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Advisory ID: SYSS-2019-034
Product: Surface Keyboard
Manufacturer: Microsoft
Affected Version(s): WS2-00005
Tested Version(s): WS2-00005
Vulnerability Type: Insufficient Protection of Code (Firmware) and 
                    Data (Cryptographic Key)
Risk Level: Medium
Solution Status: Open
Manufacturer Notification: 2019-07-31
Solution Date: -
Public Disclosure: 2019-10-10
CVE Reference: Not assigned yet
Author of Advisory: Matthias Deeg (SySS GmbH)

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Overview:

Microsoft Surface Keyboard is a Bluetooth Low Energy (LE) keyboard.

The manufacturer describes the product as follows (see [1]):

"Meticulously crafted, just like your Surface
Enjoy the solid feel of the keyboard under your fingers as you work.
And it pairs seamlessly with your Surface with Wireless Bluetooth - at a
range of up to 50 feet - and battery power to last a full year."

Due to the insufficient protection of the flash memory of the keyboard,
an attacker with physical access has read and write access to the
firmware and the used cryptographic key.

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Vulnerability Details:

SySS GmbH found out that the embedded flash memory of the Bluetooth LE
Microsoft Surface Keyboard can be read and written via the SWD (Serial
Wire Debug) interface of the used nRF51822 Bluetooth SoC [2] as the
flash memory is not protected by the offered readback protection
feature.

Thus, an attacker with physical access to the keyboard can simply read
and write the nRF51822 flash memory contents and either extract the
cryptographic key (Bluetooth LE Long Term Key), for instance, to
perform further attacks against the wireless communication, or modify
the firmware.

However, even if the readback protection of the nRF51822 was enabled,
an attacker would be able to read and write the flash memory contents by
bypassing the security feature as described in [3] and [4] with
slightly more effort.

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Proof of Concept (PoC):

SySS GmbH could successfully read the nRF51822 flash memory contents of
the Microsoft Surface Keyboard via the SWD interface using a SEGGER
J-Link PRO [5] debug probe in combination with SEGGER J-Link Commander
and extract the currently used cryptographic key (Long Term Key).

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Solution:

According to Microsoft, the reported security issue does not meet
the bar for servicing via a security update [6].

The described security issue may be fixed in future versions of the
product.

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Disclosure Timeline:

2019-07-31: Vulnerability reported to manufacturer
2019-08-01: Microsoft confirms receipt of security advisory
2019-08-06: Microsoft responds that the reported issue does not meet
            the bar for servicing via a security update
2019-10-10: Public release of SySS security advisory

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References:

[1] Product website for Microsoft Surface Keyboard
    https://www.microsoft.com/en-us/p/surface-keyboard/8r3rqvvflp4k
[2] nRF51822 Product Specification v3.1
    https://infocenter.nordicsemi.com/pdf/nRF51822_PS_v3.1.pdf  
[3] Kris Brosch, Include Security, Firmware dumping technique for an ARM Cortex-M0 SoC, 2015
    https://blog.includesecurity.com/2015/11/NordicSemi-ARM-SoC-Firmware-dumping-technique.html
[4] Andrew Tierney, Pen Test Partners, NRF51822 code readout protection bypass - a how-to, 2018
    https://www.pentestpartners.com/security-blog/nrf51822-code-readout-protection-bypass-a-how-to/
[5] Product website for Segger J-Link PRO
    https://www.segger.com/products/debug-probes/j-link/models/j-link-pro/
[6] Microsoft Vulnerability Severity Classification for Windows
    https://aka.ms/windowsbugbar
[7] SySS Security Advisory SYSS-2019-034
    https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2019-034.txt
[8] SySS GmbH, SySS Responsible Disclosure Policy
    https://www.syss.de/en/news/responsible-disclosure-policy/

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Credits:

This security vulnerability was found by Matthias Deeg of SySS GmbH.

E-Mail: matthias.deeg (at) syss.de
Public Key: https://www.syss.de/fileadmin/dokumente/Materialien/PGPKeys/Matthias_Deeg.asc
Key fingerprint = D1F0 A035 F06C E675 CDB9 0514 D9A4 BF6A 34AD 4DAB

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Disclaimer:

The information provided in this security advisory is provided "as is"
and without warranty of any kind. Details of this security advisory may
be updated in order to provide as accurate information as possible. The
latest version of this security advisory is available on the SySS Web
site.

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Copyright:

Creative Commons - Attribution (by) - Version 3.0
URL: http://creativecommons.org/licenses/by/3.0/deed.en

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