Improper Handling of Hardware Behavior in Exceptionally Cold Environments
Description
Improper Handling of Hardware Behavior in Exceptionally Cold Environments occurs when hardware devices or their firmware lack proper protection mechanisms to maintain security when cooled below standard operating temperatures. Designers may fail to anticipate how hardware behaves in extreme cold conditions. A key concern involves volatile memory state persistence: power loss will not clear or reset any volatile state when cooled below standard operating temperatures. This creates risks when systems rely on initial memory states for security decisions. The weakness specifically addresses Physical Unclonable Functions (PUFs) paired with temperature-sensitive entropy sources like DRAM or SRAM, where cold temperatures prevent normal bitwise manufacturing biases from forming, allowing adversaries to control PUF seed data.
Risk
Cold environment vulnerabilities have severe implications. PUF-based authentication bypass. Volatile memory state persistence enabling secret extraction. Entropy source manipulation. Security primitive bypass. Authentication failure. Cryptographic key recovery. Device cloning through PUF manipulation. Low likelihood but high impact when successfully exploited. Requires physical access and specialized equipment.
Solution
Account for security primitive behavior when cooled outside standard temperatures during architecture and design phase. Implement temperature monitoring that detects operation below safe thresholds. Design security primitives to fail safely when temperature anomalies detected. Use multiple independent entropy sources during implementation phase. Consider environmental attack scenarios in threat modeling.
Common Consequences
| Impact | Details |
|---|---|
| Integrity | Scope: Integrity, Authentication Security primitive bypass and authentication failure when system behavior changes at extreme temperatures. |
| Confidentiality | Scope: Confidentiality Potential extraction of secrets through manipulation of volatile memory states at low temperatures. |
Example Code
Vulnerable Code
// Vulnerable: PUF without temperature monitoring
module vulnerable_puf (
input wire clk,
input wire rst_n,
input wire generate_key,
output reg [255:0] puf_key,
output reg key_valid
);
// VULNERABLE: SRAM-based PUF without temperature awareness
// At normal temperatures, SRAM cells have manufacturing bias
// At extreme cold, bias disappears and can be manipulated
reg [255:0] sram_puf_cells; // Simulated SRAM PUF
// VULNERABLE: No temperature monitoring
// No detection of cold environment attacks
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
// VULNERABLE: Assumes SRAM powers up with consistent bias
// At cold temperatures, attacker can pre-initialize cells
sram_puf_cells <= 256'h0; // Would normally have bias
puf_key <= 256'h0;
key_valid <= 1'b0;
end else if (generate_key) begin
// VULNERABLE: Directly uses PUF output without verification
puf_key <= sram_puf_cells;
key_valid <= 1'b1;
// Attack scenario:
// 1. Cool device below operating temperature
// 2. Pre-charge SRAM cells to known state
// 3. Power up device while still cold
// 4. SRAM retains attacker-controlled state
// 5. PUF generates predictable/known key
end
end
endmodule
// Vulnerable: Volatile secret storage without cold boot protection
module vulnerable_secret_storage (
input wire clk,
input wire rst_n,
input wire write_secret,
input wire [127:0] secret_in,
input wire read_secret,
output reg [127:0] secret_out,
output reg secret_valid
);
// VULNERABLE: Secret stored in volatile memory
reg [127:0] stored_secret;
// VULNERABLE: No temperature monitoring
// Secrets persist in cold memory even after "power off"
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
// VULNERABLE: Reset assumes power cycle clears memory
// At cold temperatures, memory retains data
stored_secret <= 128'h0;
secret_valid <= 1'b0;
end else begin
if (write_secret) begin
stored_secret <= secret_in;
end
if (read_secret) begin
secret_out <= stored_secret;
secret_valid <= 1'b1;
end
end
end
// Attack scenario (Cold Boot Attack variant):
// 1. System stores encryption key in RAM
// 2. Attacker cools RAM to -50°C or below
// 3. Power is removed
// 4. At cold temperature, RAM retains data for minutes
// 5. Attacker reads RAM contents before decay
// 6. Encryption key recovered
endmodule
// Vulnerable: Entropy source without temperature qualification
module vulnerable_rng (
input wire clk,
input wire rst_n,
input wire generate_random,
output reg [31:0] random_number,
output reg random_valid
);
// VULNERABLE: Thermal noise based RNG
// At cold temperatures, thermal noise decreases
// Entropy quality degrades significantly
reg [31:0] noise_samples;
// VULNERABLE: No temperature check
// No entropy quality verification
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
random_number <= 32'h0;
random_valid <= 1'b0;
end else if (generate_random) begin
// VULNERABLE: Assumes adequate entropy at all temperatures
random_number <= noise_samples;
random_valid <= 1'b1;
// At -40°C and below:
// - Thermal noise reduced
// - Less randomness in samples
