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AA-95.04 AUSCERT Advisory
June 2, 1995
wu-ftpd misconfiguration vulnerability
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A problem exists with certain configurations of the Washington
University ftpd which may allow root access from any account on the
system. This vulnerability was described in the AA-94.01 Advisory,
which is available from:
ftp://ftp.auscert.org.au/pub/auscert/advisory/
AA-94.01.ftpd.Configuration.Advice
Please note that AUSCERT previously operated as SERT. AUSCERT contact
details (below) supercede the SERT details included in AA-94.01.
Note: This Advisory contains new and updated information.
*** The Australian Computer Emergency Response Team has received information
*** that some pre-compiled wu-ftpd-2.4 binaries distributed with Linux have
*** a vulnerable configuration by default.
All other users of wu-ftpd should take this opportunity to verify the
configuration of their daemons.
Versions of wu-ftpd prior to 2.4 contain serious security
vulnerabilities and should be updated immediately.
1. Description
A vulnerability exists in certain configurations of wu-ftpd which may
allow users to gain root access. The vulnerability has been described
previously in the AA-94.01 Advisory.
In its original form, the vulnerability was not enabled by default.
However, certain distributions of Linux contain a wu.ftpd that has been
compiled with a vulnerable configuration. This vulnerable
configuration is distributed and run by default.
This vulnerability has been confirmed for Linux Slackware-2.1 and 2.2.
It has been claimed that Linux Slackware-2.0 and 2.3 are also affected.
Other versions may similarly be affected. To test whether your system
is affected, see Section 3 (Detection).
Non-Linux systems running wu-ftpd should also be checked to determine
if the configuration is vulnerable. See Section 3 (Detection).
The pre-compiled binaries shipped for Linux Slackware distributions are
vulnerable. The variable _PATH_EXECPATH has been set to "/bin" in the
configuration file src/pathnames.h when the distribution binary was
built. _PATH_EXECPATH should be set to "/bin/ftp-exec" or a similar
directory that does not contain a shell or command interpreter. The
source code shipped with the Linux distributions contains the correct
value ("/bin/ftp-exec") (which should be verified before recompiling),
despite the incorrect distribution binary. See Section 4.2 for further
information.
The documentation states that the directory defined by _PATH_EXECPATH
is relative to ~ftp. This is misleading. The pathname is relative to
~ftp for anonymous users only. It is relative to "/" for normal user
sessions.
Floppy-only distributions of Linux do not contain source code. The
latest version of the wu-ftpd source code can be obtained from:
ftp://ftp.auscert.org.au/pub/mirrors/wuarchive.wustl.edu/
packages/wuarchive-ftpd/wu-ftpd-2.4.tar.Z
A diff(1) file exists to modify the wu-ftpd source code to allow it
to compile on Linux. The application of this patch will cause the
vulnerable configuration to exist.
*** The patch file wu-ftpd-2.4.diff.gz for Linux contains incorrect
*** information. This should be corrected and verified before recompiling.
2. Impact
Anyone who has a local account on the system offering ftp services with
the vulnerable configuration may gain root access. Support for
anonymous ftp access is not required to exploit this vulnerability.
3. Detection
Vulnerable systems can be detected by executing (as a user) the
commands below or by running strings(1) against the wu-ftpd daemon.
Both tests are recommended.
3.1 Detection using user commands
To test your configuration to see if you are vulnerable, you can
execute the following commands:
srchost> ftp ftphost
Connected to ftphost
220 ftphost FTP server (Version wu-2.4(2) Mon Apr 18 09:12:35 GMT+1000 1994) ready.
Name (srchost:user):
331 Password required for user.
Password:
230 User user logged in.
ftp> quote site exec echo problem
200-echo problem
200-problem
200 (end of 'echo problem')
ftp> quit
221 Goodbye.
srchost>
If you receive the line "200-problem", then your site is vulnerable.
Note that this does not work for anonymous ftp access, or for all
vulnerable configurations.
3.2 Detection using strings(1)
Determine the location of the SITE EXEC path by executing the following
command on the src/pathnames.h file:
$ grep _PATH_EXECPATH pathnames.h
#define _PATH_EXECPATH "/bin/ftp-exec"
$
Use the output of this command to verify that the currently running
binary is configured the same as the source code. Note, you should
consult your documentation for strings(1) to determine the correct
switch for examining the entire binary:
$ strings -a wu.ftpd | grep "/bin/ftp-exec"
/bin/ftp-exec
$
If the binary contains the same pathname for _PATH_EXECPATH, then you
have determined the correct location for the SITE EXEC commands.
