Apple on Monday released an update for Mac OS X Leopard that fixes almost 40 issues related to the operating system.

The update improves Safari compatibility with Web proxy servers, improved reliability of Time Machine with Time Capsule and fixed a bug that could cause Time Machine to state the backup volume could not be found.

The update also improves printing under CS3 and improves printing for users of USB-based Brother and Canon printers.

A big change has been made for users of Apple’s MobileMe syncing service. According to Apple, contacts, calendars, and bookmarks on a Mac automatically sync within a minute of the change being made on the computer, another device, or the web at me.com.

Apple’s Mail app fixes an issue that could prevent Mail from quitting and improves reliability when printing PDF attachments.

An iChat issues that caused pasting text from Microsoft Office has been resolved. A big that could cause an encryption alert to appear in the chat window has also been fixed.

The update includes a critical update for Adobe’s Flash Player, fixing bugs that were disclosed last month. (Apple also tackled that problem in \a separate security update for Tiger users.)

Mac OS X 10.5.6 also includes patches for several Mac OS libraries, the operating system kernel, and system utilities such as the BOM (Bill of Materials) 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.

Mac OS X Leopard 10.5.6 can be downloaded from the software update mechanism in Mac OS X or from the company’s Web site.

Robert McMillan of IDG News Service contributed to this report.

Updated at 12:17 p.m. to include information about the update for Adobe’s Flash Player from IDG News Service.