A flaw in the software used to remotely access computers running Windows XP could leave users vulnerable to a denial of service attack.

The vulnerability does not allow attackers to gain control of Windows systems, but it could be used to cause affected computers to crash repeatedly. This would be done by creating specially crafted messages using the Remote Desktop Protocol (RDP), which is used by Windows' Terminal Services and Remote Desktop Sharing features, according to Microsoft.

Microsoft is advising users to either block the port that uses RDP (port 3389) or to disable the remote access features that use them. Terminal Services is used by the Windows 2000 and Windows Server 2003 operating systems. Remote Desktop Sharing is a feature of Windows XP.

As far as Microsoft knows, the vulnerability has not yet been used in an attack, and it is rated as "moderately critical" by Danish security vendor Secunia.

Microsoft will issue a patch for the vulnerability, but has not yet decided whether it will be part of the company's next scheduled group of security patches, scheduled for 9 August, a Microsoft spokeswoman said.