Both programs that MicroB is comprised of (browser and browserd), run as the user user. For the two points above to be possible, a vulnerability would need to be exploited in the kernel or some other software. This may be possible indirectly through some other MircoB exploit, otherwise MicroB itself is completely safe from these.

As has already been mentioned, the main threat comes from MitM attacks but the problem is not only limited to wireless networks. Given these vulnerabilities in MicroB, I'd assume that pretty much all communications including passwords and other sensitive data are being intercepted. Global surveillance programmes have been well documented.