Apple has released a major set of security patches for its Mac OS X operating system, fixing a number of critical flaws in the software.

The Mac OS X v10.5.6 update includes a critical update for Adobe Systems' Flash Player, fixing bugs that were disclosed last month. It also includes patches for several Mac OS libraries, the operating system kernel, and system utilities such as the BOM archiving software. In total, 21 bugs are patched in the update.

Many of the flaws could possibly be exploited to run unauthorized software on a victim's PC, but only the Adobe flaw had been publicly disclosed prior to Monday's patches, said Andrew Storms, director of security operations at nCircle Network Security.

The timing of Apple's patches is noteworthy, Storms said. That's because cybercriminals often like to launch attacks in December, when IT staff are on vacation and computer emergency response teams are often slower to respond to threats.

Over the weekend, Microsoft warned that it had seen a spike in attacks targeting an unpatched flaw in Internet Explorer, and security experts expect more such attacks as the holidays approach. "It's not a month we can let our guards down," he said.

The IDG News Service is a Network World affiliate.