can not be 100% true as far as my work since last 10 years on various security solutions especially sandboxing (real-time) and inprivate filtering or incognito browsing (non-real-time)
part 1
Just consider a case
A person uses UC browser and downloads a malicious file on its SD card then sandboxing will not delete this file without user's permission.
Even if then your logic is not perfectly correct because please check the Nokia' official announcement about Nokia lumia 630 HERE
http
/www.nokia.com/gb-en/support/product/lumia630/userguidance/?action=singleTopic&topic=S-HARMFUL-CONTENT
where point no.4 says that
"Install antivirus and other security software on your device and any connected computer. Only use one antivirus app at a time. Using more may affect performance and operation of the device and/or computer"
Part 2
on some of the forums folllowing are the points
1)you have an operating system that cannot be overwritten by an external program, and also nothing can be installed without your knowledge, so you are quite safe from viruses
2) Lumia phones can only get apps from the Windows Phone Store, which means that everything you install has been checked before we allowed it to be published, and this is why it is the most secure platform on the market.
This is a perfect example of intellectual bankruptcy because here, this person is completely forgetting the interaction of his Lumia device with the whole, huge sea of WWW i.e. internet using any web browser
But still i can not answer one fact of yours
"Finally there is no such software for windows phone 8."