Georgi Guninski, a well-known Bulgarian bug hunter, on Monday posted a security advisory on his Web site. Guninski ranks the problem as high risk.
Malicious Web masters can exploit so-called ".chm" files, a compressed help file format, to execute arbitrary programs on a user's computer, Guninski said. The bug also allows viewing of temporary Internet files stored on the user's hard drive.
Guninski said he reported a similar vulnerability to Microsoft "some time ago". The software was fixed by allowing .chm files to run programs only if the .chm was loaded from the local system. Guninski says this is no longer a limitation. He discovered a way Web masters could access temporary Internet files on other users' machines. Explorer saves pages in a folder called "temporary Internet files."
"It is possible to find the temporary Internet files folder," Guninski said.
Explorer creates several of these folders on a PC using random names. With a special HTML (Hypertext Markup Language) document, a malevolent Web master can reveal the name and location of a temporary Internet files folder.
Once a folder name is known it is possible to cache a .chm file in any folder and execute it, Guninski said.
Until Microsoft has issued a patch for the problem users can protect their computer by disabling active scripting, a browser function that has repeatedly been associated with security issues.
Affected programs are Microsoft's Internet Explorer versions 5 and higher. Because Explorer is integrated in Microsoft's e-mail clients Outlook and Outlook Express are also affected. Other versions may also be unsafe, but those have not been tested, Guninski said in his advisory.
Microsoft could not be immediately reached for comment.
Georgi Guninski is at http://www.guninski.com/