Apple has released a security update for its Mac OS X operating system, fixing a critical Internet security flaw that the company had failed to properly patch in late July.

The Mac OS X v. 10.5.5 security update was released Monday, fixing security bugs in Apple's software as well as several open-source components that ship with the operating system. In all, more than 25 bugs have been patched.

But the Internet flaw, which has to do with the Domain Name System (DNS), is the most widely publicized issue.

Apple, like many other operating-system vendors, was forced to patch its DNS software after security researcher Dan Kaminsky discovered a fundamental bug in the way this type of software is built.

On July 31, Apple had attempted to patch the flaw in Mac OS X, but security experts quickly discovered that while Apple's bug fix worked on the server side, it did not fix the issue on the client software.

With Monday's patch, Apple has fixed a flaw in the Mac OS X Libresolv DNS software that could have allowed attackers to trick victims into visiting malicious Web sites using what's known as a cache poisoning attack, said Andrew Storms, director of security operations with security vendor nCircle.

Libresolv is maintained by the Internet Systems Consortium (ISC). Although ISC had patched Libresolv by the time of Apple's last security update, the company did not include this bug fix in its July security update, Storms said.

After testing the 10.5.5 update Monday, he said that the Mac OS X client is now doing the required address port randomization that was added in ISC's bug fix. This is needed to make a cache poisoning attack much more difficult to pull off.

Also patched Monday were common Mac OS components such as Finder, Time Machine and the Mac OS kernel, as well as open-source components including Ruby ClamAV and OpenSSH.

At least nine of the patches fix flaws that could possibly be exploited by attackers to run unauthorized software on a victim's computer.