CERT Advisory CA-1995-16 wu-ftpd Misconfiguration Vulnerability

Original issue date: November 30, 1995
Last revised: September 23, 1997
Updated copyright statement

A complete revision history is at the end of this file.

A vulnerability exists with certain configurations of the SITE EXEC command in the Washington University ftpd, also known as wu-ftpd. Exploitation of this vulnerability may allow root access from any account on the system.

The vulnerable configuration is known to exist in numerous Linux distributions and is currently being actively exploited by intruders.

It should be noted that this vulnerability is not necessarily limited to Linux but may exist on any wu-ftpd installation. Thus, all users of the wu-ftpd program, not just the Linux users, should take this opportunity to verify the configuration of their daemons. Note that versions of wu-ftpd before the 2.4 release contain serious security vulnerabilities and should be updated immediately.

Section III contains instructions for disabling ftpd and correcting the configuration.

We will update this advisory as we receive additional information. Please check advisory files regularly for updates that relate to your site.

I. Description

There is a problem with the default configuration of the Washington University FTP Server version 2.4 in major Linux distributions, including but not limited to Slackware 2.0, 2.1, 2.2, 2.3, Yggdrasil Plug&Play Fall'94, and the Debian Distribution. By exploiting this problem, any user who is able to log into a system having the vulnerable configuration via FTP using their login, and not the anonymous login, may gain root access.

Other systems besides Linux can be configured to be vulnerable although the standard wu-ftpd 2.4 source code as distributed is not vulnerable.

The problem is that the variable _PATH_EXECPATH was 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, for example. The source code shipped with the Linux distributions contains the correct value ("/bin/ftp-exec") despite the incorrect distribution binary. You should verify that _PATH_EXECPATH has the correct value before recompiling.

Note that the documentation for wu-ftpd states that the directory defined by _PATH_EXECPATH is relative to ~ftp, the ftp home directory as specified in the password file. This is misleading. The pathname is relative to ~ftp for anonymous users only. This pathname is relative to "/" for other user sessions.

II. Impact

Any user with a local account on a system offering FTP services with the vulnerable configuration may gain root access. Support for anonymous FTP access is not required to exploit this vulnerability.

III. How to determine if you are vulnerable

All systems running wu-ftpd should be checked to determine if the configuration is vulnerable.

To test your configuration, access the FTP server using a legitimate user account (not an anonymous FTP login) and login to your FTP server. For example:

    srchost> ftp ftphost
    Connected to ftphost
    220 ftphost FTP server (Version wu-2.4(2) Mon Apr 18 09:135 [...]
    ready.
    Name (srchost:joe):
    331 Password required for joe.
    Password:
    230 User joe logged in.

Then type:

    ftp> quote site exec echo problem

If you see the following response, then you are not vulnerable:

    200-echo problem
    200  (end of 'echo problem')

However, if you see this following response, then you are vulnerable (note the additional '200-problem' entry):

    200-echo problem
    200-problem
    200  (end of 'echo problem')

IV. Solution

If you have the vulnerability, we recommend that you turn off ftpd immediately using the method described in Section A below. Once you have done that, you can then decide whether to rebuild or fetch a new ftpd binary.

If you have built wu-ftpd from a source distribution, follow the steps in Sections B.2 and B.3 below.

Once you have eliminated this vulnerability, turn on ftpd with the method described in Section C below.

A. Disable ftpd

To disable ftpd, do the following as root.

Shut down the FTP server using the ftpshut command. This command blocks all connections to the FTP server.
For ftpshut to work correctly, the ftpaccess(5) file will need a shutdown directive that names a file used by wu-ftpd to indicate that the server is shutdown. If your ftpaccess file does not have such a directive, add one to that file. When added, use ftpshut(8) to shut down the server. Once the server has been shutdown, all new incoming FTP requests will fail.
Here is an example of the ftpshut command:
```
    ftpshut now
```
Verify that the FTP service has been shut down by attempting to connect to it. You should see a message that contains a line similar to the following:
```
    hostname FTP server shut down -- please try again later
```
where hostname is the host from which you are requesting FTP service.

B. Correct the configuration

Item 1 below applies to those running Debian Linux. Item 2 applies to all other Linux systems. Item 3 applies to those who are building wu-ftpd from source on systems other than Linux.

