Improper Restriction of Security Token Assignment

What is CWE-1259?

Angreiferpfad Schritt für Schritt

1. 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.

2. 2

   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 |

3. 3

   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".

4. 4

   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)

How to prevent CWE-1259

• Architecture and Design / Implementation - 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.

Frequently asked questions

Further reading

• MITRE — offizielle CWE-1259 https://cwe.mitre.org/data/definitions/1259.html