The product implements a decoding mechanism to decode certain bus-transaction signals to security identifiers. If the decoding is implemented incorrectly, then untrusted agents can now gain unauthorized access to the asset.

In a System-On-Chip (SoC), various integrated circuits and hardware engines generate transactions such as to access (reads/writes) assets or perform certain actions (e.g., reset, fetch, compute, etc.). Among various types of message information, a typical transaction is comprised of source identity (to identify the originator of the transaction) and a destination identity (to route the transaction to the respective entity). Sometimes the transactions are qualified with a security identifier. The security identifier helps the destination agent decide on the set of allowed actions (e.g., access an asset for read and writes). A decoder decodes the bus transactions to map security identifiers into necessary access-controls/protections.
A common weakness that can exist in this scenario is incorrect decoding because an untrusted agent's security identifier is decoded into a trusted agent's security identifier. Thus, an untrusted agent previously without access to an asset can now gain access to the asset.

Resultant

Implementation

Architecture and Design

Confidentiality
Integrity
Availability
Access Control
Modify Memory
Read Memory
DoS: Resource Consumption (Other)
Execute Unauthorized Code or Commands
Gain Privileges or Assume Identity
Quality Degradation
High

Architecture and Design
Security identifier decoders must be reviewed for design consistency and common weaknesses.

Implementation
Access and programming flows must be tested in pre-silicon and post-silicon testing in order to check for this weakness.

Consider a system that has four bus masters and a decoder. The decoder is supposed to decode every bus transaction and assign a corresponding security identifier. The security identifier is used to determine accesses to the assets. The bus transaction that contains the security information is Bus_transaction [15:14], and the bits 15 through 14 contain the security identifier information. The table below provides bus masters as well as their security identifiers and trust assumptions:

Bus MasterSecurity Identifier DecodingTrust Assumptions
Master_0"00"Untrusted
Master_1"01"Trusted
Master_2"10"Untrusted
Master_3"11"Untrusted

The assets are the AES-Key registers for encryption or decryption. The key is 128 bits implemented as a set of four 32-bit registers. The AES_KEY_ACCESS_POLICY is used to define which agents with a security identifier in the transaction can access the AES-key registers. The size of the security identifier is 4 bits (i.e., bit 3 through 0). Each bit in these 4 bits defines a security identifier. There are only 4 security identifiers that are allowed accesses 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. If clear (i.e., "0"), disallows the respective action to that corresponding agent.

RegisterField description
AES_ENC_DEC_KEY_0AES key [0:31] for encryption or decryptionDefault 0x00000000
AES_ENC_DEC_KEY_1AES key [32:63] for encryption or decryptionDefault 0x00000000
AES_ENC_DEC_KEY_2AES key [64:95] for encryption or decryptionDefault 0x00000000
AES_ENC_DEC_KEY_3AES key [96:127] for encryption or decryptionDefault 0x00000000
AES_KEY_ACCESS_POLCY[31:4] Default 0x00000000[3:0]-0x01 agent with Security Identified "1" has access to AES_ENC_DEC_KEY_0 through AES_ENC_DEC_KEY_3 registers

The following Pseudo code outlines the process of checking the value of the Security Identifier within the AES_KEY_ACCESS_POLICY register:

If (AES_KEY_ACCESS_POLICY[Security_Identifier] == "1")

Allow access to AES-Key registers

Else

Deny access to AES-Key registers

Below is a decoder's Pseudo code that only checks for bit [14] of the bus transaction to determine what Security Identifier it must assign.

If (Bus_transaction[14] == "1")

Security_Identifier == "1"

Else

Security_Identifier == "0"

The security identifier is two bits, but the decoder code above only checks the value of one bit. Two Masters have their bit 0 set to "1" - Master_1 and Master_3. Master_1 is trusted, while Master_3 is not. The code above would therefore allow an untrusted agent, Master_3, access to the AES-Key registers in addition to intended trusted Master_1.

The decoder should check for the entire size of the security identifier in the bus-transaction signal to assign a corresponding security identifier. The following is good Pseudo code:

If (Bus_transaction[15:14] == "00")

Security_Identifier == "0"

If (Bus_transaction[15:14] == "01")

Security_Identifier == "1"

If (Bus_transaction[15:14] == "10")

Security_Identifier == "2"

If (Bus_transaction[15:14] == "11")

Security_Identifier == "3"

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Arun Kanuparthi, Hareesh Khattri, Parbati Kumar Manna
Intel Corporation
2020-04-29
4.2
2020-08-20

CWE Content Team
MITRE
2021-07-20
updated Related_Attack_Patterns

CWE Content Team
MITRE
2021-10-28
updated Demonstrative_Examples

CWE Content Team
MITRE
2022-04-28
updated Applicable_Platforms

CWE Content Team
MITRE
2022-06-28
updated Applicable_Platforms

CWE Content Team
MITRE
2022-10-13
updated Demonstrative_Examples, Related_Attack_Patterns

CWE Content Team
MITRE
2023-01-31
updated Demonstrative_Examples

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Weakness_Ordinalities