Windows users should make sure their firewalls are properly configured so they don't fall victim to an attack by way of a new operating system flaw, Microsoft warned in an advisory Thursday. The software giant acknowledged that proof-of-concept exploit code is circulating. "Microsoft is aware of public reports of proof-of-concept code that seeks to exploit a possible vulnerability in Microsoft Windows 2000 Service Pack 4 (SP4) and in Microsoft Windows XP Service Pack 1 (SP1)," the company said. "This vulnerability could allow an attacker to levy a denial-of-service attack of limited duration." But the company stressed that the security hole isn't easy to exploit, saying, "On Windows XP Service Pack 1, an attacker must have valid logon credentials to try to exploit this vulnerability� [which] could not be exploited remotely by anonymous users." Danish vulnerability clearinghouse Secunia said the flaw, discovered by researcher Winny Thomas, "is caused due to a memory allocation error when handling UPnP GetDeviceList requests via RPC." The firm said this could be exploited to cause "services.exe" to consume a large amount of memory for a limited period of time.