Android App 安全的HTTPS 通信-自定义TrustManager

其他 2020-04-19 12:34:02 阅读次数: 0

 

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 more
Caused 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：

<html>
<head>
<title>UW Services CA Test Page</title>
</head>
<body>
<h2>
UW Services CA test page
</h2>
<p>
<b>QUESTION</b>:
Did you arrive here without any security alerts or warnings?</p>
<ul>
<p>
<li>
<b>YES</b> - This test page uses a certificate issued by the
UW Services Certificate Authority. If you reached this page
without any alerts or warnings from your browser, you
have successfully installed the UW Services CA Certificate
into your browser.
<p>
<li>
<b>NO</b> - If your browser warned you about the validity of this
test page's security certificate, or the certificate
authority is unrecognized, you may not have successfully
installed the UW Services CA Certificate.
<p>
</ul>
<form action="https://www.washington.edu/computing/ca/" method=get>
<input type=submit value="Return to the Install Page">
</form>
</body>
</html>

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);

参考资料

苹果核————Android App 安全的HTTPS 通信

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