Microsoft has moved to protect millions of networks vulnerable to a .Net exploit discovered nine months ago.

Microsoft has tried to patch the exploit since its discovery by analyst group last October. The company has remained silent on the flaw which was uncovered in the US this week as a result of Patch Tuesday.

Security consultant and researcher at Matthew Strahan said a filename which contains a null byte in the .Net environment can launch a Null byte injection attack which allows servers to be fully compromised.

He said a flaw exists in an upload file code when the .Net CLR (Common Language Runtime) considers Null bytes as data to directly call a native C function call.

"The flaw could be very dangerous when affected servers are trying to receive uploaded files; a null byte will terminate strings in lower level layers but won't for strings in higher level layers," Strahan said.

"The attack means you can upload any code you want to take over the entire server.

"If you upload a .aspx file, followed by a Null byte and an extension such as .txt, it will be saved as a txt file. [Native function] calls at the injected Null byte allows a remote user to terminate a sting parameter which can lead to a compromise."

Strahan said there are five vulnerabilities that can be exploited under the same method, and warned .Net users to review their .Net frameworks and applications and to ensure they apply the patch for the exploit which was released earlier this week.

He said uploaded files can be sanitised by adding a file extension such as .txt or .doc.

"There are stacks of businesses that need to assess the vulnerability; the code is certainly not uncommon," Strahan said. reported close to 90 percent of Web sites upon which the company penetration tested in 2006 had "critical to urgent vulnerabilities"

Microsoft's Patch Tuesday release covered vulnerabilities in Microsoft's .Net Framework, Office Excel, Office Publisher, and three for its Windows operating systems.