk?raw=true

Static Analysis

After decompressing the APK file, use a .NET decompiler like dotPeak,ILSpy or dnSpy to decompile the app dlls stored inside the 'Assemblies' folder and confirm the usage of the ServicePointManager.

For further information, please check the OWASP certificate pinning guide.

Dynamic Analysis

Dynamic analysis can be performed by launching a MITM attack with your preferred interception proxy. This will allow you to monitor the traffic between the client (the mobile application) and the backend server. If the proxy is unable to intercept the HTTP requests and responses, the SSL pinning has been implemented correctly.

Testing the Network Security Configuration settings

Overview

Network Security Configuration was introducted on Android 7 and lets apps customize their network security settings such as custom trust anchors and Certificate pinning.

Trust Anchors

When apps target API Levels 24+ and are running on an Android device with versions 7+, they use a default Network Security Configuration that doest not trust user supplied CA's, reducing the possibility of MiTM attacks by luring users to install malicious CA's.

This protection can be bypassed by using a custom Network Security Configuration with a custom trust anchor indicating that the app will trust user supplied CA's.

Pin-set Expiration Date

Pin-set contain a set of public key pins. Each set can define a expiration date. When the expiration date is reached, the network communication will continue to work, but the Certificate Pinning will be disabled for the affected domains.

Static Analysis

The Network Security Configuration should be analysed to determine what settings are configured. The file is located inside the apk in the /res/xml/ folder with the name network_security_config.xml.

If there are custom present in a or , that define a the application will trust user supplied CA's for those particular domains or for all domains. Example:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <base-config>
        <trust-anchors>
            <certificates src="system"/>
            <certificates src="user"/>
        </trust-anchors>
    </base-config>
    <domain-config>
        <domain includeSubdomains="false">owasp.org</domain>
        <trust-anchors>
            <certificates src="system"/>
            <certificates src="user"/>
        </trust-anchors>
        <pin-set expiration="2018/8/10">
            <!-- Hash of the public key (SubjectPublicKeyInfo of the X.509 certificate) of
            the Intermediate CA of the OWASP website server certificate -->
            <pin digest="SHA-256">YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=</pin>
            <!-- Hash of the public key (SubjectPublicKeyInfo of the X.509 certificate) of
            the Root CA of the OWASP website server certificate -->
            <pin digest="SHA-256">Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=</pin>
        </pin-set>
    </domain-config>
</network-security-config>

Is important to understand the precedence of entries. If a value is not set in a <domain-config> entry or in a parent <domain-config>, the configurations in place will be based on the <base-config>, and lastly if not defined in this entry, the default configuration will be used.

The default configuration for apps targeting Android 9 (API level 28) and higher is as follows:

<base-config cleartextTrafficPermitted="false">
    <trust-anchors>
        <certificates src="system" />
    </trust-anchors>
</base-config>

The default configuration for apps targeting Android 7.0 (API level 24) to Android 8.1 (API level 27) is as follows:

<base-config cleartextTrafficPermitted="true">
    <trust-anchors>
        <certificates src="system" />
    </trust-anchors>
</base-config>

The default configuration for apps targeting Android 6.0 (API level 23) and lower is as follows:

<base-config cleartextTrafficPermitted="true">
    <trust-anchors>
        <certificates src="system" />
        <certificates src="user" />
    </trust-anchors>
</base-config>

Dynamic Analysis

In a scenario where we have the proxy root CA (Ex. Burp Suite) installed on the device, this particular app sets the targetSDK to Api level 24+ and is running on a Android device with version 7+, we should not be able to intercept the communication. If we are able to, this means that there is a bypass of this mechanism.

Testing Default Network Security Configuration

Overview

As mentioned in the previous topic, apps that target API levels 24+, unless otherwise defined, will implement a default Network Security Configuration that no longer trust user supplied CA's.

In a scenario that the app is running on a Android device with version 7+, but targets API levels below 24, it will not use this feature, therefore still trusting in user supplied CA's.

Static Analysis

  • Use a decompiler (Ex. Jadx) to confirm the targetSDK present in the AndroidManifest.xml file
  • Use apktool to decode the app and confirm the targetSDK present in the file apktool.yml of the output folder.

Dynamic Analysis

In a scenario where we have the proxy root CA (Ex. Burp Suite) installed on the device, the app is running on a Android device with version 7+ and there is no custom Network Security Configuration implemented, this is an indicator that the targetSDK is set to API levels below 24, assuming that the app correctly validates certificates.

Testing the Security Provider

Overview

Android relies on a security provider to provide SSL/TLS-based connections. The problem with this kind of security provider (one example is OpenSSL), which comes with the device, is that it often has bugs and/or vulnerabilities. To avoid known vulnerabilities, developers need to make sure that the application will install a proper security provider. Since July 11, 2016, Google has been rejecting Play Store application submissions (both new applications and updates) that use vulnerable versions of OpenSSL.

Static Analysis

Applications based on the Android SDK should depend on GooglePlayServices. For example, in the gradle build file, you will find compile 'com.google.android.gms:play-services-gcm:x.x.x' in the dependencies block. You need to make sure that the ProviderInstaller class is called with either installIfNeeded or installIfNeededAsync. ProviderInstaller needs to be called by a component of the application as early as possible. Exceptions thrown by these methods should be caught and handled correctly. If the application cannot patch its security provider, it can either inform the API of its less secure state or restrict user actions (because all HTTPS traffic should be deemed riskier in this situation).

