Security firm Sophos has warned owners of the first- and second-generation iPhones that iOS 4.3 has left them out in the cold, protection-wise.

As was widely expected the newly-released iOS 4.3 does not support the original iPhone or the iPhone 3G, meaning that a number of security fixes included in the update are not available to those with older hardware.

The same applies to the first- and second-generation iPod touch and Sophos warned that many people would have to upgrade if they wanted full, official protection for their devices.

"If you have an earlier iPhone or iPod touch, your device is potentially vulnerable to attacks which could exploit these known security holes, and there is no official patch available for you to protect yourself," said Graham Cluley, senior technology consultant at Sophos.

"That's bad news for the very many people who still have an iPhone 3G, for instance. If you were looking for an excuse to upgrade your iPhone or iPod touch - maybe you've just been given a good one by Apple. But if you were happy with your iPhone 3G, I doubt you're feeling too good about having to reach into your pocket," Cluley continued.

Some of the security fixes in iOS 4.3 include protection against malicious TIFF image files and memory corruption issues in WebKit, which could allow to the execution of unauthorised code if you were to visit a compromised website.

Recent evidence has suggested that cybercriminals are starting to concentrate on targeting iOS devices over Windows.

Rogue apps have been to blame for some previous iPhone security breaches. However, Apple's App Store policy does make iOS a safer bet than Android where apps are concerned. Also, as Android is open source, hackers have a better chance of spotting vulnerabilities in the code, according to rival security firm Trend Micro.