Microsoft has urged Windows users to update their operating system, saying it's now seen more than 25,000 attacks targeting one of the critical bugs fixed in July's monthly security patches.
Microsoft researchers tracked a "fairly large", spike in web-based attacks that exploit the problem over the past weekend, the company said in a blog posting. "As of midnight on July 12 (GMT), over 25,000 distinct computers in over 100 countries/regions have reported this attack attempt at least one time."
On the busiest single day, Microsoft researchers tracked more than 2,500 attacks, a small number considering Windows' massive user-base. Still, Microsoft and security experts are worried about this flaw because it's been publicly known for more than a month, and has shown up in real-world attacks.
Users in Russia are now the most-targeted, Microsoft said. They have accounted for 2 percent of all attacks, which translates to about 10 times the total number of attacks per computer as the worldwide average. Portugal is the number-two most-targeted region.
Successful attacks secretly install malicious software on the victim's machine, often a program called Obitel. Once Obitel is on a PC, it enables other malware to be loaded, such as malware that can log keystrokes, send spam, or perform other nefarious task.
Two weeks ago, the total number of attacks logged by Microsoft was 10,000.
Security experts say the flaw is being exploited in drive-by web attacks that are triggered by malicious code placed on hacked or malicious web sites, although it could also be triggered in other applications - email readers, for example - that can interact with Web pages.
To protect themselves from these attacks, Windows users need to install the MS10-042 update, released on Tuesday. It fixes a bug in the Windows Help and Support Center, which ships with Windows XP. Although this flaw also affects Windows Server 2003, Microsoft has only seen Windows XP attacks used by criminals.
More-recent versions of Windows such as Vista, Server 2008 and Windows 7 are not affected.