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Home » Hacking News » Directory Traversal Vulnerabilities in FTP Clients

Directory Traversal Vulnerabilities in FTP Clients

by Nikola Strahija on December 11th, 2002 FTP clients, including those that may be embedded in web clients, can be vulnerable to certain directory traversal attacks by modified FTP servers.


Product: Multiple FTP and web clients
OS/Platform: Multiple
Vendor: Multiple
Vendor Status: Vendors informed individually and through CERT/CC
Risk: Medium. A malicious server could potentially overwrite
key files to cause a denial of service or, in some
cases, gain privileges by modifying executable files.
The risk is mitigated because non-default
configurations are primarily affected, and the user
must be convinced to access the malicious server.
However, web-based clients may be more easily exploited
using server-side vulnerabilities such as XSS.


FTP clients, including those that may be embedded in web clients, can
be vulnerable to certain directory traversal attacks by modified FTP
servers. If successful, the attacks could allow the server to
overwrite or create arbitrary files outside of the client's working
directory, subject to file/directory permissions and the privilege
level of the client.

Multiple clients are affected. See "Test Results" for the list of
clients.


___ Introduction _____________________________________________________


Inspiration:

A colleague of mine selected a bunch of files from an "ftp://" URL
and copied them to a folder en masse.

Realization:

FTP clients haven't been reported as being vulnerable to directory
traversal conditions.

Correction based on further investigation:

In 1997, some FTP clients were reported as being vulnerable to
directory traversal conditions [1].

Theory:

Some FTP clients are still vulnerable to directory traversal
conditions.

Verification:

The task would involve creating a malicious FTP server that would
send filenames with "../" and other sequences as the result of a
"LIST" request, or a "multiple GET" request. The client might then
download these files into some parent directory.


___ Testing Methodology ______________________________________________


This methodology is a simplification of the PROTOS methodology as
developed by the Oulu University Secure Programming Group [2], in
which a test suite is developed for a particular protocol, and the
suite is then used against specific implementations. The PROTOS
methodology has proven effective in finding large numbers of
vulnerabilities in many different products that implement standard
networking protocols.

For a simple test suite, the "ftp4all" FTP server was modified to
return filenames of various forms that might cause files to be created
outside a client's working directory. ftp4all was chosen because it
was easy to install and it allowed non-root users to run an FTP server
on a port other than 21.

The files src/ftps/list.c, serverd.c, and transfer.c were changed to
produce modified filenames, and to return the same test file for any
filename that the client requests.

When a client sends a LIST or NLST command, the test server returns
filenames containing the following sequences:

"../" - classic traversal
"/path" - an absolute pathname
".." - backslash traversal pattern (Windows systems)
"C:" - Drive letter traversal (Windows systems)
"..." - "triple-dot" (Windows systems, equivalent to ../..)

When downloading a group of files using wildcards, the FTP client
typically performs an "NLST" command, reads the list of files returned
by the server, and uses those filenames to make individual requests.

Note that web clients may also be affected if they can process
"ftp://" URLs.


Demonstration Session
---------------------

Following is a simulated session to demonstrate how a vulnerable
client may behave.


CLIENT> CONNECT server
220 FTP4ALL FTP server ready. Local time is Tue Oct 01, 2002 20:59.
Name (server:username): test
331 Password required for test.
Password:
230-Welcome, test - I have not seen you since Tue Oct 01, 2002 20:15 !
230 At the moment, there are 0 guest and 1 registered users logged in.

CLIENT> pwd
257 "/" is current directory.

CLIENT> ls -l
200 PORT command successful.
150 Opening ASCII mode data connection for /bin/ls.
total 1
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 ...FAKEME5.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 ../../FAKEME2.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 ../FAKEME1.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 ....FAKEME4.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 ..FAKEME3.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 /tmp/ftptest/FAKEME6.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 C:tempFAKEME7.txt
-rw-r----- 0 nobody nogroup 54 Oct 01 20:10 FAKEFILE.txt
-rw-r----- 0 nobody nogroup 0 Oct 01 20:11 misc.txt
226 Directory listing completed.


CLIENT> GET *.txt

Opening ASCII data connection for FAKEFILE.txt...
Saving as "FAKEFILE.txt"

Opening ASCII data connection for ../../FAKEME2.txt...
Saving as "../../FAKEME2.txt"

Opening ASCII data connection for /tmp/ftptest/FAKEME6.txt...
Saving as "/tmp/ftptest/FAKEME6.txt"

[etc.]


If a client is vulnerable, it saves files outside of the user's
current working directory.


