Apple’s security technologies for Mac OS X may still miss iWorm, a piece of malware discovered in late September that infected thousands of computers.

Apple released an update for its XProtect antivirus engine to detect iWorm, but the update only detects when iWorm’s installer is launched, which is a one-time operation, said Patrick Wardle, director of research with Synack, a computer security company based in Redwood City, California. He wrote a paper describing his findings.

It means that computers already infected with iWorm before the update would still be compromised.

Apple “released a signature, but it doesn’t address the problem,” Wardle said in a phone interview Tuesday. “Unless the user has another antivirus product installed that has a correct signature, those infections aren’t going to go away.”

iWorm, which is a backdoor that can steal data from a computer, infected more than 18,000 machines, according to security company Dr. Web. It does not exploit any vulnerabilities on Mac OS X but instead relies on tricking people to install it.

The malware was found wrapped into pirated copies of Adobe Systems’ Photoshop and Illustrator applications, Parallels Desktop and Microsoft Office for Mac software offered on The Pirate Bay, the infamous search engine for content shared using the BitTorrent peer-to-peer network.

Why Apple only released an update—known as a “signature”—for iWorm’s installer is unclear. Apple officials couldn’t immediately be reached for comment.

Wardle contends that’s dangerous since another Apple security technology, called “Gatekeeper,” can also fail to stop iWorm in some scenarios.

When a person downloads an application, Gatekeeper checks if it has a digital signature that indicates it comes from Apple’s Store or if it has an approved developer’s certificate. If it has neither, Gatekeeper warns that the application could pose a security risk, although users can choose to run it.

But only certain applications, including Safari, Firefox and Chrome, will flag files—known as a “quarantine attribute”—for Gatekeeper to check. If someone downloads a file using uTorrent, a popular client for downloading torrents from The Pirate Bay, it isn’t programmed to flag files for inspection by Gatekeeper, Wardle said.

Wardle said he hasn’t contacted Apple with his findings, but he said Apple is likely aware of the Gatekeeper’s weaknesses, as it appears the way it works was a conscious design decision.

Ultimately, it means that malware authors will still be able to take advantage of the method iWorm uses.

“Unfortunately, it [iWorm] is able to bypass Apple’s malware mitigations really easily,” Wardle said. “It illustrates that malware on OS X is a problem. It’s not that Macs are immune to malware.”