Stolen iPhone- can thief steal my data?

See if any of the following articles can help:

• iCloud: Lock and track your device using Lost Mode - Apple Support
• How can I track my phone even though its turned off? - ASC
• Lost or Stolen iPhone? Here's What to Do - MUO
• How to use Find My iPhone to rescue your iPhone, iPad, Mac, and AirPods - iMore

If you have Find My iPhone enabled on this phone, it should be difficult, if not impossible, for someone to access it.

Ref: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

I recently lost my iPad and wonder the same thing. I had find my iPhone on the iPad turned on and it has not been online since then. I too wonder if they can turn it on and access data. I recently connected my iPhone 7 Plus to both my MacBook Pro and my PC. In both cases there was a pop up that required input before a connection could be made. I believe if they could connect either an iPhone or iPad some level of authorization would be necessary.

What I cannot understand is considering the high cost of Apple products why in the world don’t they maintain a database that would allow for the registration of an Apple device that was lost or stolen and is accessible by everyone to see. If just the product model number and serial number we visible and there were an option to report you found it or have it they might be returned. The owner could even post that they are offering a reward for its return thus encouraging a person to return it. It would be a positive marketing tool and would further brand loyalty. Instead, Apple gladly takes our money and then says sorry, can’t help you. The only issue with my idea is how do you obtain the model and serial number if you can’t get past the password. Oh well. I’m out a $700 iPad and Apple got to collect another $1200 for an iPad Pro. No wonder their the king of worldwide capitalism.

Very sorry to hear of your loss, however this is not a Apple issue. Call your cell provider they will have the phone’s IMEI and serial number, and the ability to lockdown a stolen phone. When locked down that phone’s IMEI number cannot be used for cell service any place in the world. Again, this is the responsibility of your carrier, not Apple; Apple makes phones carriers put them on their network.

Regardless of how strongly you think it is Apple’s responsibility, it is your carrier.

They would need your passcode to gain access to the phone.