It seems to me that hiding a risky implementation is the problem. If you were directly shelling out to `mkdir -p` yourself, it would (hopefully) at least be obv

It seems to me that hiding a risky implementation is the problem. If you were directly shelling out to `mkdir -p` yourself, it would (hopefully) at least be obvious that it's a risky thing to do, and you need to make sure that either the input couldn't possibly contain any data touched by a malicious user or that it was sufficiently well sanitized to protect against that sort of thing. But if it's hidden behind a friendly-look library call, it would be less obvious that it's a risk, and it's easier to see someone absent-mindedly calling it because it's handy and passing in data from an unauthenticated web user or something.