A URL parameter during login flow was vulnerable to injection. An attacker could insert a malicious domain in this parameter, which would redirect the user after auth and send the authorization token to the redirected domain.
The product receives input that is expected to be of a certain type, but it does not validate or incorrectly validates that the input is actually of the expected type.
The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.