Affected: RubyGems: rack

## Summary

`Rack::Directory`’s path check used a string prefix match on the expanded path. A request like `/../root_example/` can escape the configured root if the target path starts with the root string, allowing directory listing outside the intended root.

## Details

In `directory.rb`, `File.expand_path(File.join(root, path_info)).start_with?(root)` does not enforce a path boundary. If the server root is `/var/www/root`, a path like `/var/www/root_backup` passes the check because it shares the same prefix, so `Rack::Directory` will list that directory also.

## Impact

Information disclosure via directory listing outside the configured root when `Rack::Directory` is exposed to untrusted clients and a directory shares the root prefix (e.g., `public2`, `www_backup`).

## Mitigation

* Update to a patched version of Rack that correctly checks the root prefix. * Don't name directories with the same prefix as one which is exposed via `Rack::Directory`.