CWE-1267: Policy Uses Obsolete Encoding

Description

The product uses an obsolete encoding mechanism to implement access controls.

Submission Date :

April 18, 2020, midnight

Modification Date :

2023-06-29 00:00:00+00:00

Organization :

Intel Corporation
Extended Description

Within a System-On-a-Chip (SoC), various circuits and hardware engines generate transactions for the purpose of accessing (read/write) assets or performing various actions (e.g., reset, fetch, compute, etc.). Among various types of message information, a typical transaction is comprised of source identity (identifying the originator of the transaction) and a destination identity (routing the transaction to the respective entity). Sometimes the transactions are qualified with a Security Token. This Security Token helps the destination agent decide on the set of allowed actions (e.g., access to an asset for reads and writes). A policy encoder is used to map the bus transactions to Security Tokens that in turn are used as access-controls/protection mechanisms. A common weakness involves using an encoding which is no longer trusted, i.e., an obsolete encoding.

Example Vulnerable Codes

Example - 1

For example, consider a system that has four bus masters. The table below provides bus masters, their Security Tokens, and trust assumptions.Bus MasterSecurity Token DecodingTrust AssumptionsMaster_0"00"UntrustedMaster_1"01"TrustedMaster_2"10"UntrustedMaster_3"11"UntrustedThe policy encoding is to be defined such that Security Token will be used in implemented access-controls. The bits in the bus transaction that contain Security-Token information are Bus_transaction [15:11]. The assets are the AES-Key registers for encryption or decryption. The key of 128 bits is implemented as a set of four, 32-bit registers.RegisterField descriptionAES_ENC_DEC_KEY_0AES key [0:31] for encryption or decryption, Default 0x00000000AES_ENC_DEC_KEY_1AES key [32:63] for encryption or decryption, Default 0x00000000AES_ENC_DEC_KEY_2AES key [64:95] for encryption or decryption, Default 0x00000000AES_ENC_DEC_KEY_4AES key [96:127] for encryption or decryption, Default 0x00000000Below is an example of a policy encoding scheme inherited from a previous project where all "ODD" numbered Security Tokens are trusted.

Trusted = "1"Trusted = "0"If (Bus_transaction[14] == "1")ElseAllow access to AES-Key registersDeny access to AES-Key registersIf (trusted)Else

The inherited policy encoding is obsolete and does not work for the new system where an untrusted bus master with an odd Security Token exists in the system, i.e., Master_3 whose Security Token is "11". Based on the old policy, the untrusted bus master (Master_3) has access to the AES-Key registers. To resolve this, a register AES_KEY_ACCESS_POLICY can be defined to provide necessary, access controls:

New Policy: AES_KEY_ACCESS_POLICY[31:0] Default 0x00000002 - agent with Security Token "1" has access to AES_ENC_DEC_KEY_0 through AES_ENC_DEC_KEY_4 registersThe AES_KEY_ACCESS_POLICY register defines which agents with a Security Token in the transaction can access 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. Thus, any bus master with Security Token "01" is allowed access to the AES-Key registers. Below is the Pseudo Code for policy encoding:

Security_Token[4:0] = Bus_transaction[15:11]Allow access to AES-Key registersDeny access to AES-Key registersIf (AES_KEY_ACCESS_POLICY[Security_Token] == "1")Else

Related Weaknesses

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined to give an overview of the different insight to similar items that may exist at higher and lower levels of abstraction.

CWE-284: Improper Access Control
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