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.