Azure AD Identity Protection includes two risk policies: sign-in risk policy and user risk policy. A sign-in risk represents the probability that a given authentication request isn't authorized by the identity owner.
There are several types of risk detection. One of them is Anonymous IP Address. This risk detection type indicates sign-ins from an anonymous IP address (for example, Tor browser or anonymous VPN). These IP addresses are typically used by actors who want to hide their login telemetry (IP address, location, device, etc.) for potentially malicious intent.
You can configure the sign-in risk policy to require that users change their password.
References:
https://docs.microsoft.com/en-us/azure/active-directory/identity-protection/howto-sign-in-risk-policy