Because the OAuth standard is intentionally ambiguous and flexible, OAuth authentication flows can occur. Each grant type has a small number of essential elements for basic functionality, but the vast rest of the implementation is entirely optional. This includes numerous setup options that are required to safeguard user data. In other words, there are several opportunities for undesirable behavior to emerge.
The general lack of integrated security features with OAuth is one of its other major drawbacks. The security is largely dependent on the developers choosing the best configuration parameters and adding their own extra security measures on top, including strong input validation. As you can undoubtedly tell, there is a lot to consider, and if you are unfamiliar with OAuth, it is very simple to make mistakes.
Highly sensitive data may also be sent via the browser, based on the grant type, which presents different options for an adversary to intercept it.
