Improper Restriction of Security Token Assignment

Incomplete Base

Structure: Simple

Description

This vulnerability occurs when a System-on-a-Chip (SoC) fails to properly secure its Security Token mechanism. These tokens control which actions different system components are allowed to perform, but inadequate protection allows them to be manipulated.

Extended Description

Security Tokens act as digital IDs within a System-on-a-Chip, determining what operations each component can execute—such as read, write, program, or reset. Each agent in the system receives one or more tokens based on its trust level and privileges. Since these tokens directly enforce security boundaries, any weakness in their assignment or protection compromises the entire system's integrity. When token assignment isn't properly restricted, malicious agents can reprogram or spoof tokens to impersonate trusted components. This allows attackers to bypass privilege checks and perform unauthorized actions, effectively breaking the hardware's security model. Proper implementation must ensure tokens are immutable and only assignable by trusted system logic.

Common Consequences 1

Scope: ConfidentialityIntegrityAvailabilityAccess Control

Impact: Modify Files or DirectoriesExecute Unauthorized Code or CommandsBypass Protection MechanismGain Privileges or Assume IdentityModify MemoryModify MemoryDoS: Crash, Exit, or Restart

Potential Mitigations 1

Phase: Architecture and DesignImplementation

- Security Token assignment review checks for design inconsistency and common weaknesses. - Security-Token definition and programming flow is tested in both pre-silicon and post-silicon testing.

Demonstrative Examples 1

For example, consider a system with a register for storing an AES key for encryption and decryption. The key is of 128 bits implemented as a set of four 32-bit registers. The key register assets have an associated control register, AES_KEY_ACCESS_POLICY, which provides the necessary access controls. This access-policy register defines which agents may engage in a transaction, and the type of transaction, with the AES-key registers. Each bit in this 32-bit register defines a security Token. There could be a maximum of 32 security Tokens that are allowed access to the AES-key registers. The number of the bit when set (i.e., "1") allows respective action from an agent whose identity matches the number of the bit and, if "0" (i.e., Clear), disallows the respective action to that 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". | Register | Description | Default | | --- | --- | --- | | AES_ENC_DEC_KEY_0 | AES key [0:31] for encryption or decryption | 0x00000000 | | AES_ENC_DEC_KEY_1 | AES key [32:63] for encryption or decryption | 0x00000000 | | AES_ENC_DEC_KEY_2 | AES key [64:95] for encryption or decryption | 0x00000000 | | AES_ENC_DEC_KEY_3 | AES key [96:127] for encryption or decryption | 0x00000000 | | AES_KEY_ACCESS_POLICY | AES key access register [31:0] | 0x00000002 |

An agent with 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".

Code Example:

Bad

Other

The Aux-controller could program its Security Token to "1" from "2".

The SoC does not properly protect the Security Token of the agents, and, hence, the Aux-controller in the above example can spoof the transaction (i.e., send the transaction as if it is coming from the Main-controller to access the AES-Key registers)

Code Example:

Good

Other

The SoC needs to protect the Security Tokens. None of the agents in the SoC should have the ability to change the Security Token.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Technologies:

Processor Hardware : UndeterminedSystem on Chip : Undetermined

Modes of Introduction

Architecture and Design

Implementation

Related Attack Patterns

Related Weaknesses

ChildOf:

Improper Access Control (CWE-284)

ChildOf:

Insecure Security Identifier Mechanism (CWE-1294)

Notes

MaintenanceThis entry is still under development and will continue to see updates and content improvements. Currently it is expressed as a general absence of a protection mechanism as opposed to a specific mistake, and the entry's name and description could be interpreted as applying to software.