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S-95-15.asc
Posted Jan 10, 2000

Subject /tmp sticky bit may allow root access Date 15-aug-95

tags | root
SHA-256 | 42711f0b3f0119e6100967f8a72de68ab9d8ee33895968875503c1bbccb9d837

S-95-15.asc

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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

===============================================================================
>> CERT-NL, 01-Mar-2000 <<
>> All CERT-NL information has been moved to http://cert.surfnet.nl. Links <<
>> to CERT-NL information contained in this advisory are therefore outdated. <<
>> <<
>> CERT-NL also has stopped the CERT-CC-Mirror service. Due to this the <<
>> links to the CERT-CC mirror are obsolete. Visit the CERT-CC site for the <<
>> complete CERT-CC advisory texts: http://www.cert.org <<
===============================================================================
===============================================================================
Security Advisory CERT-NL
===============================================================================
Author/Source : Teun Nijssen Index : S-95-15
Distribution : World Page : 1
Classification: External Version: 1
Subject : /tmp sticky bit may allow root access Date : 15-aug-95
===============================================================================

CERT-NL received the following (signed) message from our Australian sister
organisation. Their message is copied here verbatim:

=============================================================================
AA-95.07 AUSCERT Advisory
15 August, 1995
Incorrect permissions on /tmp may allow root access
- - -----------------------------------------------------------------------------

AUSCERT has received advice from the bugtraq mailing list that a
vulnerability exists in Solaris systems that allows a race condition to be
exploited to gain root access. The essential problem is that the sticky
bit is sometimes not set on the /tmp directory, allowing users access to
other user's files in /tmp.

** An exploit program for this vulnerability has been published. AUSCERT
** advises that the remedial action in Section 3 be performed immediately.

1. Description

A race condition exists in at least one Solaris 2.x system program that
can be exploited to gain root access if the user has access to the
temporary files. Access to temporary files may be obtained if the
permissions on the /tmp and /var/tmp directories are set incorrectly.
The permissions on the /tmp directory are often reset incorrectly by
the system if tmpfs (which is mounting swap as /tmp) is in use.

This vulnerability has been confirmed with Sun Microsystems.

This vulnerability affects Solaris 2.x (SunOS 5.x) systems. A similar
problem has affected SunOS 4.1.x (Solaris 1.x) systems in the past, and
these systems should also be checked for the correct permission bits.
The remainder of this Advisory uses Solaris 2.x commands as examples.
Similar commands and configurations exist for SunOS 4.1.x users.

To determine if you are running tmpfs, the following command can be
used to verify if the filesystem for /tmp is swap:

% /usr/sbin/df /tmp
Filesystem kbytes used avail capacity Mounted on
swap 28348 12 28336 0% /tmp

or look in the file /etc/vfstab for the configuration line:

#device device mount FS fsck mount mount
#to mount to fsck point type pass at boot options
swap - /tmp tmpfs - yes -

If either of these two conditions exist, then you are running tmpfs and
the system may automatically reset the permission bits of /tmp at the
next reboot.

To verify if your configuration is vulnerable, the following command
may be used:

% ls -ld /tmp
drwxrwxrwt 2 root 61 Aug 15 12:12 /tmp

If the sticky bit (t) is not set (it will be an x), then the system is
vulnerable.

2. Impact

Users logged in to the system may gain unauthorised root privileges.

** Note: An exploit script for this vulnerability has been published.

3. Workarounds

These workarounds have been verified with Sun Microsystems. Sun
Microsystems expect a patch to be released in the near future.

3.1 Immediate Workaround

The immediate workaround is to set the sticky bit on the /tmp directory
using the following command as root:

# /usr/bin/chmod 1777 /tmp

Note that this command must be performed after each reboot if you are
mounting swap as /tmp (using tmpfs).

In addition, the ownership and group membership of the /tmp directory
should be verified using ls -ld /tmp and if incorrect may be reset by:

# /usr/bin/chown root /tmp
# /usr/bin/chgrp root /tmp

The AUSCERT Unix Security Checklist already addresses this issue in
Section 5.5. The Unix Security Checklist may be obtained from:

ftp://ftp.auscert.org.au/pub/auscert/papers/unix_security_checklist_1.0

3.2 System Reboot workaround

It is possible to perform these commands automatically at reboot by
creating the following script as /etc/init.d/tmpfsfix:

- - -------------------------------------8<--------------------------------------
#!/bin/sh

if [ -d /tmp ]
then
/usr/bin/chmod 1777 /tmp
/usr/bin/chgrp root /tmp
/usr/bin/chown root /tmp
fi
- - -------------------------------------8<--------------------------------------

A symbolic link should be then be created called /etc/rc3.d/S79tmpfix
which points to /etc/init.d/tmpfsfix by issuing the following command
as root:

# /usr/bin/ln -s /etc/init.d/tmpfsfix /etc/rc3.d/S79tmpfix

3.3 /var/tmp permissions

The /var/tmp directory should be similarly checked and corrected. Note
that this directory is not usually mounted as tmpfs, and therefore is
not subject to automatic resetting of its permission bits on reboot.

% ls -ld /var/tmp
drwxrwxrwt 2 root 512 Aug 15 11:35 /var/tmp

- - ----------------------------------------------------------------------------
The AUSCERT team wishes to thank Mark Graff of Sun Microsystems, Marek
Krawus of The University of Queensland, and Scott Chasin for their advice
and cooperation in this matter.
- - ----------------------------------------------------------------------------

CERT-NL thanks AUSCERT, will buy them a beer in Karlsruhe and advises its
constituency to act as advised, pending the patch from SUN.
==============================================================================
CERT-NL is the Computer Emergency Response Team for SURFnet customers. SURFnet
is the Dutch network for educational, research and related institutes. CERT-NL
is a member of the Forum of Incident Response and Security Teams (FIRST).

All CERT-NL material is available under:
http://cert.surfnet.nl/

In case of computer or network security problems please contact your local
CERT/security-team or CERT-NL (if your institute is NOT a SURFnet customer
please address the appropriate (local) CERT/security-team).

CERT-NL is one/two hour(s) ahead of UTC (GMT) in winter/summer,
i.e. UTC+0100 in winter and UTC+0200 in summer (DST).

Email: cert-nl@surfnet.nl ATTENDED REGULARLY ALL DAYS
Phone: +31 302 305 305 BUSINESS HOURS ONLY
Fax: +31 302 305 329 BUSINESS HOURS ONLY
Snailmail: SURFnet bv
Attn. CERT-NL
P.O. Box 19035
NL - 3501 DA UTRECHT
The Netherlands

NOODGEVALLEN: 06 22 92 35 64 ALTIJD BEREIKBAAR
EMERGENCIES : +31 6 22 92 35 64 ATTENDED AT ALL TIMES
CERT-NL'S EMERGENCY PHONENUMBER IS ONLY TO BE USED IN CASE OF EMERGENCIES:
THE SURFNET HELPDESK OPERATING THE EMERGENCY NUMBER HAS A *FIXED*
PROCEDURE FOR DEALING WITH YOUR ALERT AND WILL IN REGULAR CASES RELAY IT
TO CERT-NL IN AN APPROPRIATE MANNER. CERT-NL WILL THEN CONTACT YOU.
===============================================================================

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