grist-core is a spreadsheet hosting server. Prior to version 1.7.7, a user with access to any document on a Grist installation can use a feature for fetching from a URL that is executed on the server. The privileged network access of server-side requests could offer opportunities for attack escalation. This issue is fixed in version 1.7.7. The mitigation was to use the proxy for untrusted fetches intended for such purposes. As a workaround, avoid making http/https endpoints available to an instance running Grist that expose credentials or operate without credentials.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.