In further confirmation that Internet crooks tend to grab for the low-hanging fruit, a new Microsoft report reveals that the most common browser-based attacks tend to go after old software flaws. Making sure you've closed those holes can go a long way towards keeping your PC safe.

Browser-based exploits form the basis for some of the sneakiest and most dangerous attacks out there today. Crooks insert hidden attack code on a hijacked Web site that searches for a software vulnerability whenever anyone views the poisoned site. If the attack code finds a flaw, it will attempt to surreptitiously download and install a Trojan or other malicious software. If an antivirus app doesn't manage to catch it, the malware gets installed with nary a clue for the hapless victim.

These drive-by-download attacks sometimes go after the latest software flaws, but as revealed by Microsoft's new Security Intelligence Report v7, most of the attacks against Windows XP go after old Windows and third-party software flaws going as far back as 2006. Of the top 10 attacks, only one was from 2009. That's good news, since it means that basic maintenance and security measures will go a long way towards keeping your PC safe.

These are the most common browser-based exploits, as determined from Microsoft's analysis of "data from customer-reported incidents, submissions of malicious code, and Microsoft Windows error reports."

Third-party software:

CVE-2008-2992 - flaw in Adobe Acrobat and ReaderCVE-2006-5198- WinZipCVE-2007-0015- QuickTimeCVE-2007-5659- Adobe Reader

Windows holes:

MS08-041- Microsoft Office Snapshot ViewerMS09-002- Internet ExplorerMS06-057- Internet ExplorerMS08-078- Internet ExplorerMS06-01- Microsoft Data Access ComponentsMS06-055- Microsoft VML

The names here tell when the flaw was discovered (MS06 = 2006, for example), and as you can see, crooks love the golden oldies. Many of these attacks probably go after pirated Windows installs that never get updates.

Enabling Automatic updates in Windows will guard from attacks against any of the listed Windows flaws, and to protect against the third-party software flaws, make sure you have the latest software versions available. For vulnerable older software such as the vulnerable, three-year-old WinZip, that might require a manual version check and update. Or you can take the easy route and use the free Secunia PSI software, which will scan your system for outdated vulnerable software and provide simple links to update it.

For Vista attacks, only one of the most common exploits listed went after a Windows flaw (Internet Explorer). The rest targeted third-party software such as Adobe Reader or RealPlayer, with old flaws again providing a common target. As with XP, running Automatic updates and Secunia PSI should safeguard any PC from the most common exploits.

Another good protection step is to apply the patch to turn off AutoRun for USB drives. As noted by the Washington Post, Microsoft's report also shows that some of the most common malware will infect thumb drives and wait to be connected to another PC. When that happens, the malware takes advantage of AutoRun to run automatically and attempt to infect the new PC.

A Microsoft patch - which doesn't distribute via Automatic Updates, per the Washington Post - will turn off AutoRun for USB drives and guard against this infection vector. You'll need to download and install this patch yourself.

And finally, for other simple security steps that can go a long way towards keeping you safe, see The Five Most Dangerous Security Myths.