How secure is iOS?

The only legitimate way of using the iPhone with a forgotten passcode is to erase its content and restore the device from a backup.

Forgot passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

I bet there're some tricks that make it possible to bypass this security feature, however, I am not sure they can be officially used by the police or any other organisation.

In any case, we are not allowed to discuss such things at this forum.

There is nothing being "bypassed" here. If a passcode is forgotten then, yes, that can be solved by restoring the phone. But the phone is still useless to anyone who does not know the password of the iCloud account it was activated with.

The topic starter asked about the passcode, not about iCloud account.

As to the iCloud account, you are absolutely correct.

Yes...that's my point. Forgetting the passcode can be solved by restoring the device Given the numbers of people posting here who seem unable to keep this piece of information in their heads, this is just as well. But your content and the useability of the phone is still protected by your iCloud account password if your phone should fall into the wrong hands. So no compromising of secuirity is taking place.

Well OK....but it is part of the answer to your question. But the simple answer to your question is - yes - the passcode can be bypassed by restoring, but the phone would still be useless to anyone who didn't know the password of "that which cannot be mentioned", which was used to activate the phone!

The reason AKRBTN brought up iCloud is that, if you're concerned about someone restoring the iPhone and getting access to the device, that's where iCloud's Find My iPhone feature comes in. Find My iPhone includes a feature called Activation Lock, which is turned on automatically when you set up Find My iPhone. With Activation Lock, your Apple ID and password are required before anyone can erase and reactivate your device. Without the password for your iCloud's Activation Lock, the phone becomes a paperweight and no one, not even the NSA, can change that.

iCloud: Activation Lock