Cybercrooks are using a yet-to-be-patched security flaw in certain Windows versions to attack computers running the operating systems, Microsoft warned late Thursday.

The attacks target Windows 2000 Server and Windows Server 2003 systems through a hole in the domain name system, or DNS, service, Microsoft said in a security advisory. The attacks happen by sending rigged data to the service, which by design is meant to help map text-based Internet addresses to numeric Internet Protocol addresses.

"An anonymous attacker could try to exploit the vulnerability by sending a specially crafted RPC packet to an affected system," Microsoft said in the advisory. RPC, or Remote Procedure Call, is a protocol that applications use to request services from programs on another computer in a network. RPC has been involved in several security bugs before, including in the vulnerability that let the Blaster worm spread.

The French Security Incident Response Team deems the Windows DNS vulnerability "critical," its highest rating.

The DNS and RPC warning comes days after Microsoft issued its April security patches. At the same time security experts have issued warnings on multiple zero-day flaws in Office and another one in Windows.

The latest vulnerability is a stack-based buffer overrun, Microsoft said. This is a common type of coding problem that has caused many headaches for Microsoft and Windows users. A successful attack will give full control over a vulnerable machine without any user interaction, Microsoft said.

There are "limited attacks" that exploit the issue, Microsoft said. The software maker said it is finishing a security update for Windows to repair the problem. Microsoft did not say when it plans to release the update. The company's next "Patch Tuesday" is on May 8, though if attacks increase a patch could be released out of that cycle.

While it works on the fix, Microsoft suggests several work-arounds for users of affected Windows versions. These include disabling remote management over RPC capability for DNS servers, blocking specific data ports using a firewall and enabling advanced filtering. Security firm Symantec on Thursday urged users to apply the work-arounds.

"Customers are advised to…apply the appropriate work-arounds as soon as possible, in the event that the attacks become more widespread," Symantec said in an alert sent to subscribers of its DeepSight security intelligence service.

Windows XP and Windows Vista are not impacted by the DNS flaw. Windows 2000 Server Service Pack 4, Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2 are vulnerable, Microsoft said.

Joris Evers