95-07
f9e8e37de76a8c95d1e97313b6b37e512ce58d2d3b72feb4978b463b4ab95079
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AA-95.07 AUSCERT Advisory
15 August, 1995
Incorrect permissions on /tmp may allow root access
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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
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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.
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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: auscert@auscert.org.au
Facsimile: (07) 3365 4477
Telephone: (07) 3365 4417 (International: +61 7 3365 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