Umbraco is an ASP.NET CMS. In versions 13.0.0 through 13.9.2, 15.0.0 through 15.4.1 and 16.0.0 through 16.1.0, the content delivery API can be restricted from public access where an API key must be provided in a header to authorize the request. It's also possible to configure output caching, such that the delivery API outputs will be cached for a period of time, improving performance. There's an issue when these two things are used together, where caching doesn't vary by the header that contains the API key. As such, it's possible for a user without a valid API key to retrieve a response for a given path and query if it has recently been requested and cached by request with a valid key. This is fixed in versions 13.9.3, 15.4.4 and 16.1.1.

Umbraco Delivery API allows for cached requests to be returned with an invalid API key