SpiceDB: Caveat structures with nested lists can result in improper cache reuse

Impact

Users are impacted if:

• They have a caveat structure with a nested list, e.g.:

  caveat shape(x list<any>) {
    x == [["a"], "b"]
  }

• Their system exercises that caveat with either CheckBulkPermission or else LookupResources running with the --experimental-lookup-resources-version flag set to lr3, implying they are using the experimental version 3 of LookupResources
• An attacker can cause the system to craft a request to SpiceDB where either:
  1. It's a CheckBulk request where there are two check items that are identical except for their combined caveat context, and one of the caveat contexts evaluates positively and the other evaluates negatively
  2. It's a LookupResources request where two resources have the same evaluation contents except for their caveat context, and one would evaluate positively and the other would evaluate negatively

If all of the above are true, it would be possible for SpiceDB to erroneously return that a user has access to a resource that they do not have access to.

Patches

This problem was addressed in https://github.com/authzed/spicedb/pull/3065 and released in version v1.52.0.

Workarounds

If using v3 of LookupResources, turn the flag off.

If possible, refactor the caveat declaration structure so that it does not operate on a list of lists, but rather any other composite structure.