Faint brown dot whenever I'm using my iPhone 5's camera

There seems to be a faint brown dot whenever I'm using my iPhone's camera.

Nothing wrong with the front camera, just the back camera.

I've tried wiping the lens but to no avail. And I also checked if it's a screen protector issue but it's not.

It only happened recently (unless I didn't notice it before). But I looked through my photo album, the dot only started appearing a few weeks back.

Is it an internal hardware issue or something else?

Thank you so much.