Network administrators were busy on Tuesday as Microsoft released its largest collection of security patches in\u00a0more than\u00a0a year.The monthly security update includes 21 vulnerabilities on 12 updates, the most since February 2005, said Jonathan Bitle, product manager with Qualys.The update is so large because hackers are exploiting "client side" weaknesses instead of automated services that run in an operating system. That type of attack relies on PC users\u2019 tendency to open e-mail attachments and other files from unknown senders."It\u2019s a who\u2019s who of what applications are installed on an end-user PC, from Internet Explorer to PowerPoint to Word to Media Player," Bitle said.The risk is much greater than a virus sending e-mail to all the names in a user\u2019s address book. Instead, 19 of the patches correct problems that allow "remote code executions," the programmers\u2019 term for a hacker\u2019s program that can gain full control over a user\u2019s PC.With such control, a hacker could steal or corrupt data, or even use the host computer to launch additional attacks on other networks.The sole consolation is that hackers cannot exploit most of those weaknesses unless a user opens an infected file, such as a PowerPoint slideshow, Word document or Media Player picture, said Amol Sarwate, manager of the vulnerability research lab at Qualys.Still, system administrators must install all 21 patches, he warned. "You can\u2019t rely on end users not going to a malicious website or not opening an e-mail attachment."These client-side vulnerabilities also contain a host of lesser threats, said Oliver Friedrichs, director of Symantec Security Response.A malicious website can easily install crimeware, spyware or adware on a visitor\u2019s PC.So, the Microsoft security update focuses on four main areas: the Internet Explorer Web browser, Outlook Express, PowerPoint and Windows Media Player.Many of these vulnerabilities can execute without a user even opening the infected file, so Symantec recommended that IT administrators should implement their top security practices, back up sensitive data and remind users to avoid opening unexpected e-mail attachments or following Web links from unknown sources.Likewise, consumers should run Windows Update and install all the latest security updates, and use security software, Symantec said.Microsoft\u2019s security bulletin can be found here.This article is posted on our Microsoft Informer page.\u00a0For more news on the Redmond, Wash.-based powerhouse, keep checking in.Check out our CIO News Alerts and Tech Informer pages for more updated news coverage.