Symantec raised its Threat Con security alert level from one to two because of the attacks, with two denoting "increased alertness." But other vendors, including Arbor Networks and McAfee, said they were seeing no such activity.

The attacks spotted by Symantec target a in the Windows Server Service that Microsoft says could be exploited to create a self-copying worm attack. Late last month, Microsoft took the unusual step of rushing out an emergency patch for the bug after it saw a small number of online attacks that took advantage of it.

Since then, security experts and have said that the attacks have not been widespread, but that may now be changing, according to Symantec.

The security vendor said it had seen a "dramatic rise" in attacks targeting TCP (Transmission Control Protocol) port 445. A TCP port is a number assigned to packets of data sent over the Internet to help computers know what program should be processing the information. Web browsers, for example, typically use port 80. Port 445 is one of two ports used to connect with the Windows Server Service.

This activity "appears to be related to the exploitation" of the Windows Server bug, Symantec said in a note on its