Grammarly Keyboard for Android "Authorization Code with PKCE" flow implementation vulnerability that allows account takeover

Summary

@tomtenisse identified a vulnerability in Grammarly Keyboard for Android that allowed malicious application installed on the device to guess the PKCE code verifier value and consequently obtain access to OAuth access_token (grauth cookie). The vulnerability was fixed by moving from PRNG to SecureRandom

Thank you, @tomtenisse, for the detailed report and collaboration!