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ID

CWE-1290: Incorrect Decoding of Security Identifiers

Weakness ID: 1290
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
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.
+ Extended Description

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.

+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.284Improper Access Control
+ Relevant to the view "Hardware Design" (CWE-1194)
NatureTypeIDName
ChildOfClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.1294Insecure Security Identifier Mechanism
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Implementation
Architecture and Design
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)

Architectures

Class: Architecture-Independent (Undetermined Prevalence)

Technologies

Bus/Interface IP (Undetermined Prevalence)

Class: Technology-Independent (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Confidentiality
Integrity
Availability
Access Control

Technical Impact: Modify Memory; Read Memory; DoS: Resource Consumption (Other); Execute Unauthorized Code or Commands; Gain Privileges or Assume Identity; Quality Degradation

High
+ Demonstrative Examples

Example 1

Consider a system that has four bus masters and a decoder. 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 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 in formation.

The assets are the AES-Key register’s AES key for encryption or decryption. The key is128 bits implemented as a set of four 32-bit registers. The key registers are assets, and register AES_KEY_ACCESS_POLICY is defined to provide the necessary access controls. The access-policy register defines 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 and, if “0” (i.e., Clear), 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_POLICY [31:4] Default 0x000000 [3:0] – 0x02 agent with Security Identifier “1” has access to AES_ENC_DEC_KEY_0 through AES_ENC_DEC_KEY_4 registers

Pseudo Code
If (AES_KEY_ACCESS_POLICY[Security_Identifier] == “1”)
Allow access to AES-Key registers

Else
Deny access to AES-Key registers

(bad code)
Example Language: 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.
If (Bus_transaction[14] == “1”)
Security_Identifier == “1”

Else
Security_Identifier == “0”

Upon close observation of the security identifiers and the above code, it looks like the Master_3, an untrusted agent, has access to the AES-Key registers in addition to the intended trusted Master_1 because both have their bit “0” set to “1”.

(good code)
Example Language: Other 
The decoder should check for the entire size of the security identifier in the bus-transaction signal to assign acorresponding 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”
+ Potential Mitigations

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.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2020-04-29Arun Kanuparthi, Hareesh Khattri, Parbati Kumar MannaIntel Corporation
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Page Last Updated: August 20, 2020