A flaw in Windows XP has led to 10,000 machines falling victim to hack attacks, Microsoft has admitted.
The vulnerability was identified by a Google engineer last month and had been largely ignored by the cyber criminal community.
However, since then, it has gained the attention of the bad guys and thousands of attacks have been launched.
"At first, we only saw legitimate researchers testing innocuous proof-of-concepts," said Holly Stewart, senior program manager at Microsoft's malware protection centre, in a blog post.
"Then, early on June 15th, the first real public exploits emerged. Those initial exploits were targeted and fairly limited. In the past week, however, attacks have picked up and are no longer limited to specific geographies or targets, and we would like to ensure that customers are aware of this broader distribution."
The malware allows hackers to download other malware programmes such as viruses and Trojans.
The largest number of attacks occurred on machines in Portugal, followed closely by Russia, but it has been known to affect machines in almost 20 countries.
Advice on how to fix the vulnerability can be found on Microsoft's security advisory.
