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:
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
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.