2010/12/21 Miloslav Trmač:

If an attacker were controlling a process running with uid 0 and no
capabilities at all, and /bin/sh were 0555, nothing prevents the
attacker from chmod()ing /bin/sh to 0755 and overwriting it. This makes
any attempts to change the file permissions rather pointless.

You don't even need to change permissions for root to be able to delete or change the contents of the directory.

Dick