Generation of Incorrect Security Tokens
The product implements a Security Token mechanism to differentiate what actions are allowed or disallowed when a transaction originates from an entity. However, the Security Tokens generated in the system are incorrect.
Systems-On-a-Chip (SoC) (Integrated circuits and hardware engines) implement Security Tokens to differentiate and identify actions originated from various agents. These actions could be "read", "write", "program", "reset", "fetch", "compute", etc. Security Tokens are generated and assigned to every agent on the SoC that is either capable of generating an action or receiving an action from another agent. Every agent could be assigned a unique, Security Token based on its trust level or privileges. Incorrectly generated Security Tokens could result in the same token used for multiple agents or multiple tokens being used for the same agent. This condition could result in a Denial-of-Service (DoS) or the execution of an action that in turn could result in privilege escalation or unintended access.
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.
Consider a system with a register for storing an AES key for encryption or decryption. The key is 128 bits long implemented as a set of four 32-bit registers. The key registers are assets, and register, AES_KEY_ACCESS_POLICY, is defined to provide necessary access controls. The access-policy register defines which agents, using a Security Token, may access the AES-key registers. Each bit in this 32-bit register is used to define a Security Token. There could be a maximum of 32 Security Tokens that are allowed access to the AES-key registers. When set (bit = "1") bit number allows action from an agent whose identity matches that bit number. If Clear (bit = "0") the action is disallowed for the corresponding agent.
Let"s assume the system has two agents: a Main-controller and an Aux-controller. The respective Security Tokens are "1" and "2".
AES key [0:31] for encryption or decryption
AES key [32:63] for encryption or decryption
AES key [64:95] for encryption or decryption
AES key [96:127] for encryption or decryption
AES key access register [31:0]
An agent with a Security Token "1" has access to AES_ENC_DEC_KEY_0 through AES_ENC_DEC_KEY_3 registers. As per the above access policy, the AES-Key-access policy allows access to the AES-key registers if the security Token is "1".
Both agents have access to the AES-key registers.
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