will it be able to access data from Ubuntu and steal it
Yes, it can, e.g. if you use something like ext2fsd and have your Linux partitions mounted in Windows.
Even if you don't mount anything, advanced malware could read your disks directly and search for certain patterns in disk images and extract necessary files but I doubt such malware even exists.
Advanced malware may also modify UEFI firmware itself and UEFI system (boot) partition and stay resident even after Linux loads.
The bottom line is: malware under Windows can do everything you can do. If you're worried about the safety of your data in Linux make sure your Windows partition is secure as well. If you're paranoid enough, delete Windows altogether.