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Incorrect Decoding of Security Identifiers

Incomplete Base
Structure: Simple
Description

This vulnerability occurs when a hardware decoder incorrectly interprets security identifiers in bus transactions, allowing untrusted agents to gain unauthorized access to protected assets.

Extended Description

In a System-on-Chip (SoC), hardware components communicate through bus transactions that include security identifiers. These identifiers determine what actions an agent can perform—like reading, writing, or controlling a resource. A decoder maps these identifiers to specific access privileges. If this decoding logic is flawed, the system's fundamental access control can be bypassed. The core risk arises when the decoder mistakenly maps an untrusted agent's identifier to a trusted one's privileges. This error effectively promotes an unauthorized agent's access level, letting it interact with assets it shouldn't. Developers must ensure the decoding logic is rigorously verified to prevent such privilege escalation in hardware security mechanisms.
Common Consequences 1
Scope: ConfidentialityIntegrityAvailabilityAccess Control

Impact: Modify MemoryRead MemoryDoS: Resource Consumption (Other)Execute Unauthorized Code or CommandsGain Privileges or Assume IdentityQuality Degradation
Potential Mitigations 2
Phase: Architecture and Design
Security identifier decoders must be reviewed for design consistency and common weaknesses.
Phase: Implementation
Access and programming flows must be tested in pre-silicon and post-silicon testing in order to check for this weakness.
Demonstrative Examples 1
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 Master | Security Identifier Decoding | Trust 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. | Register | Field description | | --- | --- | | AES_ENC_DEC_KEY_0 | AES key [0:31] for encryption or decryption Default 0x00000000 | | AES_ENC_DEC_KEY_1 | AES key [32:63] for encryption or decryption Default 0x00000000 | | AES_ENC_DEC_KEY_2 | AES key [64:95] for encryption or decryption Default 0x00000000 | | AES_ENC_DEC_KEY_3 | AES key [96:127] for encryption or decryption Default 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:

Code Example:

Informative
Other
other
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.

Code Example:

Bad
Other
other
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:

Code Example:

Good
Other
other
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Technologies:
Bus/Interface Hardware : UndeterminedNot Technology-Specific : Undetermined
Modes of Introduction
Implementation
Architecture and Design
Related Weaknesses
ChildOf: Improper Access Control (CWE-284)
ChildOf: Insecure Security Identifier Mechanism (CWE-1294)