Webview avoid security alert from google play upon implementation of onReceivedSslError

Question

I have a link which will open in WebView. The problem is it cannot be open until I override onReceivedSslError like this:

@Override
public void onReceivedSslError(WebView view, SslErrorHandler handler, SslError error) {
    handler.proceed();
}

I am getting security alert from Google Play saying:

Security alert Your application has an unsafe implementation of the WebViewClient.onReceivedSslError handler. Specifically, the implementation ignores all SSL certificate validation errors, making your app vulnerable to man-in-the-middle attacks. An attacker could change the affected WebView's content, read transmitted data (such as login credentials), and execute code inside the app using JavaScript.

To properly handle SSL certificate validation, change your code to invoke SslErrorHandler.proceed() whenever the certificate presented by the server meets your expectations, and invoke SslErrorHandler.cancel() otherwise. An email alert containing the affected app(s) and class(es) has been sent to your developer account address.

Please address this vulnerability as soon as possible and increment the version number of the upgraded APK. For more information about the SSL error handler, please see our documentation in the Developer Help Center. For other technical questions, you can post to https://www.stackoverflow.com/questions and use the tags “android-security” and “SslErrorHandler.” If you are using a 3rd party library that’s responsible for this, please notify the 3rd party and work with them to address the issue.

To confirm that you've upgraded correctly, upload the updated version to the Developer Console and check back after five hours. If the app hasn't been correctly upgraded, we will display a warning.

Please note, while these specific issues may not affect every app that uses WebView SSL, it's best to stay up to date on all security patches. Apps with vulnerabilities that expose users to risk of compromise may be considered dangerous products in violation of the Content Policy and section 4.4 of the Developer Distribution Agreement.

Please ensure all apps published are compliant with the Developer Distribution Agreement and Content Policy. If you have questions or concerns, please contact our support team through the Google Play Developer Help Center.

If I remove onReceivedSslError (handler.proceed()), then page won't open.

Is there any way I can open the page in WebView and avoid security alert?

Answer 1

To properly handle SSL certificate validation, change your code to invoke SslErrorHandler.proceed() whenever the certificate presented by the server meets your expectations, and invoke SslErrorHandler.cancel() otherwise.

As email said, onReceivedSslError should handle user is going to a page with invalid cert, such like a notify dialog. You should not proceed it directly.

For example, I add an alert dialog to make user have confirmed and seems Google no longer shows warning.

---

@Override
public void onReceivedSslError(WebView view, final SslErrorHandler handler, SslError error) {
    final AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setMessage(R.string.notification_error_ssl_cert_invalid);
    builder.setPositiveButton("continue", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            handler.proceed();
        }
    });
    builder.setNegativeButton("cancel", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            handler.cancel();
        }
    });
    final AlertDialog dialog = builder.create();
    dialog.show();
}

---

More explain about the email.

Specifically, the implementation ignores all SSL certificate validation errors, making your app vulnerable to man-in-the-middle attacks.

The email says the default implement ignored an important SSL security problem. So we need to handle it in our own app which used WebView. Notify user with a alert dialog is a simple way.

Answer 2

The proposed solutions so far just bypass the security check, so they are not safe.

What I suggest is to embed the certificate(s) in the App, and when a SslError occurs, check that the server certificate matches one of the embedded certificates.

So here are the steps:

1. Retrieve the certificate from the website.

   • Open the site on Safari
   • Click on the padlock icon near the website name
   • Click on Show Certificate
   • Drag and drop the certificate in a folder

see https://www.markbrilman.nl/2012/03/howto-save-a-certificate-via-safari-on-mac/

2. Copy the certificate (.cer file) into the res/raw folder of your app

3. In your code, load the certificate(s) by calling loadSSLCertificates()

   private static final int[] CERTIFICATES = {
           R.raw.my_certificate,   // you can put several certificates
   };
   private ArrayList<SslCertificate> certificates = new ArrayList<>();
   
   private void loadSSLCertificates() {
       try {
           CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
           for (int rawId : CERTIFICATES) {
               InputStream inputStream = getResources().openRawResource(rawId);
               InputStream certificateInput = new BufferedInputStream(inputStream);
               try {
                   Certificate certificate = certificateFactory.generateCertificate(certificateInput);
                   if (certificate instanceof X509Certificate) {
                       X509Certificate x509Certificate = (X509Certificate) certificate;
                       SslCertificate sslCertificate = new SslCertificate(x509Certificate);
                       certificates.add(sslCertificate);
                   } else {
                       Log.w(TAG, "Wrong Certificate format: " + rawId);
                   }
               } catch (CertificateException exception) {
                   Log.w(TAG, "Cannot read certificate: " + rawId);
               } finally {
                   try {
                       certificateInput.close();
                       inputStream.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   }
               }
           }
       } catch (CertificateException e) {
           e.printStackTrace();
       }
   }
   

