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Common Weakness Enumeration

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ID

CWE-1242: Inclusion of Undocumented Features or Chicken Bits

Weakness ID: 1242
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The chip includes chicken bits or undocumented features that can create entry points for unauthorized actors.
+ Extended Description

A common design practice is to use "chicken bits," which are bits on a chip that can be used to disable certain functional security features. They can facilitate quick identification and isolation of faulty components, features that negatively affect performance, or features that do not provide the required controllability for debug and test. Another way to achieve this is through implementation of undocumented features. An attacker might exploit these interfaces for unauthorized access.

+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.284Improper Access Control
+ Relevant to the view "Hardware Design" (CWE-1194)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1198Privilege Separation and Access Control Issues
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Architecture and Design
Implementation
Documentation
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)

Architectures

Class: Architecture-Independent (Undetermined Prevalence)

Technologies

Class: System on Chip (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Confidentiality
Integrity
Availability
Access Control

Technical Impact: Modify Memory; Read Memory; Execute Unauthorized Code or Commands; Gain Privileges or Assume Identity; Bypass Protection Mechanism

+ Demonstrative Examples

Example 1

Consider a chip that comes with various, security measures, such as secure boot. The secure-boot process performs firmware-integrity verification at boot time, and this code is stored in a separate SPI-flash chip. However, this code contains undocumented "special access features" intended to be used for performing failure analysis and can only be unlocked by the chip designer.

(bad code)
Example Language: Other 
Attackers dump the code from the chip and then perform reverse engineering to analyze the code. The undocumented, special-access features are identified, and attackers can activate them by sending specific commands via UART before secure-boot phase completes. Using these hidden features, attackers can perform reads and writes to memory via the UART interface. At runtime, the attackers can also execute arbitrary code and dump the entire memory contents.

Remove all chicken bits and hidden features that are exposed to attackers. Add authorization schemes that rely on cryptographic primitives to access any features that the manufacturer does not want to expose. Clearly document all interfaces.

+ Potential Mitigations

Phases: Architecture and Design; Implementation

Do not implement chicken bits. If implemented, ensure that they are disabled in production devices. Document all interfaces to the chip.

Effectiveness: High

+ References
[REF-1071] Ali Abbasi, Tobias Scharnowski and Thorsten Holz. "Doors of Durin: The Veiled Gate to Siemens S7 Silicon". <https://i.blackhat.com/eu-19/Wednesday/eu-19-Abbasi-Doors-Of-Durin-The-Veiled-Gate-To-Siemens-S7-Silicon.pdf>.
[REF-1072] Sergei Skorobogatov and Christopher Woods. "Breakthrough Silicon Scanning Discovers Backdoor in Military Chip". <https://www.cl.cam.ac.uk/~sps32/Silicon_scan_draft.pdf>.
[REF-1073] Chris Domas. "God Mode Unlocked: Hardware Backdoors in x86 CPUs". <https://i.blackhat.com/us-18/Thu-August-9/us-18-Domas-God-Mode-Unlocked-Hardware-Backdoors-In-x86-CPUs.pdf>.
[REF-1074] Jonathan Brossard. "Hardware Backdooring is Practical". <https://media.blackhat.com/bh-us-12/Briefings/Brossard/BH_US_12_Brossard_Backdoor_Hacking_Slides.pdf>.
[REF-1075] Sergei Skorabogatov. "Security, Reliability, and Backdoors". <https://www.cl.cam.ac.uk/~sps32/SG_talk_SRB.pdf>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2020-02-13Arun Kanuparthi, Hareesh Khattri, Parbati Kumar Manna, Narasimha Kumar V MangipudiIntel Corporation
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Page Last Updated: June 25, 2020