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CVE-2020-0396 Telephony组件信息泄露漏洞分析

一、漏洞背景

  • 在AOSP的2020-09补丁中，披露了一个Telephony eSIM模块的漏洞，编号为CVE-2020-0396并且为Critical级别。成功利用漏洞可造成eSIM的信息泄露。

二、漏洞细节

  • 漏洞位于com.android.internal.telephony.euicc.EuiccController中，在构造一个PendingIntent时没有指定接收者，导致Intent变成了一个隐式Intent，任何注册了对应intent-filter的Activity都会进入选择列表。 
    // frameworks/opt/telephony/src/java/com/android/internal/telephony/euicc/EuiccController.java
/** Add a resolution intent to the given extras intent. */
@VisibleForTesting(visibility = VisibleForTesting.Visibility.PRIVATE)
public void addResolutionIntent(Intent extrasIntent, String resolutionAction,
    String callingPackage, int resolvableErrors, boolean confirmationCodeRetried,
    EuiccOperation op, int cardId) {
Intent intent = new Intent(EuiccManager.ACTION_RESOLVE_ERROR);
// Missing receiver of PendingIntent
intent.putExtra(EuiccManager.EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_ACTION,
        resolutionAction);
intent.putExtra(EuiccService.EXTRA_RESOLUTION_CALLING_PACKAGE, callingPackage);
intent.putExtra(EuiccService.EXTRA_RESOLVABLE_ERRORS, resolvableErrors);
intent.putExtra(EuiccService.EXTRA_RESOLUTION_CARD_ID, cardId);
intent.putExtra(EuiccService.EXTRA_RESOLUTION_CONFIRMATION_CODE_RETRIED,
        confirmationCodeRetried);
intent.putExtra(EXTRA_OPERATION, op);
PendingIntent resolutionIntent = PendingIntent.getActivity(
        mContext, 0 /* requestCode */, intent, PendingIntent.FLAG_ONE_SHOT);
extrasIntent.putExtra(
        EuiccManager.EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_INTENT, resolutionIntent);
}

  • 向上梳理该函数的调用者，有好几个调用者，这里以onGetDefaultListComplete为例

    // frameworks/opt/telephony/src/java/com/android/internal/telephony/euicc/EuiccController.java
@Override
public void onGetDefaultListComplete(int cardId,
    GetDefaultDownloadableSubscriptionListResult result) {
Intent extrasIntent = new Intent();
final int resultCode;
switch (result.getResult()) {
    case EuiccService.RESULT_OK:
        resultCode = OK;
        List<DownloadableSubscription> list = result.getDownloadableSubscriptions();
        if (list != null && list.size() > 0) {
            extrasIntent.putExtra(
                    EuiccManager.EXTRA_EMBEDDED_SUBSCRIPTION_DOWNLOADABLE_SUBSCRIPTIONS,
                    list.toArray(new DownloadableSubscription[list.size()]));
        }
        break;
    case EuiccService.RESULT_MUST_DEACTIVATE_SIM:
        resultCode = RESOLVABLE_ERROR;
        addResolutionIntent(extrasIntent,
                EuiccService.ACTION_RESOLVE_DEACTIVATE_SIM,
                mCallingPackage,
                0 /* resolvableErrors */,
                false /* confirmationCodeRetried */,
                EuiccOperation.forGetDefaultListDeactivateSim(
                        mCallingToken, mCallingPackage),
                cardId);
        break;
    default:
        resultCode = ERROR;
        addExtrasToResultIntent(extrasIntent, result.getResult());
        break;
}