Testing Notes
-------------

For command-line FTP clients, the client was tested in the following
fashion:

- log onto FTP server
- set no-interactive prompt
- perform "mget" (multiple GET) or equivalent command


In some cases, an FTP client would produce an error as soon as it
encountered a suspicious filename, and skip the remaining filenames.
Thus one could not be certain if the client was vulnerable to other
filenames. If a client demonstrated this behavior, then the server
was modified to send a single suspicious filename at a time, and the
client would be executed multiple times.

Other variants of directory traversal sequences were not tested, as
there were not sufficient resources to conduct such a comprehensive
analysis in a timely fashion. See [3] for examples.

It is possible that web clients may be vulnerable to the same type of
issue from malicious HTTP servers, when the clients are used to
automatically download web pages. However, this was not tested.


___ Test Results _____________________________________________________


The following products were specifically tested by the author.
Descriptions of this class of problem were reported to CERT/CC and
major vendors. Most vendors did not report results back to the
author. Consult your vendor, or the associated CERT vulnerability
note, if your product is not listed here.


Vulnerable:

Product ../ .. C: /path ...
------------- --- --- --- ----- ---
wget 1.8.1 yes no no yes{4} no
wget 1.7.1 yes no no no{2} no
OpenBSD 3.0 FTP yes no{1} no{1} yes no
Solaris 2.6, 7 yes no no yes no

The ftp command on SGI systems is also subject to one or more
flavors of directory traversal attacks. However, details were not
available at the time that this advisory was published.


Not Vulnerable:

Product ../ .. C: /path ...
------------- --- --- --- ----- ---
Red Hat 7.1 no{3} no{3} no{3} no{3} no
Debian 2.4.16 no{3} no{3} no{3} no{3} no
NT SP5 command line no no no no no
XP command (no SP) no no no no no
lftp 2.6.2 no no{1} no{1} no no
NcFTP 3.1.4 no no no no no
Lynx 2.8.1 [FTP traversal not available]


Notes:

{1} installed the file in the current directory

{2} created subdirectories within the current directory

{3} generated error message and/or stopped downloading

{4} only with the -nH option ("Disable host-prefixed directories")


Other notes on wget:

1) "wget" was tested with the -r (recursive) option.

2) When provided with an FTP URL on the command line, it is subject
to "../" traversal

3) If there's an FTP link within a web page, it will not follow the
links, and is not subject to traversal

4) HOWEVER, if the --follow-ftp option is used, it is subject to
"../" traversal

5) When both --follow-ftp and -nH are used, wget is also subject to
"/path" traversal

6) wget 1.7.1 was not tested for the -nH absolute path issue, or for
FTP URL's in web pages


___ Patches, Workarounds, and Vendor Statements ______________________


Workarounds:

Some clients may have one or more of the following features. If so,
then enabling these features could notify the user if an attack
occurs, and allow the user to take defensive action.

These features may be explicitly disabled if the client is being
called from a script or other program that does not require user
intervention.

1) The user may be able to set the client to prompt the user when an
existing file is to be overwritten. This is typically a default
behavior.

2) A command such as "runique" may be available to force the client
to use a different filename instead of overwriting an existing
file.


Sun FTP client:

Statement from Sun
------------------

We have investigated this directory traversal issue and do not
think it is a bug.

The user has several means of protection against this issue.

1. By default prompting is turned on, so the user gets a chance to
decide if they want a file returned by mget before it is
downloaded. So files will not be overwritten without prompting the
user.

2. When running as an ordinary user, Unix access controls will stop
system files being over written. If a user must run as root, care
needs to be taken which would include not turning off interactive
mode.

3. The user may run the "runique" command to force the Solaris ftp
client to avoid overwriting files that already exist.

The Solaris ftp mget behaviour is consistent with other BSD derived
ftp clients, for example on Linux and FreeBSD. Changing the
existing behaviour will cause problems.


SGI FTP client:

SGI acknowledged the vulnerability via email and is likely to have a
public acknowledgement near the time of this disclosure.


OpenBSD:

Vendor statement (Theo de Raadt):

"I've forwarded the report to the person who copes with that
stuff. I do not consider this all that serious."

wget:

Red Hat Linux has released advisory RHSA-2002:229 at:
http://www.redhat.com/support/errata/RHSA-2002-229.html

The status of other Linux vendors was unknown at the time this
advisory was published.



___ Vulnerability Identifiers ________________________________________


The following identifiers have been assigned to these issues.

wget:

CVE - CAN-2002-1344 [4]
CERT VU - VU#210148 [5]

FTP command-line clients (UNIX):

CVE - CAN-2002-1345 [6]
CERT VU - VU#210409 [7]

The CVE numbers were assigned by the Common Vulnerabilities and
Exposures (CVE) project. These are candidates for inclusion in the
CVE list, which standardizes names for security problems
(http://cve.mitre.org).

