DRD19. Properly verify server certificate on SSL/TLS

Android apps that use SSL/TLS protocols for secure communication should properly verify server certificates. The basic verification includes:

• verify that the subject (CN) of X.509 certificate and the URL matches
• verify that the certificate is signed by the trusted CA
• verify that the signature is correct
• verify that the certificate is not expired

Android SDK  4.0 and later offers packages to implement capabilities to establish network connections. For example, by using java.net, javax.net, android.net or org.apache.http, a developer can create server sockets or HTTP connection. org.webkit offers functions necessary to implement web browsing capabilities.

A developer has the freedom to customize their SSL implementation. The developer should properly use SSL as appropriate to the intent of the app and the environment the apps are used in. If the SSL is not correctly used, a user's sensitive data may leak via the vulnerable SSL communication channel.

Fahl et al [Fahl 2012] describes the following patterns of the insecure use of SSL:

• Trusting All Certificates: The developer implements the TrustManager interface so that it will trust all the server certificate (regardless of who signed it, what is the CN etc.)
• Allowing All Hostnames: The app does not verify if the certificate is issued for the URL the client is connecting to. For example, when a client connects to example.com, it will accept a server certificate issued for some-other-domain.com.
• Mixed-Mode/No SSL: A developer mixes secure and insecure connections in the same app or does not use SSL at all.

On Android, using HttpURLConnection is recommended for HTTP client implementation.

Noncompliant Code Example

The following code implements a custom MySSLSocketFactory class that inherits javax.net.ssl.SSLContext:

public class extends SSLSocketFactory {
    SSLContext sslContext;
public MySSLSocketFactory (KeyStore truststore) throws NoSuchAlgorithmException, KeyManagementException,
KeyStoreException, UnrecoverableKeyException {
    super(truststore);
    this.sslContext = SSLContext.getInstance("TLS");
    this.sslContext.init (null, new TrustManager[] {new X509TrustManager() {
            public void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException
            {
            }
            public void checkServerTrusted(X509Certificate[] chain, String authType) throws CertificateException
            {
            }
            public X509Certificate[] getAcceptedIssuers() {
                return null;
            }
        }}, null);
}
    public Socket createSocket() throws IOException {
        return this.sslContext.getSocketFactory().createSocket();
    }
    public Socket createSocket(Socket socket, String host, int port, boolean autoClose) throws IOException
                                                                                               , UnknownHostException {
        return this.sslContext.getSocketFactory().createSocket(socket, host, port, autoClose);
    }
}
 
 public static HttpClient getNewHttpClient() {
            DefaultHttpClient v6;
            try {
                KeyStore v5 = KeyStore.getInstance(KeyStore.getDefaultType());
                v5.load(null, null);
                MySSLSocketFactory mySSLScoket = new MySSLSocketFactory(v5);
                if(DefineRelease.sAllowAllSSL) {
                    ((SSLSocketFactory)mySSLScoket).setHostnameVerifier(SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);
                }
                BasicHttpParams v2 = new BasicHttpParams();
                HttpConnectionParams.setConnectionTimeout(((HttpParams)v2), 30000);
                HttpConnectionParams.setSoTimeout(((HttpParams)v2), 30000);
                HttpProtocolParams.setVersion(((HttpParams)v2), HttpVersion.HTTP_1_1);
                HttpProtocolParams.setContentCharset(((HttpParams)v2), "UTF-8");
                SchemeRegistry v3 = new SchemeRegistry();
                v3.register(new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
                v3.register(new Scheme("https", ((SocketFactory)mySSLScoket), 443));
                v6 = new DefaultHttpClient(new ThreadSafeClientConnManager(((HttpParams)v2), v3), ((HttpParams)v2));
            }
            catch(Exception v1) {
                v6 = new DefaultHttpClient();
            }
            return ((HttpClient)v6);
}   

In the example above, checkClientTrusted()and checkServerTrusted() are overriden to make a blank implementation so that SSLSocketFactory does not verify the SSL certificate. The MySSLSocketFactory class is used to create an instance of HttpClient in another part of the application.

sAllowAllSSL, which is a static member of the DefineRelease class, is initialized to true in its static constructor. This will enable the use of SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER. As a result, host name verification that should take place when establishing an SSL connection is disabled and will lead to the same situation as all the certificate is trusted.

Compliant Solution

The compliant solution may vary, depending on the actual implementation. For examples of secure implementation such as using a self-signed server certificate, please refer to "Android Application Secure Design/Secure Coding Guidebook", Section 5.4 Communicate by HTTPS.

Related Vulnerabilities

• VU#582497 Multiple Android applications fail to properly validate SSL certificates
• JVN#39218538 Pizza Hut Japan Official Order App for Android has a problem whereby it fails to verify SSL server certificates.
• JVN#75084836 Yome Collection for Android has a problem with management of IMEI.
• JVN#68156832 Yafuoku! contains an issue where it fails to verify SSL server certificates.

Related Guidelines

Bibliography

Fahl 2012

Why Eve and Mallory Love Android: An Analysis of Android SSL (In)Security

 

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