Scareware makers have again changed their fake security software scam, while Apple issued the third signature update in as many days to combat the con.

The newest version of what's generically called "MacDefender" appeared Friday, according to a pair of security companies. The phony antivirus program now goes by the name "MacShield," the fifth title since the early-May appearance of the scheme.

Apple in turn released another signature update early Friday to XProtect, the bare bones anti-malware tool tucked into Mac OS X 10.6, aka Snow Leopard. According to logs on several Macs, Apple started pushing the update -- the third in the last three days -- a few minutes after midnight GMT, or around 7 p.m. Thursday ET.

The new signature was labeled "OSX.MacDefender.D" by Apple.

According to Peter James, a spokesman for the French antivirus firm Intego, Apple's MacDefender.D signature detects the MacShield variant, warning users after they've downloaded it and urging them to toss the file into the Trash.

Apple initially updated Snow Leopard on Tuesday with signatures to sniff out two previous versions of the "scareware" and to provide users a tool that scrubbed infected Macs of the phony software.

Also called "rogueware," scareware is bogus security software that claims a computer is heavily infected with worms, viruses, Trojan horses and the like. Once installed, the worthless program nags users with pervasive pop-ups and fake alerts until they fork over a fee. MacDefender, the first scareware to target Macs, demands $60 to $80 to stop bothering victims.

Intego first reported MacDefender in early May , but since then several variants have appeared, all with different names but only minor code changes.

Like the original MacDefender, MacShield uses the name of a legitimate Mac program.

Centurion Technologies of Fenton, Mo. sells a product called MacShield that restores hard drives to earlier states as a way for educational, enterprise and public sector IT managers to return abused or corrupted systems to working condition.

Both Intego and ESET, a San Diego, Calif. security vendor, blogged Friday about the new MacShield scareware.

The hackers responsible for creating MacDefender have reacted to every Apple signature update by tweaking their scareware, and re-releasing it into the wild. Users typically fall for the con after visiting poisoned links provided by Google Image searches, or by clicking on malicious Facebook links after being lured by promises of provocative videos.

A Mac cyber crime gang tweaked the interface of its month-old scareware, and changed the name to 'MacShield,' said security firms Friday. (Image: ESET.)