The gang responsible for a month-long plague of fake Mac security software has already updated their "scareware" to evade defenses Apple put in place late Tuesday, a security company confirmed today.

"Apple's [antivirus] did not detect the new sample," said Peter James, a spokesman for Intego, a French firm that develops Mac security software.

According to James, the new malware file is identified as "mdinstall.pkg" and if installed, plants the phony MacGuard software on the victim's Mac.

Intego confirmed what ZDNet blogger Ed Bott reported earlier Wednesday, that the scammers had created a new version that wasn't detected by Apple's new defenses.

"This isn't surprising, that there's a new variant out almost as soon as Apple released its security update Tuesday," said James. "[The attackers] are following the news, they're efficient."

On Tuesday, Apple released an update for Mac OS X 10.6, aka Snow Leopard, that warns users that they've downloaded fake Mac security software and scrubs machines already infected with the scareware.

Also called "rogueware," scareware is bogus security software that claims a computer is heavily infected with worms, viruses and other malware. Once installed, the worthless program nags users with pervasive pop-ups and fake alerts until they fork over a fee. MacDefender, the generic name for the new series that targets Macs, demands $60 to $80 to cease and desist.

Intego first reported MacDefender in early May, but since then several variants have appeared, including MacGuard, which installs without requiring a password.

The new MacDefender appeared only hours after Apple issued its update yesterday, said James.

"They'll keep doing this," James predicted, referring to the cat-and-mouse game between the hackers who create MacDefender and Apple's efforts to block them.

"It's worked well enough that they're making money off it, or they wouldn't be doing this. Coming out with a new version right after the update, and a new version as soon as Apple announced it was going to release an update, shows that they're planning on staying on top of things," James said.

The move would not have surprised another researcher.

In a Tuesday interview, Chet Wisniewski, a security researcher with U.K.-based Sophos said Apple's update would either accelerate the race or the hackers would give up.

"If they keep doing [MacDefender], it implies they are succeeding at making money," said Wisniewski. "If Apple is able to stymie [MacDefender] 90% of the time or better, does that mean the bad guys will give up or up the ante?"

It seems that they've done the latter, which in James' eyes, wasn't that difficult.

"Apple's defense is signature-based," said James, talking about the antivirus tactic that relies on individual "fingerprints" of each piece of malware for detection. "Part of the new variant's code is the same, but part of it is different."

It's different enough that Apple's MacDefender signature wasn't able to spot the new version.

"This is why you need detection that's not based on a single method," James argued, as he pitched Intego's VirusBarrier X6 antivirus software, which like most security software uses generic signatures able to detect minor variations in malware code without requiring a new fingerprint.

The update that Apple pushed to Snow Leopard users Tuesday also increased the frequency with which the operating system checks for new definitions: By default Mac OS X 10.6 will now look for new malware signatures daily.

How fast Apple reacts to the new MacDefender will show the company's commitment to stamping out scareware, said James.

"We don't know how reactive Apple will be," said James. "This will be the real litmus test.... How long is it going to take Apple to update [Snow Leopard with] a new signature?"

The timing of the newest version's release suggested that Microsoft, which previously linked MacDefender with a group responsible for a fast-spreading Windows fake antivirus scam, was on the right track. Microsoft had pointed to evidence that the gang was based in Russia.

"It suggests that they're not in the States," said James, noting that Apple updated Snow Leopard around 6 p.m. ET Tuesday, or midnight in France, where Intego is headquartered, and about 2 a.m. Wednesday in Moscow.

"They would have had a full day to get this up," James said of the new version's appearance in the early evening today, Moscow time. "It makes more sense that they're on this side of the Atlantic."

Apple's update was offered only to customers running Snow Leopard; Macs powered by the older Mac OS X 10.5, known as Leopard, will not receive the same anti-MacDefender protections.

According to Web metrics company Net Applications, nearly a third of Mac users -- 31% to be exact -- run a version of Mac OS other than Snow Leopard.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@computerworld.com.