sudo -- Flaw in Runas group matching

Todd Miller reports:

Beginning with sudo version 1.7.0 it has been possible to grant
permission to run a command using a specified group via sudo -g
option (run as group). A flaw exists in the logic that matches
Runas groups in the sudoers file when the -u option is also
specified (run as user). This flaw results in a positive match for
the user specified via -u so long as the group specified via -g
is allowed by the sudoers file.
Exploitation of the flaw requires that Sudo be configured with
sudoers entries that contain a Runas group. Entries that do not
contain a Runas group, or only contain a Runas user are not
affected.