Title:
====
Google Nexus 9 Cypress SAR Firmware Injection via I2C

Identifier:
========
CVE-2017-0563

Product:
=======
Google Nexus 9

Vulnerable Version:
================
Nexus 9 Android Builds before N4F27B - May 2017, i.e. before bootloader 3.50.0.0143.

Mitigation:
=========
Install N4F27B or later (bootloader version 3.50.0.0143).

Technical Details:
==============
The Nexus 9 device contains a sensor SoC manufactured by Cypress. The sensor is managed by a driver available under drivers/input/touchscreen/cy8c_sar.c. The driver uses the sensor's data in order to regulate the radiation level emitted by the device.

The sensor communicates with the application processor via I2C bus #1, which also provides a firmware update interface. During the platform boot, the driver samples the SoC's firmware's version via chip address 0x5{c,d}, register 0x6. If it is different than the one available under /vendor/firmware/sar{0,1}.img, it initiates with a firmware flashing process (via I2C chip address 0x6{0,1}). It seems though that the firmware is not signed by Cypress, thus anyone having access to the I2C bus, can reflash the firmware of the SoC.

On Nexus 9 before build N4F27B, the I2C buses could be accessed by an unauthorized bootloader attacker:

1. Via the USB fastboot interface, accessible by the fastboot oem {i2cr, i2cw, i2crNoAddr, i2cwNoAddr} commands.
2. Via the HBOOT interface, available through UART (exposed by the headphones jack). 

These vectors are especially significant because theoretically they can be used by either a physical attacker (rebooting the device into fastboot) or by malicious chargers / headphones. For example, a malicious charger connected to an ADB-enabled device may reboot the device into fastboot if the user authorizes the charger. As for headphones, on builds before N4F26T they could reboot the device into HBOOT by issuing 'reboot oem-42' on the FIQ debugger prompt [3]. 

Full details can be found on our vulnerability report [1].

Patch:
=====
Google patched the vulnerability on build N4F27B / bootloader 3.50.0.0143 by restricting access to the I2C buses - The I2C related bootloader commands are no longer available.
Please note that although Google published the advisory on the April 2017 Security Bulletin [4], the patch has been included only since the April 5 2017 Security Patch Level, where the April Nexus 9 image (N4F26X) has the April 1 2017 Security Patch Level, hence it does not contain the patched bootloader.

References:
==========
[1] Aleph Research Vulnerability Report. https://alephsecurity.com/vulns/aleph-2017009
[2] PoC. https://github.com/alephsecurity/PoCs/tree/master/CVE-2017-0563
[3] Attacking Nexus 9 with Malicious Headphones. https://alephsecurity.com/2017/03/08/nexus9-fiq-debugger/
[4] Google's Security Bulletin (April 2017).  https://source.android.com/security/bulletin/2017-04-01#eop-in-htc-touchscreen-driver


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