Android App 安全的HTTPS 通信-白红宇

漏洞描述

对于数字证书相关概念、Android 里 https 通信代码就不再复述了，直接讲问题。缺少相应的安全校验很容易导致中间人攻击，而漏洞的形式主要有以下3种：

自定义X509TrustManager。在使用HttpsURLConnection发起 HTTPS 请求的时候，提供了一个自定义的X509TrustManager，未实现安全校验逻辑，下面片段就是常见的容易犯错的代码片段。如果不提供自定义的X509TrustManager，代码运行起来可能会报异常（原因下文解释），初学者就很容易在不明真相的情况下提供了一个自定义的X509TrustManager，却忘记正确地实现相应的方法。本文重点介绍这种场景的处理方式。

TrustManager tm = new X509TrustManager() {
         public void checkClientTrusted(X509Certificate[] chain, String authType)            throws CertificateException {
                   //do nothing，接受任意客户端证书    }    public void checkServerTrusted(X509Certificate[] chain, String authType)            throws CertificateException {
                   //do nothing，接受任意服务端证书    }    public X509Certificate[] getAcceptedIssuers() {
             return null;    }};sslContext.init(null, new TrustManager[] {
      tm }, null);

自定义了HostnameVerifier。在握手期间，如果 URL 的主机名和服务器的标识主机名不匹配，则验证机制可以回调此接口的实现程序来确定是否应该允许此连接。如果回调内实现不恰当，默认接受所有域名，则有安全风险。代码示例。

HostnameVerifier hnv = new HostnameVerifier() {
       @Override  public boolean verify(String hostname, SSLSession session) {
         // Always return true，接受任意域名服务器    return true;  }};HttpsURLConnection.setDefaultHostnameVerifier(hnv);

信任所有主机名。

SSLSocketFactory sf = new MySSLSocketFactory(trustStore);sf.setHostnameVerifier(SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);

修复方案

分而治之，针对不同的漏洞点分别描述，这里就讲的修复方案主要是针对非浏览器App，非浏览器 App 的服务端通信对象比较固定，一般都是自家服务器，可以做很多特定场景的定制化校验。如果是浏览器 App，校验策略就有更通用一些。

自定义X509TrustManager。前面说到，当发起 HTTPS 请求时，可能抛起一个异常，以下面这段代码为例（来自官方文档）：

try {
         URL url = new URL("https://certs.cac.washington.edu/CAtest/");    URLConnection urlConnection = url.openConnection();    InputStream in = urlConnection.getInputStream();    copyInputStreamToOutputStream(in, System.out);} catch (MalformedURLException e) {
         e.printStackTrace();} catch (IOException e) {
         e.printStackTrace();}

private void copyInputStreamToOutputStream(InputStream in, PrintStream out) throws IOException {
         byte[] buffer = new byte[1024];    int c = 0;    while ((c = in.read(buffer)) != -1) {
             out.write(buffer, 0, c);    }}

它会抛出一个SSLHandshakeException的异常。

javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.    at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:322)    at com.android.okhttp.Connection.upgradeToTls(Connection.java:201)    at com.android.okhttp.Connection.connect(Connection.java:155)    at com.android.okhttp.internal.http.HttpEngine.connect(HttpEngine.java:276)    at com.android.okhttp.internal.http.HttpEngine.sendRequest(HttpEngine.java:211)    at com.android.okhttp.internal.http.HttpURLConnectionImpl.execute(HttpURLConnectionImpl.java:382)    at com.android.okhttp.internal.http.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:332)    at com.android.okhttp.internal.http.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:199)    at com.android.okhttp.internal.http.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)    at com.android.okhttp.internal.http.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:25)    at me.longerian.abcandroid.datetimepicker.TestDateTimePickerActivity$1.run(TestDateTimePickerActivity.java:236)Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.    at com.android.org.conscrypt.TrustManagerImpl.checkTrusted(TrustManagerImpl.java:318)    at com.android.org.conscrypt.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:219)    at com.android.org.conscrypt.Platform.checkServerTrusted(Platform.java:114)    at com.android.org.conscrypt.OpenSSLSocketImpl.verifyCertificateChain(OpenSSLSocketImpl.java:550)    at com.android.org.conscrypt.NativeCrypto.SSL_do_handshake(Native Method)    at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:318) ... 10 moreCaused by: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found. ... 16 more

Android 手机有一套共享证书的机制，如果目标 URL 服务器下发的证书不在已信任的证书列表里，或者该证书是自签名的，不是由权威机构颁发，那么会出异常。对于我们这种非浏览器 app 来说，如果提示用户去下载安装证书，可能会显得比较诡异。幸好还可以通过自定义的验证机制让证书通过验证。验证的思路有两种：

方案1

不论是权威机构颁发的证书还是自签名的，打包一份到 app 内部，比如存放在 asset 里。通过这份内置的证书初始化一个KeyStore，然后用这个KeyStore去引导生成的TrustManager来提供验证，具体代码如下：

