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

What is Improper Restriction of Write-Once Bit Fields?

• Overview: Improper Restriction of Write-Once Bit Fields (CWE-1224) occurs when hardware design allows software to reprogram registers that are supposed to be write-once, potentially leading to security vulnerabilities.

• Exploitation Methods:

Attackers can exploit this vulnerability by reprogramming sensitive registers after the initial configuration.
Common attack patterns include manipulating system settings to disable security features or alter system states without authorization.

• Security Impact:

Direct consequences include unauthorized modification of critical hardware settings.
Potential cascading effects can lead to system instability, bypass of security controls, and exposure of sensitive data.
Business impact may involve compromised system integrity, data breaches, and loss of customer trust.

• Prevention Guidelines:

Specific code-level fixes include ensuring proper implementation of write-once protection mechanisms in the hardware description language (HDL) code.
Security best practices involve thorough testing and verification of write-once bit fields during the hardware design phase.
Recommended tools and frameworks include using hardware verification tools that can simulate and validate write-once behaviors in registers.

Corgea can automatically detect and fix Improper Restriction of Write-Once Bit Fields in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Verilog, VHDL

Affected Technologies: System on Chip

Vulnerable Code Example

Verilog Example

module register_control (
    input wire clk,
    input wire reset,
    input wire [3:0] write_data,
    input wire write_enable,
    output reg [3:0] sticky_bits // Sticky bits should be write-once
);
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            sticky_bits <= 4'b0000;
        end else if (write_enable) begin
            sticky_bits <= write_data; // Vulnerability: sticky_bits can be overwritten
        end
    end
endmodule

Explanation of Vulnerability

Improper Restriction: The sticky_bits register is intended to be write-once. However, due to the lack of proper checks, it can be overwritten whenever write_enable is high.
Security Impact: This improper restriction could allow unauthorized changes to critical configuration states, leading to potential security risks or system malfunctions.

How to fix Improper Restriction of Write-Once Bit Fields?

To fix the vulnerability:

Implement Write-Once Logic: Ensure the sticky_bits register can only be written once after a reset. Additional writes should be ignored.
Use Bitwise Logic: Apply a bitwise AND operation with the complement of the current value to ensure that only unset bits (i.e., 0s) can be set.

Fixed Code Example

module register_control (
    input wire clk,
    input wire reset,
    input wire [3:0] write_data,
    input wire write_enable,
    output reg [3:0] sticky_bits // Sticky bits are now properly restricted to write-once
);
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            sticky_bits <= 4'b0000;
        end else if (write_enable) begin
            // Fix: Ensure sticky_bits are write-once by masking
            sticky_bits <= sticky_bits | (write_data & ~sticky_bits);
        end
    end
endmodule

Explanation of Fix

Write-Once Enforcement: By using the expression sticky_bits | (write_data & ~sticky_bits), the code ensures that only the bits in write_data that correspond to 0s in sticky_bits are set. This effectively makes sticky_bits a write-once register.
Benefits: This prevents any further modifications after the initial setting, thus maintaining the integrity of critical configuration states.

This improved example ensures that the code is properly formatted and the explanations are clear, providing a thorough understanding of the vulnerability and its fix.