A scheduled denial of service attack against Microsoft's main software update Web site did not materialise on Saturday, as computers infected with the W32.Blaster worm failed to find their target.
Blaster first appeared on last Monday and quickly spread to computers worldwide by exploiting a known security vulnerability in Microsoft's Windows operating system.
By Friday, the worm, which targets a Windows component for handling RPC (Remote Procedure Call) protocol traffic called the Distributed Component Object Model (DCOM) interface, spread to more than 423,000 systems, according to Symantec's Oliver Friedrichs.
In addition to infecting vulnerable Windows machines, Blaster worm was programmed to launch a denial of service (DOS) attack against windowsupdate.com, an Internet domain owned by Microsoft and used to distribute software updates to Windows customers beginning on Saturday.
However, an error in Blaster's design combined with last minute actions by Microsoft to change the registration of windowsupdate.com cut short that attack.
Blaster's author provided the incorrect domain address for windowsupdate. The address specified in the worm's code, windowsupdate.com, simply forwards users to the actual Windows update site, windowsupdate.microsoft.com, according to Mikko Hyppönen, head of antivirus research at F-Secure in Helsinki.
On Thursday, Microsoft delisted the windowsupdate.com domain name, calling it a "non-essential address".
That solution also removed the threat of collateral damage from the attack, because requests for windowsupdate.com would never leave infected machines, slowing down the Internet, according to Sean Sundwall, a Microsoft spokesman.
On Saturday, Microsoft didn't detect any irregular network activity associated with the Blaster worm, Sundwall said.
The SANS Institute's Internet Storm Centre reported that the DOS attack anticipated from Blaster had been avoided.
The number of Blaster infections was also down more than 80 per cent since the worm's peak last Monday, indicating that vulnerable computers were being cleaned and patched by their owners, Friedrichs said.
Symantec expected that trend to continue, he said.
In time, Blaster would join predecessors like the Code Red and NIMDA worms, inhabiting a small population of infected machines that posed a risk to new, unpatched systems, but not spreading much beyond that, he said.
For Microsoft, the Blaster worm outbreak stoked internal efforts to shore up vulnerable services.
Even though Blaster missed its target, future worms might be smarter, faster and more destructive, Friedrichs said. Microsoft is using the occasion to take "a number of steps" to protect valuable customer services such as the windowsupdate.microsoft.com Web site from attack, Sundwall said.
He declined to provide details of what changes the company was making.
Microsoft was also using the Blaster outbreak as an impetus to improve communication with consumers about the need to patch regularly, Sundwall said.
"We learn from every worm out there," he said. "What we learned from [Blaster] is that there aren't enough consumers who installed the patch and use [Windows] autoupdate."
Microsoft customers should expect a concerted effort by the vendor to reach out in coming weeks and raise awareness of the need to patch vulnerable systems, he said.