Abstract

A set of flaws in the ftpd source code can be used together to
achieve root access within an ftp session. With root file manipulation
ability, mechanisms to gain a shell are numerous, so this issue
should be considered a remote root situation.

ftpd is disabled by default in NetBSD since NetBSD-1.5.3, however
many users might have reason to provide this popular service.

Technical Details

Przemyslaw Frasunek has provided a detailed analysis.
http://www.frasunek.com/lukemftpd.txt

CVE: CAN-2004-0794

Solutions and Workarounds

Confirm that the host in question is running ftpd, by checking the ftp
entries in /etc/inetd.conf. By default, the entries look like this:

#ftp stream tcp  nowait root /usr/libexec/ftpd ftpd -ll
#ftp stream tcp6 nowait root /usr/libexec/ftpd ftpd -ll

If the comment character (#) has been removed from the start of the
lines, then ftp has been enabled on this host. Hosts not running ftpd
are not vulnerable, but ftpd should be updated to prevent future
exposure if ftpd is enabled at a later date.

If ftpd has been configured to run with the -r option, then your server
is not vulnerable. Adding -r may be an acceptable workaround for some
sites, until ftpd can be upgraded.

To determine if a host is running a vulnerable version of ftpd, compare
the version string in the login banner (if displayed).

Any version of lukemftpd,
any version of NetBSD-ftpd prior to 20040809, or
any version of tnftpd prior to 20040810 is vulnerable.

% ftp ftp.server.host
Connected to ftp.server.host.
220 ftp.server.host FTP server (tnftpd 20040810) ready.
                                ^^^^^^^^^^^^^^^
                                Patched ftp server.

* Workaround:

Disable ftpd
As root, comment out the ftp lines in /etc/inetd.conf,
and execute the following command to disable ftpd:

% /etc/rc.d/inetd reload

Even if you plan to update ftpd, it is worthwhile to
disable ftpd until it is upgraded, in case you are
distracted and do not complete the update in a timely
fashion.

* Workaround:

Drop root privileges
As root, add -r to the command line options for any
ftp entry in /etc/inetd.conf. Then run:

% /etc/rc.d/inetd reload

This option may not be acceptable at all sites, since
client compatibility issues are possible. See the
ftpd manpage for more details about -r.

If all untrusted user accounts are listed in /etc/ftpchroot, then the
root file access gained will only be effective inside the chrooted
directory. This is not a guarantee against further privilege
escalation, especially in concert with social engineering.

If you have ftp servers that run in chrooted environments, make sure to
update ftpd binaries in chrooted copies of /usr/libexec or
/usr/pkg/libexec, and ensure that inetd.conf points to the correct
executable.


The following instructions describe how to upgrade your ftpd
binaries by updating your source tree and rebuilding and
installing a new version of ftpd.

* NetBSD-current:

Systems running NetBSD-current dated from before 2004-08-09
should be upgraded to NetBSD-current dated 2004-08-10 or later.

The following directories need to be updated from the
netbsd-current CVS branch (aka HEAD):
    libexec/ftpd

To update from CVS, re-build, and re-install ftpd:
    # cd src
    # cvs update -d -P libexec/ftpd
    # cd libexec/ftpd

    # make USETOOLS=no cleandir dependall
    # make USETOOLS=no install

* NetBSD 2.0_BETA:

The binary distribution of NetBSD 2.0_BETA is vulnerable.

Systems running NetBSD 2.0_BETA dated from before 2004-08-14
should be upgraded to NetBSD 2.0_BETA dated 2004-08-15 or later.

The following directories need to be updated from the
netbsd-2-0 CVS branch:
    libexec/ftpd

To update from CVS, re-build, and re-install ftpd:
    # cd src
    # cvs update -d -P libexec/ftpd
    # cd libexec/ftpd

    # make USETOOLS=no cleandir dependall
    # make USETOOLS=no install

* NetBSD-1-6:

The binary distribution of NetBSD 1.6, 1.6.1 and 1.6.2 is vulnerable.

Systems running NetBSD-1-6 dated from before 2004-08-30
should be upgraded to NetBSD 2.0_BETA dated 2004-08-31 or later.

The following directories need to be updated from the
netbsd-1-6 CVS branch:
    libexec/ftpd

To update from CVS, re-build, and re-install ftpd:
    # cd src
    # cvs update -d -P libexec/ftpd
    # cd libexec/ftpd

    # make USETOOLS=no cleandir dependall
    # make USETOOLS=no install

* NetBSD-1-5:

The binary distribution of NetBSD 1.5, 1.5.1, 1.5.2 and 1.5.3 is
vulnerable.

Systems running NetBSD-1-5 dated from before 2004-08-26
should be upgraded to NetBSD 2.0_BETA dated 2004-08-27 or later.

The following directories need to be updated from the
netbsd-1-5 CVS branch:
    libexec/ftpd

To update from CVS, re-build, and re-install ftpd:
    # cd src
    # cvs update -d -P libexec/ftpd
    # cd libexec/ftpd

    # make USETOOLS=no cleandir dependall
    # make USETOOLS=no install

* Optional upgrade from pkgsrc:

* NetBSD 1.6, 1.6.1, 1.6.2:
* NetBSD 1.5, 1.5.1, 1.5.2, 1.5.3:
* NetBSD prior to 1.5:

The binary distribution of NetBSD 1.6.2 and all prior releases
are vulnerable.

Pullups will be issued to the release branches of NetBSD-1-6,
and NetBSD-1-5.

Systems with these releases which need to run ftpd prior to
those pullups should be updated from pkgsrc using
    net/tnftpd-20040810 or later.

    % rm /usr/libexec/ftpd
    % cd /usr/pkgsrc/net/tnftpd
    % cvs update -dP
    % make update

Then modify the relevant lines in /etc/inetd.conf to refer to
/usr/pkg/libexec/tnftpd instead of /usr/libexec/ftpd as follows:

    #ftp stream tcp nowait root /usr/pkg/libexec/tnftpd ftpd -ll
    #ftp stream tcp6 nowait root /usr/pkg/libexec/tnftpd ftpd -ll

Thanks To

Przemyslaw Frasunek for notification, analysis, and discussion

Luke Mewburn for patches

Revision History

2004-08-17 Initial release
                2004-08-17 Clarify Workarounds
                2004-08-17 Add Przemyslaw's analysis URL
                2004-08-18 Path correction from Gilbert Fernandes
                2004-08-19 Add CVE id
                2004-08-31 Note pullups to 1-5, 1-6.

More Information
 
Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at
ftp://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2004-009.txt.asc

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/  and http://www.NetBSD.org/Security/ .

Copyright 2004, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.