Keystone is a content management system for Node.js. Prior to version 6.5.0, "{field}.isFilterable" access control can be bypassed in "update" and "delete" mutations by adding additional unique filters. These filters can be used as an oracle to probe the existence or value of otherwise unreadable fields. Specifically, when a mutation includes a "where" clause with multiple unique filters (e.g. "id" and "email"), Keystone will attempt to match records even if filtering by the latter fields would normally be rejected by "field.isFilterable" or "list.defaultIsFilterable". This can allow malicious actors to infer the presence of a particular field value when a filter is successful in returning a result. This affects any project relying on the default or dynamic "isFilterable" behavior (at the list or field level) to prevent external users from using the filtering of fields as a discovery mechanism. While this access control is respected during "findMany" operations, it was not completely enforced during "update" and "delete" mutations when accepting more than one unique "where" values in filters. This has no impact on projects using "isFilterable: false" or "defaultIsFilterable: false" for sensitive fields, or for those who have otherwise omitted filtering by these fields from their GraphQL schema. This issue has been patched in "@keystone-6/core" version 6.5.0. To mitigate this issue in older versions where patching is not a viable pathway, set "isFilterable: false" statically for relevant fields to prevent filtering by them earlier in the access control pipeline (that is, don't use functions); set "{field}.graphql.omit.read: true" for relevant fields, which implicitly removes filtering by these fields from the GraphQL schema; and/or deny "update" and "delete" operations for the relevant lists completely.