"They have a lot of resources," said Rod Soto, principal security researcher in the company's business security unit.
For example, they were able to target a group of individuals congregating in an Asian country based on their communications, and then used man-in-the-middle and social engineering to trick users into installing the Xsser mobile remote access Trojan on their mobile devices.
The activity was first spotted in September.
Soto said he wasn't allowed to reveal any more information about that particular attack, but did say that the criminal group involved was highly sophisticated.
"This requires a lot of coordination and skills," he said.
The criminals were able to impersonate or bypass the official Google and Apple app stores, he said, used text messages and social media, and tampered with local Wifi access points.
Cell phone signals may have been intercepted as well.
In addition, by monitoring traffic the criminals would be able to see what users were searching for -- and then provide them with malware disguised as the searched-for item.
"If you want to download a game, they will give you an infected version of the game," he said. "It was very sophisticated and multi-vector the way these mobile phones were targeted and that is remarkable."
The criminals were specifically targeting iPhones that were jailbroken, and using the Cydia third-party app store, he added.
"By jailbreaking your phone, you are exposing yourself to these types of attacks," he warned. "When you go to third party app stores, they don't go through the security vetting, so it's easier for malicious actors to put rogue malware in there."
Previously, the Xsser Trojans only targeted Android devices.
Once installed, the malware embeds itself so that it cannot be deleted, the steals data from the phone and carries out other commands as directed by its command and control server. It could be used for surveillance, to launch distributed denial of service attacks, or to steal login credentials.
It is difficult to detect whether a phone has already been infected with Xsser mRat, Akamai warned, and suggested that prevention was a better bet.
In the threat advisory, the company recommended that users avoid free Internet hot spots, disable automatic WiFi connections in public places, use a virtual private network, distrust any unexpected communications or upgrade requests, install only applications from known and trusted sources, and use two-factor authentication when available.
Akamai also warned users not to jailbreak their phones -- according to the company, 14 percent of of China's 60 million iOS devices have been jailbroken.
Finally, the company suggested that users install mobile security applications that protect against malware.