GHSA-3c93-f73f-qc9h

### Impact

The `vk-app` backend accepted VK application callback data without verifying the callback signature when the `auth_key` parameter was omitted.

Applications using this backend could treat unsigned attacker-controlled data as a verified VK identity. An attacker could choose callback fields such as `viewer_id`, `access_token`, `api_id`, and `api_result`, potentially allowing authentication as an arbitrary VK user ID.

The issue affects only applications using the `vk-app` backend.

### Patches

The issue has been fixed by requiring `auth_key` to be present and valid before callback data is trusted.

Users should upgrade to a patched version.

Fix:

* https://github.com/python-social-auth/social-core/pull/1811

### Workarounds

Applications that cannot upgrade immediately should disable the `vk-app` backend by removing `social_core.backends.vk.VKAppOAuth2` from `SOCIAL_AUTH_AUTHENTICATION_BACKENDS`.

There is no complete workaround while continuing to use the vulnerable backend.

### Credits

Reported by @lalalala5678 through GitHub private vulnerability reporting.