CWE-1224: Improper Restriction of Write-Once Bit Fields

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 define 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 the settings write-once or "sticky." This allows writing to such registers only once, whereupon they become read-only. This is useful to allow initial boot software to configure systems settings to secure values while blocking runtime software from modifying such hardware settings. Failure to implement write-once restrictions in hardware design can expose such registers to being re-programmed by software and written multiple times. For example, write-once fields could be implemented to only be write-protected if they have been set to value "1", wherein they would work as "write-1-once" and not "write-once".

Code Examples & CVEs

Demonstrative Examples

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. This implementation can be for a register that is defined by specification to be a write-once register, since the write_once_status field gets written by input data bit 0 on first write.

The above example only locks further writes if write_once_status bit is written to one. So it acts as write_1-Once instead of the write-once attribute.

BadVerilog

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) 			begin 			 			  Data_out <= 16'h0000; 			  Write_once_status <= 1'b0;  			 			end  			else if (write & ~Write_once_status) 			begin 			 			  Data_out <= Data_in & 16'hFFFE; 			  Write_once_status <= Data_in[0]; // Input bit 0 sets Write_once_status 			 			end 			else if (~write) 			begin  			 			  Data_out[15:1] <= Data_out[15:1];  			  Data_out[0] <= Write_once_status;  			 			end  			 			endmodule

The above example only locks further writes if write_once_status bit is written to one. So it acts as write_1-Once instead of the write-once attribute.

GoodVerilog

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)  			  begin  			   			    Data_out <= 16'h0000;  			    Write_once_status <= 1'b0;  			   			  end  			  else if (write & ~Write_once_status)  			  begin  			   			    Data_out <= Data_in & 16'hFFFE;  			    Write_once_status <= 1'b1; // Write once status set on first write, independent of input  			   			  end  			  else if (~write)  			  begin  			   			    Data_out[15:1] <= Data_out[15:1];  			    Data_out[0] <= Write_once_status;  			   			  end  			 			 			endmodule

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-1224 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-1224, Improper Restriction of Write-Once Bit Fields, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Improper Restriction of Write-Once Bit Fields during Architecture and Design

MITRE associates mitigation with Architecture and Design; documented detection approaches include Automated Analysis; recorded impacts include Varies by Context. Use these source-defined anchors to turn CWE-1224 into implementation, review, and verification checks for the affected component.

CWE-1224: Improper Restriction of Write-Once Bit Fields — MITRE CWE

Triage CWE-1224 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-1224, Improper Restriction of Write-Once Bit Fields, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-1224

Apply MITRE's full root-cause mapping guidance when using CWE-1224, Improper Restriction of Write-Once Bit Fields. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-1224 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-1224, Improper Restriction of Write-Once Bit Fields. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-1224: Improper Restriction of Write-Once Bit Fields?+

CWE-1224: Improper Restriction of Write-Once Bit Fields is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The hardware design control register "sticky bits" or write-once bit fields are improperly implemented, such that they can be reprogrammed by software. 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 define 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 the settings write-once or "sticky." This allows writing to such registers only once, whereupon they become read-only. This is useful to allow initial boot software to configure systems settings to secure values while blocking runtime software from modifying such hardware settings. Failure to implement write-once restrictions in hardware design can expose such registers to being re-programmed by software and written multiple times. For example, write-once fields could be implemented to only be write-protected if they have been set to value "1", wherein they would work as "write-1-once" and not "write-once".

What are the security consequences of Improper Restriction of Write-Once Bit Fields?+

If exploited, CWE-1224 (Improper Restriction of Write-Once Bit Fields) it can compromise Confidentiality, Integrity, Availability and Access Control, leading to outcomes such as Varies by Context.

How do you prevent or mitigate Improper Restriction of Write-Once Bit Fields?+

Recommended mitigations for CWE-1224 include: During hardware design, all register write-once or sticky fields must be evaluated for proper configuration.

How is Improper Restriction of Write-Once Bit Fields detected?+

CWE-1224 can be detected using Automated Analysis. Combining automated tooling with manual review typically yields the best coverage.

Which programming languages are affected by Improper Restriction of Write-Once Bit Fields?+

CWE-1224 commonly affects Verilog and VHDL. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-1224 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-1224: Improper Restriction of Write-Once Bit Fields

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