try {
       CertificateFactory cf = CertificateFactory.getInstance("X.509");  // uwca.crt 打包在 asset 中，该证书可以从https://itconnect.uw.edu/security/securing-computer/install/safari-os-x/下载  InputStream caInput = new BufferedInputStream(getAssets().open("uwca.crt"));  Certificate ca;  try {
           ca = cf.generateCertificate(caInput);      Log.i("Longer", "ca=" + ((X509Certificate) ca).getSubjectDN());      Log.i("Longer", "key=" + ((X509Certificate) ca).getPublicKey();  } finally {
           caInput.close();  }  // Create a KeyStore containing our trusted CAs  String keyStoreType = KeyStore.getDefaultType();  KeyStore keyStore = KeyStore.getInstance(keyStoreType);  keyStore.load(null, null);  keyStore.setCertificateEntry("ca", ca);  // Create a 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 our TrustManager  SSLContext context = SSLContext.getInstance("TLSv1","AndroidOpenSSL");  context.init(null, tmf.getTrustManagers(), null);  URL url = new URL("https://certs.cac.washington.edu/CAtest/");  HttpsURLConnection urlConnection =          (HttpsURLConnection)url.openConnection();  urlConnection.setSSLSocketFactory(context.getSocketFactory());  InputStream in = urlConnection.getInputStream();  copyInputStreamToOutputStream(in, System.out);} catch (CertificateException e) {
       e.printStackTrace();} catch (IOException e) {
       e.printStackTrace();} catch (NoSuchAlgorithmException e) {
       e.printStackTrace();} catch (KeyStoreException e) {
       e.printStackTrace();} catch (KeyManagementException e) {
       e.printStackTrace();} catch (NoSuchProviderException e) {
       e.printStackTrace();}

这样就可以得到正确的输出内容：

UW Services CA Test Page

UW Services CA test page

QUESTION:Did you arrive here without any security alerts or warnings?

YES - This test page uses a certificate issued by theUW Services Certificate Authority. If you reached this pagewithout any alerts or warnings from your browser, youhave successfully installed the UW Services CA Certificateinto your browser.

NO - If your browser warned you about the validity of thistest page's security certificate, or the certificateauthority is unrecognized, you may not have successfullyinstalled the UW Services CA Certificate.

如果你用上述同样的代码访问 https://www.taobao.com/ 或者 https://www.baidu.com/ ，则会抛出那个SSLHandshakeException异常，也就是说对于特定证书生成的TrustManager，只能验证与特定服务器建立安全链接，这样就提高了安全性。如之前提到的，对于非浏览器 app 来说，这是可以接受的。

方案2

同方案1，打包一份到证书到 app 内部，但不通过KeyStore去引导生成的TrustManager，而是干脆直接自定义一个TrustManager，自己实现校验逻辑；校验逻辑主要包括：

服务器证书是否过期

证书签名是否合法

try {
       CertificateFactory cf = CertificateFactory.getInstance("X.509");  // uwca.crt 打包在 asset 中，该证书可以从https://itconnect.uw.edu/security/securing-computer/install/safari-os-x/下载  InputStream caInput = new BufferedInputStream(getAssets().open("uwca.crt"));  final Certificate ca;  try {
           ca = cf.generateCertificate(caInput);      Log.i("Longer", "ca=" + ((X509Certificate) ca).getSubjectDN());      Log.i("Longer", "key=" + ((X509Certificate) ca).getPublicKey());  } finally {
           caInput.close();  }  // Create an SSLContext that uses our TrustManager  SSLContext context = SSLContext.getInstance("TLSv1","AndroidOpenSSL");  context.init(null, new TrustManager[]{
               new X509TrustManager() {
                   @Override              public void checkClientTrusted(X509Certificate[] chain,                      String authType)                      throws CertificateException {
                   }              @Override              public void checkServerTrusted(X509Certificate[] chain,                      String authType)                      throws CertificateException {
                       for (X509Certificate cert : chain) {
                           // Make sure that it hasn't expired.                      cert.checkValidity();                      // Verify the certificate's public key chain.                      try {
                               cert.verify(((X509Certificate) ca).getPublicKey());                      } catch (NoSuchAlgorithmException e) {
                               e.printStackTrace();                      } catch (InvalidKeyException e) {
                               e.printStackTrace();                      } catch (NoSuchProviderException e) {
                               e.printStackTrace();                      } catch (SignatureException e) {
                               e.printStackTrace();                      }                  }              }              @Override              public X509Certificate[] getAcceptedIssuers() {
                       return new X509Certificate[0];              }          }  }, null);  URL url = new URL("https://certs.cac.washington.edu/CAtest/");  HttpsURLConnection urlConnection =          (HttpsURLConnection)url.openConnection();  urlConnection.setSSLSocketFactory(context.getSocketFactory());  InputStream in = urlConnection.getInputStream();  copyInputStreamToOutputStream(in, System.out);} catch (CertificateException e) {
       e.printStackTrace();} catch (IOException e) {
       e.printStackTrace();} catch (NoSuchAlgorithmException e) {
       e.printStackTrace();} catch (KeyManagementException e) {
       e.printStackTrace();} catch (NoSuchProviderException e) {
       e.printStackTrace();}

同样上述代码只能访问 certs.cac.washington.edu 相关域名地址，如果访问 https://www.taobao.com/ 或者 https://www.baidu.com/ ，则会在cert.verify(((X509Certificate) ca).getPublicKey());处抛异常，导致连接失败。

自定义HostnameVerifier，简单的话就是根据域名进行字符串匹配校验；业务复杂的话，还可以结合配置中心、白名单、黑名单、正则匹配等多级别动态校验；总体来说逻辑还是比较简单的，反正只要正确地实现那个方法。

HostnameVerifier hnv = new HostnameVerifier() {
       @Override  public boolean verify(String hostname, SSLSession session) {
         //示例    if("yourhostname".equals(hostname)){
             return true;      } else {
             HostnameVerifier hv =            HttpsURLConnection.getDefaultHostnameVerifier();      return hv.verify(hostname, session);    }  }};

主机名验证策略改成严格模式

SSLSocketFactory sf = new MySSLSocketFactory(trustStore);sf.setHostnameVerifier(SSLSocketFactory.STRICT_HOSTNAME_VERIFIER);

漏洞描述

修复方案

方案1

UW Services CA test page

方案2

参考资料