Microsoft has released software patches fixing a handful of critical bugs in the Windows kernel, as well as flaws in the Windows Directory Name System and SChannel security software.

The software vendor rates the three Windows kernel bugs as critical because they could be used to create attack code that would run unauthorised software on a victim's machine. The SChannel and DNS bugs are given the less-scary rating of "important" by Microsoft, meaning the company doesn't think they're as likely to be used to take over a PC.

The Windows kernel update, MS09-006, should be the first one applied, according to Eric Schultze, chief technology officer with Shavlik Technologies. That's because it "could allow an attacker to take complete control of your computer if you view a website, email, or document that contains an evil graphic or picture," he said via email.

This update is rated critical for all versions of Windows.

Schultze also considers the four DNS vulnerabilities patched in the MS09-008 update as a top priority because they could be used in man-in-the-middle attacks, "to redirect Internet traffic to look-alike websites in hopes of gathering sensitive user information," Schultze said.

This kind of flaw was a top security concern last year after security researcher Dan Kaminsky showed how a bug in almost all of the world's DNS software could be exploited in this type of man-in-the-middle attack.

In fact, two of the DNS bugs relate to Kaminsky's attack, fixing Windows flaws that could help an attacker place inaccurate information on a DNS server. Both bugs would be hard to exploit in real-word scenarios, however, according to Matt Watchinski, senior director of vulnerability research with security vendor Sourcefire. "This is Microsoft being diligent," he said. "They found a few new ways to do a few of these [attacks]."

Watchinski agreed that the Windows kernel update should be applied first, saying that one of the bugs it fixes is probably easier to exploit than Microsoft realises. "We're pretty confident that consistent exploitation code is likely," he said.

Microsoft also patched a bug in the SChannel software used to create SSL (Secure Sockets Layer) connections on Windows systems. The bug could allow an attacker to spoof a digital certificate, Microsoft said.

The SANS Internet Storm Center has provided its own overview of Tuesday's patches, rating all of them critical.