Source:
Two pieces of malicious software affecting Apple's Mac OS X appeared this week: a Trojan horse with the ability to download and install malicious code of an attacker's choice, and a hacker tool for creating backdoors, according to security vendors.

The Trojan — called 'OSX.RSPlug.D' by Intego, the Mac security specialist that discovered the threat — is a variant on an older piece of malicious code but with a new installer, Intego said.

"It is a downloader, and it contacts a remote server to download the files it installs," Intego said in an advisory. "This means that, in the future, the downloader may be able to install payloads [other] than the one it currently installs." . . .

OSX.Lamzev.A is a hacker tool designed primarily to allow attackers to install backdoors in a user's system, according to Intego. However, the company dismissed the tool as a serious threat because a potential hacker has to have physical access to a system to install the backdoor.

"Unlike true malware and Trojan horses, OSX.TrojanKit.Malez requires that a hacker already have access to a Mac in order to install the code," Intego stated.

Other antivirus vendors noted that Lamzev could be disguised as a piece of legitimate software and used to trick users into creating the backdoor themselves.

Lamzev is not related to RSPlug, despite several high-profile reports confounding the two, Intego emphasized. (Source)
Be careful out there. Upgrade your anti-virus and all that.