In the first versions of BLE specifications, there was a huge security flaw during the pairing allowing an attacker to perform a MITM attack. Since Bluetooth 4.2, the pairing use ECDH, so there isn't left much to worry (I guess...). I search for other security flaws, but even if I often read that Bluetooth (so BLE too) is not secure, I didn't find anything really interesting with the latests versions of the protocol.
There are some issue regarding privacy (for example), but is it still the case for the security?
