CVE-2023-21270 is a security vulnerability discovered in the restorePermissionState function, located in Android's PermissionManagerServiceImpl.java. This flaw can allow an app to keep permissions that should have been revoked during an app update. As a result, malicious apps can hold onto or even regain access to sensitive permissions, potentially leading to a local privilege escalation. No user interaction is necessary for this attack—just that the user executes the vulnerable app.

This article explains the vulnerability, shows how the exploit can happen, and provides code snippets to illustrate the issue. We'll also reference the official report and resources so you can dig deeper.

Background: Understanding Permissions in Android

Permissions in Android act as gatekeepers to sensitive device features and user data (like the camera, SMS, or contacts). When an app is updated via the Play Store, Android may revoke some permissions if the new version doesn't ask for them, especially if the permissions are sensitive.

However, this auto-revocation relies on the operating system correctly tracking which permissions are granted and when they should be revoked. The root of this CVE is that, under the right circumstances, that tracking fails—letting an app hold onto permission it shouldn't have.

The Vulnerability Explained

The glitch lies in restorePermissionState inside the Android Permission Manager implementation. During an app update, the system is supposed to clear certain permission flags and revoke unrequested dangerous permissions. Due to a subtle bug, those flags may not be set correctly, so permissions aren't revoked as expected.

Affected Function: restorePermissionState

- Risk: Permission revocation during app update does NOT occur as intended, letting apps keep privileged status

Let's look at a simplified/representative code example based on the vulnerability

// In PermissionManagerServiceImpl.java:

void restorePermissionState(Package pkg) {
    for (Permission permission : pkg.getPermissions()) {
        if (shouldRevoke(permission)) {
            // Suppose this flag should be set to trigger the revoke
            permission.setFlag(Permission.FLAG_PERMISSION_REVOKED_ON_UPDATE, true);

            // BUG: The actual call sometimes fails to set the flag due to logic error
            // or the flag is cleared incorrectly.
        }
    }
}

The issue: Instead of properly marking a permission as revoked, the code sometimes incorrectly preserves permission flags, so the permission remains granted even though the new APK doesn't request it or should not retain it.

Install App (v1): User installs and grants the dangerous permission.

3. Update App (v2): The attacker publishes an update with the permission *removed* from the manifest.
4. After Update: Due to this bug, instead of revoking the permission at update time, Android mistakenly leaves it granted.
5. App Kept Privilege: The app (v2) silently keeps access to, say, SMS—even without a manifest request.

Why is this dangerous?
The app can continue performing privileged actions undetected, violating Android's security promise to users. Apps may also use this to circumvent Play Store permission audits.

Real World Implications

- Everyday Users: A legitimate-looking app could ask for access, fake compliance with removals, yet keep snooping.
- Attack Surface: No user action is needed after initial grant. An attacker just persuades someone to update their app.

For illustration, here’s how an attacker might exploit the CVE

// v1 - AndroidManifest.xml requests SMS permission
<uses-permission android:name="android.permission.READ_SMS"/>

// v2 - Manifest removes the permission, but core code keeps access due to CVE bug!
// Malicious code continues reading SMS messages
Cursor cursor = contentResolver.query(
    Uri.parse("content://sms"), null, null, null, null);

while (cursor.moveToNext()) {
    String msg = cursor.getString(cursor.getColumnIndex("body"));
    Log.d("MaliciousApp", msg); // Do something nefarious
}

References

- Google Android Security Bulletin June 2023
- NVD Record for CVE-2023-21270
- Android Open Source Project - PermissionManagerServiceImpl.java
- Android Security - Permission Model Overview

Conclusion

CVE-2023-21270 exposes how subtle bugs in permission management can have major consequences for smartphone security. By understanding this bug, we see the importance of robust permission handling in user safety—and why fast patching is crucial.

If you want to know more, check out the original sources or the Android Security Center. Stay vigilant, keep your device updated, and review app permissions regularly!

Timeline

Published on: 11/19/2024 18:15:19 UTC
Last modified on: 11/20/2024 17:35:18 UTC