Impact

fosite#400 (released as v0.30.2) introduced a new feature for handling redirect URLs pointing to loopback interfaces (rfc8252#section-7.3). As part of that change new behavior was introduced which failed to respect the redirect URL's (only for loopback interfaces!) query parameters

1. Registering a client with allowed redirect URL http://127.0.0.1/callback
2. Performing OAuth2 flow and requesting redirect URL http://127.0.0.1/callback?bar=foo
3. Instead of an error, the browser is redirected to http://127.0.0.1/callback?bar=foo with a potentially successful OAuth2 response.

as well as the host parameter (as long as the host is a loopback interface):

1. Registering a client with allowed redirect URL https://example.com/callback
2. Performing OAuth2 flow and requesting redirect URL http://127.0.0.1/callback
3. Instead of an error, the browser is redirected to http://127.0.0.1/callback with a potentially successful OAuth2 response.

These bugs are only applicable in scenarios where the attacker has control over the loopback interface (localhost, 127.0.0.1, [::1]) where the browser performing the OAuth2 flow is running.

References

• GHSA-rfq3-w54c-f9q5
• https://nvd.nist.gov/vuln/detail/CVE-2020-15233
• ory/fosite#400
• ory/fosite@cdee51e