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Directory Traversal Vulnerabilities in FTP Clients

SUMMARY

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.

DETAILS

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

Vendor response:
Vendors informed individually and through CERT/CC

Risk:
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.

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:\temp\FAKEME7.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.

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&gt;
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.

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

ADDITIONAL INFORMATION

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

[2] OUSPG: Oulu University Secure Programming Group
<http://www.ee.oulu.fi/research/ouspg/index.html&gt;
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&amp;m=103911851613670&amp;w=2&gt;
http://marc.theaimsgroup.com/?l=secprog&amp;m=103911851613670&amp;w=2

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

[5] <http://www.kb.cert.org/vuls/id/210148&gt;
http://www.kb.cert.org/vuls/id/210148

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

[7] <http://www.kb.cert.org/vuls/id/210409&gt;
http://www.kb.cert.org/vuls/id/210409

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

The information has been provided by <mailto:[email protected]> Steve
Christey.

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