In the marshmallow library before 2.15.1 and 3.x before 3.0.0b9 for Python, the schema "only" option treats an empty list as implying no "only" option, which allows a request that was intended to expose no fields to instead expose all fields (if the schema is being filtered dynamically using the "only" option, and there is a user role that produces an empty value for "only").
{ "nvd_published_at": null, "cwe_ids": [ "CWE-358" ], "severity": "MODERATE", "github_reviewed": true, "github_reviewed_at": "2020-06-16T21:29:19Z" }