sudo -- Potential bypass of Runas user restrictions

Todd C. Miller reports:

When sudo is configured to allow a user to run commands as an
arbitrary user via the ALL keyword in a Runas specification, it
is possible to run commands as root by specifying the user ID -1
or 4294967295.
This can be used by a user with sufficient sudo privileges to
run commands as root even if the Runas specification explicitly
disallows root access as long as the ALL keyword is listed first
in the Runas specification.
Log entries for commands run this way will list the target user
as 4294967295 instead of root. In addition, PAM session modules
will not be run for the command.