CVE-2026-57178: social-auth-core: VK App backend accepts unsigned callback data when auth_key is missing

### 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.