sendResult(mCallbackIntent, resultCode, extrasIntent);
}
  • sendResult方法中可以看到是将extrasIntent添加到了callbackIntent中（Intent.fillIn），并且发送了callbackIntent这个PendingIntent
// frameworks/opt/telephony/src/java/com/android/internal/telephony/euicc/EuiccController.java
/** Dispatch the given callback intent with the given result code and data. */
@VisibleForTesting(visibility = VisibleForTesting.Visibility.PRIVATE)
public void sendResult(PendingIntent callbackIntent, int resultCode, Intent extrasIntent) {
    try {
        callbackIntent.send(mContext, resultCode, extrasIntent);
    } catch (PendingIntent.CanceledException e) {
        // Caller canceled the callback; do nothing.
    }
}
  • 通过上述代码可以梳理出来这个callbackIntent的结构，首先callbackIntent自身有Action等字段，这里以[Original PendingIntent Part]来表示，然后extrasIntent是通过Intent.fillIn方法添加进去的，这里因为没有重复的字段，所以就是简单的组合，而extrasIntent中包含了一个字段名为EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_INTENT的PendingIntent，也就是addResolutionIntent方法中的resolutionIntent，这个PendingIntent携带的Intent就是造成漏洞的那个Intent，Action为ACTION_RESOLVE_ERROR，extras域有六个字段，包含eSIM的Card ID、Confirmation Code，这些应该是敏感信息。 
    callbackIntent
(PendingIntent)
|
[Original PendingIntent Part]
extrasIntent(Intent.fillIn)
(Intent)
    Extras:
    EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_INTENT(resolutionIntent)
    (PendingIntent)
        |
        Action: ACTION_RESOLVE_ERROR
        Extras:
            EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_ACTION
            EXTRA_RESOLUTION_CALLING_PACKAGE
            EXTRA_RESOLVABLE_ERRORS
            EXTRA_RESOLUTION_CARD_ID
            EXTRA_RESOLUTION_CONFIRMATION_CODE_RETRIED
            EXTRA_OPERATION
  • 那么这个复杂的PendingIntent在send之后传递给谁了呢，查找EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_INTENT字段之后我们发现，是发送给了android.telephony.euicc.EuiccManager这边的startResolutionActivity方法，最终以startIntentSenderForResult的形式发送出去了 
    // frameworks/base/telephony/java/android/telephony/euicc/EuiccManager.java
public void startResolutionActivity(Activity activity, int requestCode, Intent resultIntent, PendingIntent callbackIntent) throws IntentSender.SendIntentException {
PendingIntent resolutionIntent =
    resultIntent.getParcelableExtra(EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_INTENT);
if (resolutionIntent == null) {
    throw new IllegalArgumentException("Invalid result intent");
}
Intent fillInIntent = new Intent();
fillInIntent.putExtra(
    EuiccManager.EXTRA_EMBEDDED_SUBSCRIPTION_RESOLUTION_CALLBACK_INTENT,
        callbackIntent);
activity.startIntentSenderForResult(resolutionIntent.getIntentSender(), requestCode, fillInIntent, 0 /* flagsMask */, 0 /* flagsValues */, 0 /* extraFlags */);
}
  • 这个startIntentSenderForResult是什么接口呢？查看定义发现，如果是在Activity对象上调用的，其效果和startActivityForResult相似，只是参数变成了PendingIntent的内部实现IntentSender。 
    Like startActivityForResult(android.content.Intent, int), but allowing you to use a IntentSender to describe the activity to be started. If the IntentSender is for an activity, that activity will be started as if you had called the regular startActivityForResult(android.content.Intent, int) here; otherwise, its associated action will be executed (such as sending a broadcast) as if you had called IntentSender#sendIntent on it.
  • 所以最终eSIM的Card ID、Confirmation Code这些敏感信息，会以隐式Intent的形式被发送出去，这样一来，只要恶意应用编写一个Action为ACTION_RESOLVE_ERROR的intent-filter，就有机会拿到这些数据。

三、漏洞验证

  • 漏洞需要eSIM，现在Pixel 3、Pixel 4系列是支持eSIM的，然而国内并没有，所以没办法复现。

四、漏洞影响

  • 造成Card ID、Confirmation Code信息泄露，这个Card ID不知道是什么概念，如果类比实体SIM卡的话，应该类似于IMSI，也就是SIM卡的唯一ID，该ID可用于追踪用户，这可能是Google将漏洞定为Critical的原因。

五、漏洞补丁

  • 在com.android.internal.telephony.euicc.EuiccController的addResolutionIntent方法显式添加了接收者，使Intent成为显式Intent。 
    intent.setPackage(RESOLUTION_ACTIVITY_PACKAGE_NAME);
intent.setComponent(new ComponentName(RESOLUTION_ACTIVITY_PACKAGE_NAME, RESOLUTION_ACTIVITY_CLASS_NAME));
  • 这里可以看到实际的接收者应是com.android.phone.euicc.EuiccResolutionUiDispatcherActivity 
    /** Restrictions limiting access to the PendingIntent */
private static final String RESOLUTION_ACTIVITY_PACKAGE_NAME = "com.android.phone";
private static final String RESOLUTION_ACTIVITY_CLASS_NAME =
"com.android.phone.euicc.EuiccResolutionUiDispatcherActivity";
  • 另外Google还给一大堆Telephony模块中的其他PendingIntent增加了PendingIntent.FLAG_IMMUTABLE标志，修改位置达到7处之多，该标志的作用是让PendingIntent在send时候添加的额外Intent失效，也就是不允许二次向原本的Intent中添加内容（不允许Intent.fillIn），这几个修改看起来和EuiccController并不直接相关，可能是为了消减一类问题，防止PendingIntent被二次编辑。另外给BluetoothPairingDialog加了一个PRIVATE_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS标志，防止蓝牙配对对话框被第三方应用使用顶部显示特权遮挡。

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