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FreeBSD-SA-01:59                                           Security Advisory
                                                                FreeBSD, Inc.

Topic:          rmuser contains a race condition exposing /etc/master.passwd

Category:       core
Module:         rmuser
Announced:      2001-09-04
Credits:	dynamo@harvard.net
Affects:        FreeBSD 4.2-RELEASE, 4.3-RELEASE
                FreeBSD 4.3-STABLE prior to the correction date.
Corrected:      2001-07-28 12:10:15 UTC (4.3-STABLE)
                2001-09-04 07:46:57 UTC (RELENG_4_3)
FreeBSD only:   Yes

I.   Background

rmuser is a perl script used to completely remove users from a system.

II.  Problem Description

When removing a user from the system with the rmuser utility, the
/etc/master.passwd file and it's corresponding database /etc/spwd.db
must be updated.  The rmuser script was incorrectly doing this by
creating a new master.passwd file with an unsafe umask and then using
chmod to set its permissions to 0600.  Between the time that the file
was created and the time that its permissions were changed the file is
world-readable.

This is only a minor security vulnerability since the rmuser command
is only used infrequently on most systems, and the attack is highly
timing-dependent.

All versions of FreeBSD prior to the correction date including FreeBSD
4.3 contain this problem.  The base system that will ship with FreeBSD
4.4 does not contain this problem since it was corrected prior to the
release.

III. Impact

For a brief amount of time while running rmuser, a world-readable copy
of /etc/master.passwd is available.  A local attacker who reads this
file can extract password hashes from the copy of /etc/master.passwd.
This information could be used by attackers to escalate their
privileges, possibly yielding root privileges on the local system, by
mounting an offline dictionary attack in order to guess the plaintext
passwords of the accounts on the local system.

IV. Workaround

Use the pw(8) utility to remove users instead of rmuser.
    
    - "pw userdel <username>" will only remove the user from
      /etc/passwd, /etc/master.passwd and /etc/group
    - "pw -r userdel <username>" will also remove the user's home
      dirrectory

V. Solution

1) Upgrade your vulnerable system to 4.3-STABLE or the RELENG_4_3
security branch, dated after the respective correction dates.

2) To patch your present system: download the relevant patch from the
below location, and execute the following commands as root:

# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:59/rmuser.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:59/rmuser.patch.asc

Verify the detached PGP signature using your PGP utility.

This patch has been verified to apply to FreeBSD 4.2-RELEASE and
4.3-RELEASE.  It may or may not apply to older, unsupported releases
of FreeBSD.

# cd /usr/src/usr.sbin/adduser
# patch -p < /path/to/patch
# make depend && make all install

3) FreeBSD 4.3-RELEASE systems:

An experimental upgrade package is available for users who wish to
provide testing and feedback on the binary upgrade process.  This
package may be installed on FreeBSD 4.3-RELEASE systems only, and is
intended for use on systems for which source patching is not practical
or convenient.

If you use the upgrade package, feedback (positive or negative) to
security-officer@FreeBSD.org is requested so we can improve the
process for future advisories.

During the installation procedure, backup copies are made of the files
which are replaced by the package.  These backup copies will be
reinstalled if the package is removed, reverting the system to a
pre-patched state.

# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:59/security-patch-rmuser-01.59.tgz
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/packages/SA-01:59/security-patch-rmuser-01.59.tgz.asc

Verify the detached PGP signature using your PGP utility.

# pkg_add security-patch-rmuser-01.59.tgz

VI.  CVS Revisions

The following $FreeBSD$ CVS revision contain the fixes for this
vulnerability.  The $FreeBSD$ revision of installed sources can be
examined using the ident(1) command.  These revision IDs are not
updated by applying the patch referenced above.

[FreeBSD 4.3-STABLE]

  Revision      Path
  1.8.2.5       src/usr.sbin/rmuser.perl

[RELENG_4_3]

  Revision      Path
  1.8.2.2.2.1   src/usr.sbin/rmuser.perl
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