Here are two examples from the Android Developer documentation that show how to update Security Provider to prevent SSL exploits. In both cases, the developer needs to handle the exceptions properly, and reporting to the backend when the application is working with an unpatched security provider may be wise.

Patching Synchronously:

//this is a sync adapter that runs in the background, so you can run the synchronous patching.
public class SyncAdapter extends AbstractThreadedSyncAdapter {

  ...

  // This is called each time a sync is attempted; this is okay, since the
  // overhead is negligible if the security provider is up-to-date.
  @Override
  public void onPerformSync(Account account, Bundle extras, String authority,
      ContentProviderClient provider, SyncResult syncResult) {
    try {
      ProviderInstaller.installIfNeeded(getContext());
    } catch (GooglePlayServicesRepairableException e) {

      // Indicates that Google Play services is out of date, disabled, etc.

      // Prompt the user to install/update/enable Google Play services.
      GooglePlayServicesUtil.showErrorNotification(
          e.getConnectionStatusCode(), getContext());

      // Notify the SyncManager that a soft error occurred.
      syncResult.stats.numIOExceptions++;
      return;

    } catch (GooglePlayServicesNotAvailableException e) {
      // Indicates a non-recoverable error; the ProviderInstaller is not able
      // to install an up-to-date Provider.

      // Notify the SyncManager that a hard error occurred.
      //in this case: make sure that you inform your API of it.
      syncResult.stats.numAuthExceptions++;
      return;
    }

    // If this is reached, you know that the provider was already up-to-date,
    // or was successfully updated.
  }
}

Patching Asynchronously:

//This is the mainactivity/first activity of the application that's there long enough to make the async installing of the securityprovider work.
public class MainActivity extends Activity
    implements ProviderInstaller.ProviderInstallListener {

  private static final int ERROR_DIALOG_REQUEST_CODE = 1;

  private boolean mRetryProviderInstall;

  //Update the security provider when the activity is created.
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ProviderInstaller.installIfNeededAsync(this, this);
  }

  /**
   * This method is only called if the provider is successfully updated
   * (or is already up-to-date).
   */
  @Override
  protected void onProviderInstalled() {
    // Provider is up-to-date, app can make secure network calls.
  }

  /**
   * This method is called if updating fails; the error code indicates
   * whether the error is recoverable.
   */
  @Override
  protected void onProviderInstallFailed(int errorCode, Intent recoveryIntent) {
    if (GooglePlayServicesUtil.isUserRecoverableError(errorCode)) {
      // Recoverable error. Show a dialog prompting the user to
      // install/update/enable Google Play services.
      GooglePlayServicesUtil.showErrorDialogFragment(
          errorCode,
          this,
          ERROR_DIALOG_REQUEST_CODE,
          new DialogInterface.OnCancelListener() {
            @Override
            public void onCancel(DialogInterface dialog) {
              // The user chose not to take the recovery action
              onProviderInstallerNotAvailable();
            }
          });
    } else {
      // Google Play services is not available.
      onProviderInstallerNotAvailable();
    }
  }

  @Override
  protected void onActivityResult(int requestCode, int resultCode,
      Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == ERROR_DIALOG_REQUEST_CODE) {
      // Adding a fragment via GooglePlayServicesUtil.showErrorDialogFragment
      // before the instance state is restored throws an error. So instead,
      // set a flag here, which will cause the fragment to delay until
      // onPostResume.
      mRetryProviderInstall = true;
    }
  }

  /**
   * On resume, check to see if we flagged that we need to reinstall the
   * provider.
   */
  @Override
  protected void onPostResume() {
    super.onPostResult();
    if (mRetryProviderInstall) {
      // We can now safely retry installation.
      ProviderInstall.installIfNeededAsync(this, this);
    }
    mRetryProviderInstall = false;
  }

  private void onProviderInstallerNotAvailable() {
    // This is reached if the provider cannot be updated for some reason.
    // App should consider all HTTP communication to be vulnerable, and take
    // appropriate action (e.g. inform backend, block certain high-risk actions, etc.).
  }
}

Make sure that NDK-based applications bind only to a recent and properly patched library that provides SSL/TLS functionality.

Dynamic Analysis

When you have the source code:

  • Run the application in debug mode, then create a breakpoint where the app will first contact the endpoint(s).
  • Right click the highlighted code and select Evaluate Expression.
  • Type Security.getProviders() and press enter.
  • Check the providers and try to find GmsCore_OpenSSL, which should be the new top-listed provider.

When you do not have the source code:

  • Use Xposed to hook into the java.security package, then hook into java.security.Security with the method getProviders (with no arguments). The return value will be an array of Provider.
  • Determine whether the first provider is GmsCore_OpenSSL.

References

OWASP Mobile Top 10 2016

OWASP MASVS
  • V5.3: "The app verifies the X.509 certificate of the remote endpoint when the secure channel is established. Only certificates signed by a trusted CA are accepted."
  • V5.4: "The app either uses its own certificate store or pins the endpoint certificate or public key, and subsequently does not establish connections with endpoints that offer a different certificate or key, even if signed by a trusted CA."
  • V5.6: "The app only depends on up-to-date connectivity and security libraries."
CWE
Android Developer Documentation
Xamarin Certificate Pinning