The Computer Emergency Response Team (CERT/CC) on Monday issued an advisory for a buffer overflow vulnerability in Microsoft IIS 5.0 running on Microsoft Windows 2000, warning sysadmins that an exploit is already circulating.

Microsoft issued a "critical" rating on the flaw and issued a patch while warning that the vulnerability may allow a remote attacker to run arbitrary code on an infected machine.

"An exploit is publicly available for this vulnerability, which increases the urgency that system administrators apply a patch," the Center warned. IIS 5.0 is installed and running by default on Microsoft Windows 2000 server products.

CERT/CC said the unchecked buffer was detected in a Windows component of the World Wide Web Distributed Authoring and Versioning (WebDAV) protocol, which is supported by Windows 2000. An attacker could send a specially formed HTTP request to a machine running IIS, causing the server to fail or to execute code of the attacker's choice.

WebDAV uses IIS to pass requests to and from Windows 2000. Microsoft explained that when IIS receives a WebDAV request, it typically processes the request and then acts on it. However, if the request is formed in a particular way, a buffer overrun can result because one of the Windows components called by WebDAV does not correctly check parameters.