Users of Microsoft's Internet Information Services 6 web-server software are being warned that a new online attack could put their data at risk.

The US Computer Emergency Response Team has revealed that a flaw discovered last week is being exploited in online attacks. The vulnerability, discovered by security researcher Nikolaos Rangos, works by sending a specially crafted HTTP request to the server he was able to view and upload files on the machine. The attack takes advantage of a bug in the way that Microsoft's software processes Unicode tokens, he said.

In a statement, Microsoft said it hadn't heard of any such attacks, but that it was investigating the claims. "We are working on a security advisory to provide customers with guidance," said the company said the company.

The bug affects IIS 6 users who have enabled the WebDAV (Web-based Distributed Authoring and Versioning) protocols, used to share documents via the web.

It gives attackers a way to view protected files on the server without authorisation and it could be used to upload files as well, according to Thierry Zoller, an independent security researcher who confirmed Rangos' findings. However, Zoller said he had found no way to use this flaw to run unauthorised software on an IIS server.

Zoller said that IIS 5 and IIS 7 do not appear to be vulnerable to the attack, but that it could affect other Microsoft products that use the WebDAV technology. "Better safe than sorry," he said via instant message, "Disable WebDAV temporarily and wait for Microsoft to patch."

In an interview, Rangos said that even with WebDAV enabled, Exchange Server running on IIS 6 and SharePoint Server were not affected by the flaw.

Cisco sounded a similar warning. "Administrators of sites that are hosting sensitive information on IIS servers that use WebDAV are advised to put effective mitigations into place immediately because exploit code is publicly available," Cisco said in a security alert posted to its website.