OVERVIEW A flaw in Microsoft Internet Explorer allows a malicious website to spoof file extensions in the download dialog to make an executable program file look like a text, image, audio, or any other file. If the user chooses to open the file from its current location, the executable program will be run, circumventing Security Warning dialogs, and the attacker could gain control over the user's system. A piece of HTML can be used to cause a normal download dialog to pop up. The dialog would prompt the user to choose whether he/she wants to "open this file from its current location" or "save this file to disk". The file name and extension may be anything the malicious website administrator (or a user having access there) wishes, e.g. README.TXT, index.html, or sample.wav. If the user chooses the first alternative, "open the file from its current location", an .EXE application is actually run without any further dialogs. This happens even if downloading a normal .EXE file from the server causes a Security Warning dialog. The user has no way of detecting that the file is really an .EXE program and not a text, html, or other harmless file. The program could quietly backdoor or infect the user's system, and then pop up a window which does what the user expected, ie. show a text document or play an audio file. No active scripting is necessary in order to exploit the flaw. The malicious website can be refered e.g. in an iframe, in a normal link, or by javascript. DETAILS The flaw is in the way Internet Explorer processes certain kind of URLs and HTTP headers. No further technical details are disclosed this time, as there is no proper workaround and the vulnerability could be relatively easily and unnoticeably exploited to spread virii, install DDoS zombies or backdoors, format harddisks, and so on. The flaw has been successfully exploited with Internet Explorer 5.5 and 6. An IE5 with the latest updates shows the spoofed file name and extension without a sign of EXE, and issue no Security Warning dialog after the file download dialog. Internet Explorer 6 is exploitable in a slightly different way, but the effect is the same. The user gets a download dialog with the spoofed file name and extension, and can choose between "Open" and "Save". Opening the file causes the program to be run. Older versions such as IE5.0 behave somewhat differently. The dialog indicates the user is about to execute an application; the dialog has the word "execute" instead of "open", and a Security Warning dialog appears after choosing "execute". It still shows the spoofed file name and extension instead of "EXE". Any way to skip all dialogs, ie. to run an application without ANY dialog with this vulnerability has NOT been found. In all variations of the exploit there is always the normal file download dialog, but the following Security Warning dialog is skipped. Technical details of the vulnerability will be revealed later. WORKAROUNDS Opening a file type previously considered safe, e.g. plain text or HTML file isn't safe with IE. Users of the browser should avoid opening files directly and save them to disk instead (if opening them is necessary at all). If this flaw is being exploited, the file save dialog will reveal that the file is actually an executable program. Dealing with files from an untrusted source isn't advisable anyway. Another workaround is switching to another browser such as Opera or Netscape which don't seem to have this vulnerability. VENDOR STATUS Microsoft was contacted on November 19th. The company doesn't currently consider this is a vulnerability; they say that the trust decision should be based on the file source and not type. The origin of the file, ie. the web server's hostname can't be spoofed with this flaw. It's not known whether a patch is going to be produced. Microsoft is currently investigating the issue. -- Jouko Pynnonen Online Solutions Ltd Secure your Linux - joukoat_private http://www.solutions.fi http://www.secmod.com
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