And exploitation happened, all right, on a scale that hadn't been seen before.

On the afternoon of Friday, July 13, 2001, security engineers at eEye Digital Security received reports of a worm that was spreading rapidly through its customers' networks. Fueled by a limited edition, crimson, caffeinated, high-fructose corn syrup-based beverage, Mark Maiffret and Ryan Permeh spent a weekend reverse-engineering the worm, and alerted the world to its presence.

What the worm did was probe vulnerable IIS servers, infect them, and create 100 threads of itself, which then spread to other computers. If the date was between the 20th of the month and the end of the month, it would attempt to spew data at www.whitehouse.gov. Permeh and Maiffret estimated that the worm could infect approximately 500,000 unique IP addresses per day.

Upshot: Code Red really drove home the importance of patching bugs soon after Microsoft released the patch, because the patches themselves give malware authors clues to exactly where they should look for new vulnerabilities.

Fastest infection. Ever.
Bug identifier: MS02-039
Description: Buffer overruns in SQL Server 2000 Resolution Service could enable remote code execution
Alias: The SQL Slammer bug
Date published: July 24, 2002

While technically not an OS bug, the SQL Slammer bug deserves honorary mention due to the sheer velocity with which vulnerable systems were infected. The bug targeted Microsoft's database server. Vulnerable computers were subject to buffer overflows that, if properly crafted, could place commands into memory to cause the targeted system to execute those commands with the permissions of the database service.

Patching was complicated by the fact that admins needed to run an earlier patch before they could run the MS02-039 fix. The bug affected primarily corporate server systems, but also affected home users who had MSDE (Microsoft SQL Server Desktop Engine) installed. That made a number of home users, some of whom didn't even know they had MSDE on their machines, unwitting participants in the carnage to come.

Because the Slammer worm primarily targeted servers running databases, it didn't infect millions of machines. It did, however, spread rapidly—so rapidly, in fact, that it had infected roughly 9 out of 10 vulnerable machines within 10 minutes of being released on Jan. 25, 2003. The entire worm was only 376 bytes, and fit into a single packet of data.

The MS02-039 bug was "one of the biggest oversights of all time," says Steve Manzuik, senior manager of security research at Juniper Networks, "not because it was an easy or obvious bug to find—it wasn't."