The CERT VU numbers were assigned by CERT/CC and will be accessible in
the Vulnerability Notes database (http://www.kb.cert.org/vuls).

Additional identifiers will be assigned to separate packages if
necessary.


___ Research and Disclosure History __________________________________

The disclosure of this issue has been conducted in accordance with the
Christey/Wysopal "Responsible Vulnerability Disclosure Process" draft,
which has expired [8].

Since multiple vendors and products were affected, the research and
disclosure history for this issue is extensive. The total amount of
time required for research, vendor notification, and coordination is
estimated to be 50 hours.


Research and General Notification
---------------------------------
Sep 25, 2002 - issue theorized
Sep 27, 2002 - modified FTP server created; initial tests
Oct 1, 2002 - notified vendor-sec with various responses
Oct 10, 2002 - sent update to CERT/CC
Oct 11, 2002 - CERT/CC reply
Dec 2, 2002 - notified CERT/CC of status
Dec 2, 2002 - set release date of December 10, notified all parties
Dec 5, 2002 - received CERT ID (VU#210409) for issue
Dec 9, 2002 - more edits
Dec 9, 2002 - CVE ID's sent to vendor-sec


Sun (CVE: CAN-2002-1345)
------------------------
Sep 27, 2002 - Sun ftp client issue discovered
Sep 30, 2002 - Notified Sun
Sep 30, 2002 - CERT/CC notified of Sun issue
Oct 1, 2002 - initial response from Sun (within 1 day)
Oct 1, 2002 - provided fake FTP server to Sun
Oct 7, 2002 - additional info from Sun
Nov 18, 2002 - response from Sun; will not address issue, as other
protections are already available
Dec 2, 2002 - suggested "vendor statement" to Sun
Dec 4, 2002 - Sun provides final statement


SGI (CVE: CAN-2002-1345)
------------------------
Oct 1, 2002 - provided fake FTP server to SGI
Nov 5, 2002 - inquiry by SGI on release status
Nov 27, 2002 - SGI inquires about release date
Dec 2, 2002 - response to SGI; set release to Dec 10?
Dec 9, 2002 - CVE candidate acquired, sent to SGI


OpenBSD (CVE: CAN-2002-1345)
----------------------------
Oct 1, 2002 - OpenBSD client issue discovered
Oct 1, 2002 - notified OpenBSD ([email protected])
Dec 2, 2002 - second notification to OpenBSD
Dec 2, 2002 - response from Theo de Raadt (original message was lost)
Dec 2, 2002 - report forwarded to other OpenBSD maintainers


wget (CVE: CAN-2002-1344)
--------------------------
Note: notification and resolution of the wget issue was handled
primarily through Mark Cox of Red Hat Linux, not the package
maintainer.

Sep 30, 2002 - wget issue discovered
Sep 30, 2002 - notified Mark Cox (Red Hat) of wget issue
Oct 1, 2002 - found wget absolute path issue
Oct 2, 2002 - provided fake web server to Red Hat
Oct 6, 2002 - notified wget developer ([email protected])
Nov 7, 2002 - inquiry by Red Hat on release status for wget; still
haven't heard back from [email protected], need to
consider other options
Nov 25, 2002 - Red Hat notifies that wget patches are ready
Dec 2, 2002 - notification to wget developer; new email address
found by Red Hat; developer is mostly inactive
Dec 9, 2002 - CVE ID acquired, sent to Red Hat


Other Activities
----------------
Oct 1, 2002 - provided fake FTP server to Solar Designer
Oct 1, 2002 - briefly tested lftp
Oct 9, 2002 - received report that ncftp is vulnerable to /abs/path
in the -R option; checked 3.1.4, doesn't seem to be an
issue - "-R /" is interpreted as / on local system, so
all pathnames would be "legal"; no response to followup


___ References _______________________________________________________


[1] "security hole in mget (in ftp client)"
[email protected]
Bugtraq mailing list
August 5, 1997
http://marc.theaimsgroup.com/?l=bugtraq&m=87602746719482&w=2

[2] OUSPG: Oulu University Secure Programming Group
http://www.ee.oulu.fi/research/ouspg/index.html

[3] "A 'straw man' vulnerability auditing checklist"
Steve Christey
SecProg mailing list
December 5, 2002
http://marc.theaimsgroup.com/?l=secprog&m=103911851613670&w=2

[4] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2002-1344

[5] http://www.kb.cert.org/vuls/id/210148

[6] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2002-1344

[7] http://www.kb.cert.org/vuls/id/210409

[8] http://www.wiretrip.net/rfp/p/doc.asp/i2/d73.htm


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