The warning comes after a iPhone jailbreaking group called Comex claims to have developed Frash, an unauthorised version of Adobe's Flash player that runs on the Apple iPad, and is now being ported to the increasingly popular iPhone 4.
Barmak Meftah, Fortify's chief products officer, said that jailbreaking refers to the act of cracking a vendor's smartphone operating system to allow it to work with almost any mobile network.
"As you might imagine, it's frowned up on by the cellular carriers as it drives a steamroller through their handset subsidy schemes", he said.
"Whilst Frash may look attractive to iPhone 4 and 3GS users wanting to surf to extra websites, the reality is that to install this software, users will have to jailbreak their handsets, so allowing the loading of apps from almost any source", he added.
Everyone, he explained, loves free software for their mobile phone and, as a result, the normal guard that people have when viewing websites promising free software for their desktop or laptop computer tends to be lowered when it comes to their iPhone, or any other smartphone.
"Smartphone users to think very carefully before downloading an app for their handset from an untried or unknown source, no matter how attractive the free app looks", he said.
"You wouldn't be so silly to download a 'free version' of Microsoft Office from a Ukrainian website to your desktop PC, so why do the same for your smartphone? Think before you click and download", he added.