Attack code raises Windows DNS zero-day risk

At least four exploits for the vulnerability in the Windows domain name system, or DNS, service were published on the Internet over the weekend, Symantec said in an alert Monday. In response, the Cupertino, Calif., company raised its ThreatCon to level 2, which means an increase in attacks is expected.

The security vulnerability affects Windows 2000 Server and Windows Server 2003. Microsoft last week warned that it had already heard of a "limited attack" exploiting the flaw. However, exploit code wasn't yet publicly available. Exploits may help miscreants craft malicious code that uses the vulnerability to compromise Windows systems.

Microsoft continues to work on a fix for the problem, and attacks are still limited, Christopher Budd, a Microsoft Security Response Center staffer, wrote on a corporate blog Sunday.

"Attacks are still limited. We are aware though of public disclosure of proof-of-concept code to exploit the vulnerability," Budd wrote. Microsoft urges users of the vulnerable systems to apply the workarounds it has suggested.

McAfee on Monday afternoon said it had spotted a variant of Nirbot that appears to exploit the DNS vulnerability. Nirbot is a typical botworm that gives an attacker full control over an infected computer via an Internet Relay Chat channel.

"An attacker can gain control over the compromised computer and use it to send spam, install adware or launch a DDos attack on internet systems," according to McAfee's description of the pest. There are multiple versions of the Nirbot family, which is also known as Rinbot.

The attacks on the DNS service happen when someone sends rigged data to it. The service is meant to help map text-based Internet addresses to numeric Internet Protocol addresses. The vulnerability affects the DNS RPC interface. RPC, or Remote Procedure Call, is a protocol used by applications to send requests across a network.

The vulnerability is not exploitable over the standard DNS ports TCP/UDP 53, according to Microsoft. The RPC Interface is typically bound to network ports between 1024 and 5000, Symantec said. This mitigates the risk, according to the SANS Internet Storm Center, which tracks network threats.

"Networks obliging to basic secure perimeter design would only allow port 53 UDP/TCP to the authoritative DNS servers, and definitely not the additional RPC ports required for exploitation," a SANS ISC staffer wrote on the organization's blog Monday.

Still, the issue is significant, according to SANS ISC. Web hosting companies may run various network services on a single server, and Active Directory servers often also run DNS and may be exposed, according to the blog post.

The DNS flaw does not affect Windows XP or Windows Vista. Windows 2000 Server Service Pack 4, Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2 are vulnerable, Microsoft said.