[Security Discussion] jcase:
Subject:

Race condition in Sprite Software's backup software, installed by OEM on LG Android devices.

Author:

Justin Case - jcase@cunninglogic.com

CVE ID:

CVE-2013-3685

Effect:

Locally exploited vulnerability with minimal device user interaction which results in executing code as the root user. Under specific circumstances, it is possible to exploit this vulnerability without the device user's knowledge

Products:

"Backup"

"spritebud"

Vendors:

Sprite Software

LG Electronics

Potentially other vendors

Affected Versions:

spritebud 1.3.24

backup 2.5.4105

Likely others versions as well

Affected Devices (Subject to firmware configuration):

LG-E971 LG Optimus G

LG-E973 LG Optimus G

LG-E975 LG Optimus G

LG-E975K LG Optimus G

LG-E975T LG Optimus G

LG-E976 LG Optimus G

LG-E977 LG Optimus G

LG-F100K LG Optimus Vu

LG-F100L LG Optimus Vu

LG-F100S LG Optimus Vu

LG-F120K LG Optimus Vu

LG-F120L LG Optimus LTE Tag

LG-F120S LG Optimus LTE Tag

LG-F160K LG Optimus LTE 2

LG-F160L LG Optimus LTE 2

LG-F160LV LG Optimus LTE 2

LG-F160S LG Optimus LTE 2

LG-F180K LG Optimus G

LG-F180L LG Optimus G

LG-F180S LG Optimus G

LG-F200K LG Optimus Vu 2

LG-F200L LG Optimus Vu 2

LG-F200S LG Optimus Vu 2

LG-F240K LG Optimus G Pro

LG-F240L LG Optimus G Pro

LG-F240S LG Optimus G Pro

LG-F260K LG Optimus LTE 3

LG-F260L LG Optimus LTE 3

LG-F260S LG Optimus LTE 3

LG-L21 LG Optimus G

LG-LG870 LG (Unknown)

LG-LS860 LG Mach

LG-LS970 LG Optimus G

LG-P760 LG Optimus L9

LG-P769 LG Optimus L9

LG-P780 LG Optimus L7

LG-P875 LG Optimus F5

LG-P875h LG Optimus F5

LG-P880 LG Optimus 4X HD

LG-P940 LG Prada

LG-SU540 LG Prada 3.0

LG-SU870 LG Optimus 3D Cube

LG-US780 LG Lollipop

Potentially other devices as well.

Product Information:

"Backup" and "spritebud" are a setting and application backup/restore system written by Sprite Software and deployed on LG Android smartphones. "Backup" is the end user front end app, and "spritebud" is the service that preforms the backup and restore functions.

Details:

The "spritebud" daemon is started by the init scripts and runs as the root user. Listening on a unix socket, the daemon accepts instructions from the "Backup" app. Using a crafted backup, we can write to, change permission and change ownership of any file, being that "spritebud" is running under the root user.

The crafted backup contains restore data for our exploiting application, "com.cunninglogic.lgpwn". The data includes a 50mb dummy file (a) used to increase our exploit window, su binary (b), a script (c) to install su, and a text file (d) containing the path to our script. All files are owned by the application, and are world write/read/execute. All files are restored in alphabetical order. The entire backup, after compress, is approximately 2mb. The structure of this backup is as follows:

drwxrwxrwx u0_a114 u0_a114 2013-05-28 20:13 files

./files:

- -rwxr-xr-x u0_a114 u0_a114 52428800 2013-05-22 20:06 a

- -rwxr-xr-x u0_a114 u0_a114 91992 2013-05-22 20:07 b

- -rwxr-xr-x u0_a114 u0_a114 251 2013-05-22 20:12 c

- -rwxr-xr-x u0_a114 u0_a114 42 2013-05-22 20:07 d

Prior to restoration, our exploit app runs, watches the process and waits. During restoration, the spritebud daemon first creates the files directory, then sets it's permission and owner. Next it decompresses and restores the "a" file, our 50mb dummy files. During the restoration of "a", our exploit application has time to symlink "d", our text file containing the full path to our script (c), to /sys/kernel/uevent_helper. Upon restoration of file "d", our path is written to uevent_helper. When a hotplug even occurs (which occur every few seconds), the path contained in uevent_helper is execute by the kernel and our script (c) is executed and installs the su binary (b).