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CWE-1223: Race Condition for Write-Once Attributes

Weakness ID: 1223
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
A write-once register in hardware design is programmable by an untrusted software component earlier than the trusted software component, resulting in a race condition issue.
+ Extended Description

Integrated circuits and hardware IP software programmable controls and settings are commonly stored in register circuits. These register contents have to be initialized at hardware reset to defined default values that are hard coded in the hardware description language (HDL) code of the hardware unit. A common security protection method used to protect register settings from modification by software is to make them write-once. This means the hardware implementation only allows writing to such registers once, and they become read-only after having been written once by software. This is useful to allow initial boot software to configure systems settings to secure values while blocking runtime software from modifying such hardware settings.

Implementation issues in hardware design of such controls can expose such registers to a race condition security flaw. For example, consider a hardware design that has two different software/firmware modules executing in parallel. One module is trusted (module A) and another is untrusted (module B). In this design it could be possible for Module B to send write cycles to the write-once register before Module A. Since the field is write-once the programmed value from Module A will be ignored and the pre-empted value programmed by Module B will be used by hardware.

+ 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)
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.362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
+ Relevant to the view "Hardware Design" (CWE-1194)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1199General Circuit and Logic Design Concerns
+ 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.

Architecture and DesignThis weakness can appear in designs that use register write-once attributes with two or more software/firmware modules with varying levels of trust executing in parallel.
+ 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.


Verilog (Undetermined Prevalence)

VHDL (Undetermined Prevalence)


Class: System on Chip (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.

Access Control

Technical Impact: Bypass Protection Mechanism

System configuration cannot be programmed in a secure way
+ Demonstrative Examples

Example 1

consider the example design module system verilog code shown below. register_write_once_example module is an example of register that has a write-once field defined. Bit 0 field captures the write_once_status value.

(bad code)
Example Language: Verilog 
module register_write_once_example
input [15:0] Data_in,
input Clk,
input ip_resetn,
input global_resetn,
input write,
output reg [15:0] Data_out

reg Write_once_status;

always @(posedge Clk or negedge ip_resetn)
if (~ip_resetn)
Data_out <= 16'h0000;
Write_once_status <= 1'b0;
else if (write & ~Write_once_status)
Data_out <= Data_in & 16'hFFFE; // Input data written to register after masking bit 0
Write_once_status <= 1'b1; // Write once status set after first write.
else if (~write)
Data_out[15:1] <= Data_out[15:1];
Data_out[0] <= Write_once_status;


The first system component that sends a write cycle to this register can program the value. This could result in a race condition security issue in SoC design, if an untrusted agent is running in the system in parallel with the trusted component that is expected to program the register.

Trusted firmware or software trying to set the write-once field.
- Must confirm the Write_once_status (bit 0) value is zero, before programming register. If another agent has programmed the register before, then Write_once_status value will be one.
- After writing to the register, the trusted software can issue a read to confirm that the valid setting has been programmed.
+ Potential Mitigations

Phase: Architecture and Design

During hardware design all register write-once or sticky fields must be evaluated for proper configuration.

Phase: Testing

The testing phase should use automated tools to test that values are not reprogrammable and that write-once fields lock on writing zeros.
+ Content History
Submission DateSubmitterOrganization
2019-12-12Hareesh KhattriIntel Corporation
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Page Last Updated: February 19, 2020