Deep validation of redirect URI

[nabokihms]

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Problem Description

https://security.lauritz-holtmann.de/post/sso-security-redirect-uri/

Proposed Solution

Validate schema and query parameters. Do not allow the following:

• Schema: data, javascript, vbscript
• Query params: code, state, response

Alternatives Considered

No response

Additional Information

No response

[abhisek]

@nabokihms I was looking at this issue and it seems to me that the attack will require a malicious client registration, either as static client or through the gRPC API.

From documentation:

The Dex API does not provide any authentication or authorization beyond TLS client auth.

Since there is no AuthZ at the API level, access to the gRPC API can be practically considered as administrative access to dex. So we cannot consider the case where a malicious client exploits this issue to gain additional privilege. Also dex doesn't really have any session so there isn't anything to protect even if malicious client side code is executed.

To me it doesn't seem like a risk worth fixing at this point. If we need to harden the redirect URI validation then we should probably consider an allow list approach because I see there are possibility of attacks using the ws:// scheme as well.