Unintended directory traversal via symlink replacement

A TOCTOU race in Go standard library path/filepath Walk and WalkDir allows attackers to traverse unintended directories via symlink replacement. The functions do not bind later traversal to the path entry already checked, so replacing a checked component with a symlink redirects the walk before use. Exploitation requires control of part of the walked filesystem and precise timing; subsequent reads or writes depend on the consuming callback.