I was mucking around with my ZTE a while back and found a few vulnerabilties. Reported to the manufacturer via Telstra, no fixes appear to be planned for release so there you go.
Note also that by default, the MF91 (at least the Telstra version) uses a easily idenitifiable wireless network key when in its default state. The WPA2 key is always 8847[last four octets of the device's MAC address], so if your MF91 has a MAC address of F4:D6:E2:DB:76:D8, then the default wireless key will be 8847E2DB76D8 (credit to Topaz Aral for discovering this).