wu-ftpd ftpaccess `restricted-uid'/`restricted-gid' directive may be bypassed

Glenn Stewart reports a bug in wu-ftpd’s ftpaccess
restricted-uid'/restricted-gid’ directives:

Users can get around the restriction to their home
directory by issuing a simple chmod command on their home
directory. On the next ftp log in, the user will have ‘/’
as their root directory.

Matt Zimmerman discovered that the cause of the bug was a
missing check for a restricted user within a code path that
is executed only when a certain error is encountered.