Sony is the problem, not Mediatek. Sony made the decision to lock the bootloader on this device, all other MTK devices comes with unlocked bootloader.
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I emailed Sony many times but every time they send me back that they don't have any information about it because it's not in the US market and one time one of them seid to me that the xperia c would have the locked bootloader forever and Sony doesn't even think about to unlock the bootloader but it's on the 4.3 update in next winter.
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