VTAdmin users that can create shards can deny access to other functions

Impact

Users can either intentionally or inadvertently create a shard containing / characters from VTAdmin such that from that point on, anyone who tries to create a new shard from VTAdmin will receive an error.
Attempting to view the keyspace(s) will also no longer work.
Creating a shard using vtctldclient does not have the same problem because the CLI validates the input correctly.

Patches

v16.0.2, corresponding to 0.16.2 on pkg.go.dev

Workarounds

• Always use vtctldclient to create shards, instead of using VTAdmin
• Disable creating shards from VTAdmin using RBAC
• Delete the topology record for the offending shard using the client for your topology server. For example, if you created a shard called a/b in keyspace commerce, and you are running etcd, it can be deleted by doing something like

% etcdctl --endpoints "http://${ETCD_SERVER}" del /vitess/global/keyspaces/commerce/shards/a/b/Shard

References

https://github.com/vitessio/vitess/issues/12842

Found during a security audit sponsored by the CNCF and facilitated by OSTIF.