7a69Adv#17 - Internet Explorer FTP download path disclosure

From: Albert Puigsech Galicia (ripe_at_7a69ezine.org)
Date: 12/30/04

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    Date: Thu, 30 Dec 2004 15:56:41 +0100
To: bugtraq@securityfocus.com

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    - ------------------------------------------------------------------
           7a69ezine Advisories 7a69Adv#17
    - ------------------------------------------------------------------
      http://www.7a69ezine.org [23/12/2004]
    - ------------------------------------------------------------------

    Title: Internet Explorer FTP download path disclosure

    Author: Albert Puigsech Galicia - <ripe@7a69ezine.org>

    Software: Microsoft Internet Explorer

    Versions: >= 6.0.3790.0

    Remote: yes

    Exploit: yes

    Severity: Medium-High

    - ------------------------------------------------------------------

    I. Introduction.

     Internet Explorer is a well-known HTTP browser, and like others it can use
    more protocols, for example FTP. The security historial of this navigator is
    really cool and we are glad for the excelent work done by Microsoft. We love
    your (in)security features.

    II. Description.

     When you save a file from an FTP server to a local folder it is saved on
    'local_folder/file_name', consequently if the name of the file contains
    '../', the real destination of the file changes. Despite it is imposible to
    create a file with '../' characters an FTP server can reply a LIST request
    with this char on filenames, so a malicious FTP server can modify the folder
    where the downloaded file will be stored.

     Of the three possible ways of downloading a file only two are affected by
    this bug; the left-click and save-as procedure and dragging the file. The one
    that is unaffected is the double-click procedure.

     
    III. Exploit

     You can use the very-tiny and malicious FTP server attached in this mail to
    check the vulnerability. It's easy to use './ftpd-iexpl localfile
    remotefile', where localfile is a file on the FTP server and localfile is the
    path where the will be stored on the attacked system. If you try to overwrite
    a file the Internet Explorer show a confirmation message, so is better for
    explotation purposes to create new files, but It's not a problem to execute
    arbitrari code because you can create, for example, startup files on 'C:
    \Documents and settings\All Users\Start menu\Programs\Start' that will be
    executed after next user login.

     However Internet Explorer shows the complete name of the file, including the
    '../' characters so It's unlikely that it will not arise suspicions, but you
    can put the malicious file into a FTP folder and waiting for victim dragging
    the folder.

    IV. Patch

     Don't use Internet Explorer and turn to Firefox world.

    V. Timeline

    06/12/2004 - Bug discovered
    23/12/2004 - Advisor released
    25/12/2004 - Noel; uoh! ouh! ouh!

    VI. Extra data

     You can find more 7a69ezine advisories on this following link:

        http://www.7a69ezine.org/avisos/propios [spanish info]

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