Sharing itself as a channel, independent of any misconfiguration
Least common mechanism holds that the infrastructure shared between different users or processes should be minimized, because whatever is shared becomes a potential channel through which one can affect, observe, or interfere with another — a shared cache, a shared queue, a shared resource pool — even when every individual party's own access control is configured perfectly correctly on its own terms.
This is what makes it a subtler category of problem than an ordinary permission bug: nothing about either side's specific configuration is wrong, and the interference still happens, because the path runs through something both sides depend on rather than through either side's own access boundary.
Reducing sharing rather than policing every interaction across it
The practical response is to look for what's genuinely necessary to share versus what's shared purely for convenience or efficiency, and to separate the latter wherever the cost of doing so is reasonable. This is more durable than trying to monitor and police every possible way the shared mechanism could be misused after the fact — reducing what's actually shared closes the path structurally, rather than depending on catching every attempt to exploit it.