parse-server: Relation `$relatedTo` query bypasses `protectedFields` and owning-object ACL

Impact

A relation query using the $relatedTo operator could read the membership of a Relation field even when that field was hidden from the requesting client by protectedFields, and even when the object owning the relation was not readable by the client under its ACL or class-level permissions. The request requires only the public API credentials that Parse clients normally carry — no user session, master key, or Cloud Code is needed.

As a result, an unauthenticated client who knows or obtains the owning object's objectId could enumerate the objects linked through a protected relation, or combine the operator with an objectId constraint to use it as a membership oracle — confirming whether a specific object is linked to a private parent. This affects applications that rely on protectedFields or object ACLs to keep Relation membership confidential, such as private group memberships, block lists, or account-to-resource associations.

Patches

The relation query path now authorizes $relatedTo against the owning object before reading the relation join table, using the caller's authentication context. The relation key is checked against the owning class's protectedFields (the query is rejected if the key is protected), and the owning object must be readable by the caller under its class-level permissions, ACL, and pointer permissions; otherwise the relation returns no results. Master and maintenance requests are unaffected. The check is applied consistently whether $relatedTo is used at the top level or nested within $or, $and, or $nor.

Workarounds

There is no complete workaround without upgrading. As mitigation, applications can avoid exposing sensitive membership through Relation fields to untrusted clients, or enforce access on the queried class in a beforeFind trigger.