Incorrect Chaining or Granularity of Debug Components
The product's debug components contain incorrect chaining or granularity of debug components.
For debugging and troubleshooting a chip, several hardware design elements are often implemented, including:
Various Test Access Ports (TAPs) allow boundary scan commands to be executed.
For scanning the internal components of a chip, there are scan cells that allow the chip to be used as a "stimulus and response" mechanism.
Chipmakers might create custom methods to observe the internal components of their chips by placing various tracing hubs within their chip and creating hierarchical or interconnected structures among those hubs.
Logic errors during design or synthesis could misconfigure the interconnection of the debug components, which could allow unintended access permissions.
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.
The following example shows how an attacker can take advantage of incorrect chaining or missing granularity of debug components.
In a System-on-Chip (SoC), the user might be able to access the SoC-level TAP with a certain level of authorization. However, this access should not also grant access to all of the internal TAPs (e.g., Core). Separately, if any of the internal TAPs is also stitched to the TAP chain when it should not be because of a logic error, then an attacker can access the internal TAPs as well and execute commands there.
As a related example, suppose there is a hierarchy of TAPs (TAP_A is connected to TAP_B and TAP_C, then TAP_B is connected to TAP_D and TAP_E, then TAP_C is connected to TAP_F and TAP_G, etc.). Architecture mandates that the user have one set of credentials for just accessing TAP_A, another set of credentials for accessing TAP_B and TAP_C, etc. However, if, during implementation, the designer mistakenly implements a daisy-chained TAP where all the TAPs are connected in a single TAP chain without the hierarchical structure, the correct granularity of debug components is not implemented and the attacker can gain unauthorized access.
Weaknesses in this category are related to access control.
Weaknesses in this category are related to hardware debug and test interfaces such as JTAG and scan chain.
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