Inclusion of Undocumented Features or Chicken Bits
The device includes chicken bits or undocumented features that can create entry points for unauthorized actors.
Description
A common design practice is to use undocumented bits on a device that can be used to disable certain functional security features. These bits are commonly referred to as "chicken bits". 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.
Demonstrations
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
Example One
Consider a device 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 device. However, this code contains undocumented "special access features" intended to be used only for performing failure analysis and intended to only be unlocked by the device designer.
Attackers dump the code from the device 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.
See Also
Weaknesses in this category are related to access control.
Weaknesses in this category are related to the "Poorly Documented or Undocumented Features" category from the SEI ETF "Categories of Security Vulnerabilities in ICS" a...
Weaknesses in this category are related to features and mechanisms providing hardware-based isolation and access control (e.g., identity, policy, locking control) of s...
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This view (slice) lists weaknesses that can be introduced during design.
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