RE: Windows Platform Binary Table (WPBT) - BIOS PE backdoor

["Limanovski, Dimitri" <dimitri.limanovski@xxxxxxxxxxxxx>]

Hi Kevin,
I too was looking at this, and it does look absolutely horrendous. More so, 
that Microsoft does not provide a good measure to control WPBT: in the official 
doc there's some watered down paragraph about "good security measures", but 
there's no way to enforce binary signing, or CA-like validation of the 
signature. One thing is not clear is whether Windows 10 is vulnerable to the 
same functionality, and whether the malicious actors can write to WPBT 
directly, or, like the case with Lenovo, have to hijack "trusted" OEM apps that 
are allowed to do so.

Dimitri

-----Original Message-----
From: Kevin Beaumont [mailto:kevin.beaumont@xxxxxxxxx] 
Sent: Wednesday, August 12, 2015 7:45 AM
To: bugtraq@xxxxxxxxxxxxxxxxx
Subject: Windows Platform Binary Table (WPBT) - BIOS PE backdoor

PRECURSOR

There will be debate about if this is a vulnerability.  It affects a majority 
of user PCs -- including all Enterprise editions of Windows, there is no way to 
disable it, and allows direct code execution into secure boot sequences.  I 
believe it is worth discussing.

SCOPE

Microsoft documented a feature in Windows 8 and above called Windows Platform 
Binary Table.  Up until two days ago, this was a single Word document not 
referenced elsewhere on Google:

 
http://webcache.googleusercontent.com/search?q=cache:H-SSYRAB0usJ:download.microsoft.com/download/8/A/2/8A2FB72D-9B96-4E2D-A559-4A27CF905A80/windows-platform-binary-table.docx+&cd=1&hl=en&ct=clnk&gl=us

This feature allows a BIOS to deliver the payload of an executable, which is 
run in memory, silently, each time a system is booted.  The executable code is 
run under under Session Manager context (i.e.
SYSTEM).

This technique is being used by Lenovo and HP to silently deliver software, 
even after systems are completely wiped.  This issue came to light in this 
forum thread:
http://arstechnica.com/civis/viewtopic.php?p=29551819#p29551819

Additionally, the code is injected and executed in Windows after the Windows 
kernel has booted - meaning hard drives are accessible.  In a HP document - 
http://h10032.www1.hp.com/ctg/Manual/c03857419.pdf page
18 - they reference they use Windows Platform Binary Table to inject their code 
into encrypted systems (e.g. BitLocker) (!!!!).

MITIGATIONS

It is not possible to disable this functionality.  If you can gain access to 
the BIOS, you can inject code into the Windows boot sequence using the 
documentation linked above.  The BIOS delivered PE code is not countersigned by 
Microsoft.

Microsoft say: "If partners intentionally or unintentionally introduce malware 
or unwanted software though the WPBT, Microsoft may remove such software 
through the use of antimalware software.  Software that is determined to be 
malicious may be subject to immediate removal without notice."

However, you are relying on Microsoft being aware of attacks.  Since the code 
is executed in memory and not written to disk prior to activation, Windows 
Defender does not even scan the executed code.


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