4. When a SslError occurs, check that the server certificate matches one embedded certificate. Note that it is not possible to directly compare certificates, so I use SslCertificate.saveState to put the certificate data into a Bundle, and then I compare all the bundle entries.

   webView.setWebViewClient(new WebViewClient() {
   
       @Override
       public void onReceivedSslError(WebView view, final SslErrorHandler handler, SslError error) {
   
           // Checks Embedded certificates
           SslCertificate serverCertificate = error.getCertificate();
           Bundle serverBundle = SslCertificate.saveState(serverCertificate);
           for (SslCertificate appCertificate : certificates) {
               if (TextUtils.equals(serverCertificate.toString(), appCertificate.toString())) { // First fast check
                   Bundle appBundle = SslCertificate.saveState(appCertificate);
                   Set<String> keySet = appBundle.keySet();
                   boolean matches = true;
                   for (String key : keySet) {
                       Object serverObj = serverBundle.get(key);
                       Object appObj = appBundle.get(key);
                       if (serverObj instanceof byte[] && appObj instanceof byte[]) {     // key "x509-certificate"
                           if (!Arrays.equals((byte[]) serverObj, (byte[]) appObj)) {
                               matches = false;
                               break;
                           }
                       } else if ((serverObj != null) && !serverObj.equals(appObj)) {
                           matches = false;
                           break;
                       }
                   }
                   if (matches) {
                       handler.proceed();
                       return;
                   }
               }
           }
   
           handler.cancel();
           String message = "SSL Error " + error.getPrimaryError();
           Log.w(TAG, message);
       }
   
   
   });
   

Answer 3

I needed to check our truststore before show any message to the user so I did this:

public class MyWebViewClient extends WebViewClient {
private static final String TAG = MyWebViewClient.class.getCanonicalName();

Resources resources;
Context context;

public MyWebViewClient(Resources resources, Context context){
    this.resources = resources;
    this.context = context;
}

@Override
public void onReceivedSslError(WebView v, final SslErrorHandler handler, SslError er){
    // first check certificate with our truststore
    // if not trusted, show dialog to user
    // if trusted, proceed
    try {
        TrustManagerFactory tmf = TrustManagerUtil.getTrustManagerFactory(resources);

        for(TrustManager t: tmf.getTrustManagers()){
            if (t instanceof X509TrustManager) {

                X509TrustManager trustManager = (X509TrustManager) t;

                Bundle bundle = SslCertificate.saveState(er.getCertificate());
                X509Certificate x509Certificate;
                byte[] bytes = bundle.getByteArray("x509-certificate");
                if (bytes == null) {
                    x509Certificate = null;
                } else {
                    CertificateFactory certFactory = CertificateFactory.getInstance("X.509");
                    Certificate cert = certFactory.generateCertificate(new ByteArrayInputStream(bytes));
                    x509Certificate = (X509Certificate) cert;
                }
                X509Certificate[] x509Certificates = new X509Certificate[1];
                x509Certificates[0] = x509Certificate;

                trustManager.checkServerTrusted(x509Certificates, "ECDH_RSA");
            }
        }
        Log.d(TAG, "Certificate from " + er.getUrl() + " is trusted.");
        handler.proceed();
    }catch(Exception e){
        Log.d(TAG, "Failed to access " + er.getUrl() + ". Error: " + er.getPrimaryError());
        final AlertDialog.Builder builder = new AlertDialog.Builder(context);
        String message = "SSL Certificate error.";
        switch (er.getPrimaryError()) {
            case SslError.SSL_UNTRUSTED:
                message = "O certificado não é confiável.";
                break;
            case SslError.SSL_EXPIRED:
                message = "O certificado expirou.";
                break;
            case SslError.SSL_IDMISMATCH:
                message = "Hostname inválido para o certificado.";
                break;
            case SslError.SSL_NOTYETVALID:
                message = "O certificado é inválido.";
                break;
        }
        message += " Deseja continuar mesmo assim?";

        builder.setTitle("Erro");
        builder.setMessage(message);
        builder.setPositiveButton("Sim", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                handler.proceed();
            }
        });
        builder.setNegativeButton("Não", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                handler.cancel();
            }
        });
        final AlertDialog dialog = builder.create();
        dialog.show();
    }
}
}

Answer 4

Fix which works for me is just disable onReceivedSslError function defined in AuthorizationWebViewClient. In this case handler.cancel will be called in case of SSL error. However it works good with One Drive SSL certificates. Tested on Android 2.3.7, Android 5.1.