If you are running Debian Linux, obtain a fixed binary, available from the following location, and install this binary.
ftp://ftp.debian.org/debian/debian-0.93/binary/net/wu-ftpd-2.4-14.deb
MD5 (wu-ftpd-2.4-14.deb) = c00a0aac75216bf83568aee4c2e7d168
If you are running any version of Linux, there is a version of the source code available that has been improved to compile more cleanly. It too is correctly configured for SITE EXEC. It is available from (file wu-ftpd-2.4-fixed.tar.gz)
ftp://bach.cis.temple.edu/pub/Linux/security/wu-ftpd-2.4-fix/
MD5 (wu-ftpd-2.4-fixed.tar.gz) = 3e1c6fd7cd6757e45894df0d3638b524
This version is also correctly configured for the SITE EXEC command and can be compiled and installed. Consult Section IV below for suggestions on how to configure wu-ftpd.
If you are running a version of wu-ftpd before version 2.4, you should upgrade to version 2.4 first. That version is available from
ftp://wuarchive.wustl.edu/packages/wuarchive-ftpd/wu-ftpd-2.4.tar.Z
MD5 (wu-ftpd-2.4.tar.Z) = 57f1a962c90a9b12825d39af518df433
Version 2.4 is correctly configured for the SITE EXEC command and can be compiled and installed. Consult Section IV below for suggestions on how to configure wu-ftpd.

C. Enabling ftpd

To turn ftpd back on, delete the file referenced by the shutdown directive in your ftpaccess file.
Verify that the FTP service has been enabled by attempting to connect to it. You should see a message that contains lines similar to the following:
```
srchost> ftp ftphost
Connected to ftphost
220 ftphost FTP server (Version wu-2.4(3) Mon Apr 3 16:53:11 EDT 1995) ready.
Name (srchost:joe):
```

IV. Advice on configuring the FTP Daemon for SITE EXEC

Here are some configuration guidelines for the directories named by the _PATH_EXECPATH variable.

Directories used by SITE EXEC: The documentation for wu-ftpd states that the directory defined by the _PATH_EXECPATH variable is relative to ~ftp, the ftp home directory as specified in the password file. This is misleading. The pathname is relative to ~ftp for anonymous users only. The pathname is relative to "/" for all other user sessions.
Therefore, you need to check the two directories used by the SITE EXEC command. For example, if the _PATH_EXECPATH variable is set to /bin/ftp-exec, then wu-ftpd searches the ~ftp/bin/ftp-exec directory for programs specified by SITE EXEC when the anonymous login is used, and the /bin/ftp-exec directory specified by SITE EXEC when any other login is used.
Contents of the directories used by SITE EXEC: The commands installed in these directories can be executed by the SITE EXEC command. We strongly recommend that this directory contain only those programs that you wish to be executed by those users who connect to your FTP server. An example of a program to install in these directories is the ls program. Programs that should not be installed in these directories are shells, for example sh or csh, and command interpreters, for example awk and perl.

The CERT Coordination Center thanks AUSCERT, the Australian response team, and Alexander O. Yuriev, Temple University, author of Linux Security Updates, for their support in responding to this problem. Linux Security Updates are available from http://bach.cis.temple.edu/linux/linux-security/

UPDATES

Information for Solaris 2.4

After the advisory was originally issued, Charles Jardine <cj10@cam.ac.uk> provided the following information.

The problem with the SITE EXEC command is that programs spawned by wu-ftpd are run as the effective user and group id of the logged in user but real user and group id of root (or however wu-ftpd is started by inetd, usually root).

To address this, the following can be used as a basis for a patch. (Note that this patch works for Solaris 2.4 compiled with gcc-2.7.2.)

    *** /tmp/T0a001YI       Mon Dec  4 10:22:13 1995
    --- popen.c     Mon Dec  4 10:22:08 1995
    ***************
    *** 141,146 ****
    --- 141,158 ----
                  }
                  (void) close(pdes[1]);
              }
    + /*
    +  * This fixes the ``real'' problem with SITE EXEC
    +  */
    +       {
    +               uid_t u = geteuid();
    +               gid_t g = getegid();
    +
    +               setuid(0);
    +               setgid(g);
    +               setuid(u);
    +       }
    +
              execv(gargv[0], gargv);
              _exit(1);
          }

Revision History

Sep. 23, 1997  Updated copyright statement
Aug. 30, 1996  Information previously in the README was inserted
               into the advisory.
Jan. 19, 1996  Updates - Added code that can be used as the basis for a patch
               for the SITE EXEC command for Solaris 2.4.
Dec. 19, 1995  Sec. III - Expanded the explanation of how to determine if
               you are vulnerable.

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