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OAuth 2.0 Authorisation Endpoint Threats

This is part of a larger series on OAuth 2.0 Threats. The other parts are linked below.

  1. OAuth 2.0 Threat Model and Security Considerations
  2. OAuth 2.0 Client Threats
  3. OAuth 2.0 Authorisation Endpoint Threats
  4. OAuth 2.0 Token Endpoint Threats
  5. OAuth 2.0 Refresh Token Threats
  6. OAuth 2.0 Protected Resource Threats

The front-door of your OAuth 2.0 authorisation system is the Authorisation Endpoint. This should be well-known and documented (ideally with an OpenID Discovery Document), so don’t even think about security by obscurity.

Your first line of defense is TLS and Certificates. Use a public-signed certificate for your OAuth 2.0 Endpoints. Never use self-signed certificates. This is your primary defense against DNS poisoning, ARP spoofing, doppleganger domains, etc. If a bad actor can encourage your users to hit a counterfit authorisation server, they could use this to steal password information. In your client, always verify your TLS certificates, never allow bad or self-signed.

An Authorisation Server may ‘cache’ information on previously authorised clients. This can be used to reduce the computational cost, to reduce the user complexity. But, it leaves a replay attack open. You can protect against this by limiting the number of access tokens which may be (re) issued, by forcing pre-registered public redirect URI.

As with all services, use a set of best practices. Never use wildcard TLS certificates. Use read-only fileystems. Protect against the east-west threat from other services in the same domain or scope or cluster.

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