The directory defined by _PATH_EXECPATH should not contain a shell or
command interpreter (such as perl) and should not be world or group
writeable, nor should any directory back to the root directory (/) be
group or world writeable. Permissions 511 are acceptable.
4. Recovery
If you have the vulnerability and you are unsure how to rectify it
immediately, you should disable your ftp daemon until the configuration
can be corrected.
4.1 Temporary workaround
If you are unsure how to rebuild a new ftpd daemon, then an interim
workaround is to disable the existing service.
Note: this will cause all incoming ftp requests to fail.
1. become root
2. comment out ftp in /etc/inetd.conf by prepending # to the line, ie:
#ftp stream tcp <rest of normal ftp line here>
3. Restart the inetd process. On most systems, this is done by sending
a HUP signal to the inetd process. For example:
# /bin/ps -ef | grep inetd | grep -v grep (System V)
or
# /bin/ps -aux | grep inetd | grep -v grep (BSD)
followed by:
# kill -HUP <pid gained by above line>
You should verify that the ftp service has been disabled by
attempting to connect to it. You should see a "connection refused"
message.
4.2 Correcting the configuration
Ensure that the _PATH_EXECPATH definition in src/pathnames.h is
"/bin/ftp-exec" and not "/bin" or any other system directory containing
a shell or interpreter, and then recompile.
If the wu-ftpd-2.4.diff.gz patch has been applied on Linux systems, the
patched version of pathnames.h will be vulnerable. This file should be
edited manually before the rebuild to correct the _PATH_EXECPATH
definition to "/bin/ftp-exec".
Replace the existing ftpd binary with the newly built version.
5. Instructions to enable SITE EXEC
Once the running binary has been confirmed as not containing the
vulnerable configuration, the SITE EXEC commands can be enabled by
following the following steps.
a) Ensure that the _PATH_EXECPATH definition in pathnames.h is
"/bin/ftp-exec" and not "/bin" or any other system directory
containing a shell. This should also be checked in the binary
version (see Section 3.2).
b) Create ~ftp/bin/ftp-exec. This should be owned by root, permissions
set to 555.
c) Copy the statically linked binaries that you want available for
execution by SITE EXEC into the ~ftp/bin/ftp-exec directory. These
should be owned by root, permissions 111. The binaries should never
be a shell or command interpreter that allows arbitrary programs to
be run.
d) If you want the DIR ftp command, you will need a hard link from
~ftp/bin/ls to ~ftp/bin/ftp-exec/ls or a copy of ls in ~ftp/bin.
The instructions above enable SITE EXEC commands for anonymous users
only. To enable SITE EXEC commands for normal ftp users:
e) Create a symbolic link from /bin/ftp-exec to ~ftp/bin/ftp-exec.
You should follow file ownership, group membership and permissions
strictly according to your documentation. Note that some versions of
ftp contain incorrect information for setting file permissions and
ownership. Further information can be found in:
ftp://ftp.auscert.org.au/pub/mirrors/cert.org/tech_tips/anonymous_ftp
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AUSCERT would like to acknowledge Michel (
[email protected]), Thomas
Lundquist (
[email protected]), Aleph One (
[email protected]), Olaf Kirch
(
[email protected]), Jeff Uphoff, and Dave Barr (
[email protected]) for
information published about the Linux problem.
AUSCERT would like to thank Dr. Ian Hoyle from BHP Research and Reinhard
Uebel from QTAC for their assistance in confirming the extent of this
vulnerability.
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If you believe that your system has been compromised, contact AUSCERT or your
representative in FIRST (Forum of Incident Response and Security Teams).
AUSCERT is the Australian Computer Emergency Response Team, funded by the
Australian Academic Research Network (AARNet) for its members. It is
located at The University of Queensland within the Prentice Centre.
AUSCERT is a full member of the Forum of Incident Response and Security
Teams (FIRST).
AUSCERT maintains an anonymous FTP service which is found on:
ftp://ftp.auscert.org.au. This archive contains past SERT and AUSCERT
Advisories, and other computer security information.
AUSCERT also maintains a World Wide Web service which is found on:
http://www.auscert.org.au.
Internet Email:
[email protected]
Facsimile: (07) 365 4477
Telephone: (07) 365 4417 (International: +61 7 365 4417)
AUSCERT personnel answer during Queensland business hours
which are GMT+10:00 (AEST).
On call after hours for emergencies.
Postal:
Australian Computer Emergency Response Team
c/- Prentice Centre
The University of Queensland
Brisbane
Qld. 4072.
AUSTRALIA
