More than 600,000 Apple Inc. Mac computers were affected by a hacking attack, a sign that the once rarely targeted company is becoming a bigger focus for people intent on spreading malware, a security-research firm said.

The attack affects computers running Apple’s Mac OS X software, according to Russian antivirus software maker Doctor Web. Most of the infected computers are in the United States and Canada, the firm said. in a blog posting. Apple fixed a security hole this week that let the malicious software spread. Users who haven’t downloaded the necessary updates are vulnerable, Dr. Web said.

“This once again refutes claims by some experts that there are no cyber-threats to Mac OS X,” Doctor Web said. A message left for Tom Neumayr, a spokesman for Cupertino, California- based Apple, wasn’t immediately returned.