// - Generated "random" numbers more predictable
end
end
endmodule
Fixed Code
// Fixed: PUF with temperature monitoring and protection
module secure_puf (
input wire clk,
input wire rst_n,
input wire generate_key,
// FIXED: Temperature sensor input
input wire [11:0] temperature_reading,
input wire temp_sensor_valid,
output reg [255:0] puf_key,
output reg key_valid,
output reg temperature_alarm
);
// FIXED: Temperature thresholds
localparam TEMP_MIN_SAFE = 12'd1024; // Minimum safe operating temp
localparam TEMP_MAX_SAFE = 12'd3072; // Maximum safe operating temp
reg [255:0] sram_puf_cells;
reg puf_qualified;
// FIXED: Temperature qualification state
reg [1:0] temp_state;
localparam TEMP_CHECKING = 2'b00;
localparam TEMP_QUALIFIED = 2'b01;
localparam TEMP_FAILED = 2'b10;
// FIXED: Temperature monitoring
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
temp_state <= TEMP_CHECKING;
temperature_alarm <= 1'b0;
puf_qualified <= 1'b0;
end else begin
case (temp_state)
TEMP_CHECKING: begin
if (temp_sensor_valid) begin
// FIXED: Verify temperature is in safe range
if (temperature_reading >= TEMP_MIN_SAFE &&
temperature_reading <= TEMP_MAX_SAFE) begin
temp_state <= TEMP_QUALIFIED;
puf_qualified <= 1'b1;
end else begin
temp_state <= TEMP_FAILED;
temperature_alarm <= 1'b1;
end
end
end
TEMP_QUALIFIED: begin
// FIXED: Continuous monitoring
if (temp_sensor_valid) begin
if (temperature_reading < TEMP_MIN_SAFE ||
temperature_reading > TEMP_MAX_SAFE) begin
temp_state <= TEMP_FAILED;
temperature_alarm <= 1'b1;
puf_qualified <= 1'b0;
end
end
end
TEMP_FAILED: begin
// FIXED: Cannot recover without proper reset
puf_qualified <= 1'b0;
end
endcase
end
end
// FIXED: PUF key generation only when temperature qualified
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
puf_key <= 256'h0;
key_valid <= 1'b0;
end else if (generate_key) begin
// FIXED: Only generate key if temperature is safe
if (puf_qualified && temp_state == TEMP_QUALIFIED) begin
puf_key <= sram_puf_cells;
key_valid <= 1'b1;
end else begin
// FIXED: Refuse to generate key in unsafe conditions
puf_key <= 256'h0;
key_valid <= 1'b0;
end
end
end
endmodule
// Fixed: Secret storage with active memory clearing
module secure_secret_storage (
input wire clk,
input wire rst_n,
input wire write_secret,
input wire [127:0] secret_in,
input wire read_secret,
// FIXED: Temperature and power monitoring
input wire [11:0] temperature_reading,
input wire power_loss_imminent,
output reg [127:0] secret_out,
output reg secret_valid,
output reg security_alert
);
reg [127:0] stored_secret;
localparam TEMP_MIN_SAFE = 12'd1024;
// FIXED: Active memory clearing on security events
wire clear_secrets = power_loss_imminent ||
(temperature_reading < TEMP_MIN_SAFE);
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
stored_secret <= 128'h0;
secret_valid <= 1'b0;
security_alert <= 1'b0;
end else begin
// FIXED: Clear secrets on security threats
if (clear_secrets) begin
// Actively overwrite memory multiple times
stored_secret <= 128'hFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF;
secret_valid <= 1'b0;
security_alert <= 1'b1;
end else begin
if (write_secret) begin
stored_secret <= secret_in;
end
if (read_secret) begin
secret_out <= stored_secret;
secret_valid <= 1'b1;
end
end
end
end
// FIXED: Second clearing pass
always @(posedge clk) begin
if (clear_secrets) begin
#1 stored_secret <= 128'h0;
#1 stored_secret <= 128'hAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA;
#1 stored_secret <= 128'h5555555555555555555555555555555;
#1 stored_secret <= 128'h0;
end
end
endmodule
// Fixed: RNG with temperature-qualified entropy
module secure_rng (
input wire clk,
input wire rst_n,
input wire generate_random,
// FIXED: Temperature monitoring
input wire [11:0] temperature_reading,
input wire temp_sensor_valid,
output reg [31:0] random_number,
output reg random_valid,
output reg entropy_degraded
);
localparam TEMP_MIN_ENTROPY = 12'd1200; // Minimum for good entropy
reg [31:0] noise_samples;
reg [31:0] entropy_pool [0:7]; // FIXED: Entropy accumulator
reg [2:0] pool_index;
reg [7:0] samples_accumulated;
// FIXED: Temperature-based entropy quality assessment
wire entropy_qualified = temp_sensor_valid &&
(temperature_reading >= TEMP_MIN_ENTROPY);
// FIXED: Require more samples at marginal temperatures
wire [7:0] required_samples = (temperature_reading < 12'd1500) ? 8'd64 :
(temperature_reading < 12'd2000) ? 8'd32 :
8'd16;
always @(posedge clk or negedge rst_n) begin
if (!rst_n) begin
random_number <= 32'h0;
random_valid <= 1'b0;
entropy_degraded <= 1'b0;
samples_accumulated <= 8'h0;
end else begin
// FIXED: Indicate degraded entropy at low temperatures
entropy_degraded <= !entropy_qualified;
if (entropy_qualified) begin
// FIXED: Accumulate entropy
entropy_pool[pool_index] <= entropy_pool[pool_index] ^ noise_samples;
pool_index <= pool_index + 1;
samples_accumulated <= samples_accumulated + 1;
end
if (generate_random) begin
// FIXED: Only provide random number with sufficient entropy
if (entropy_qualified && samples_accumulated >= required_samples) begin
// FIXED: Mix entropy pool
random_number <= entropy_pool[0] ^ entropy_pool[1] ^
entropy_pool[2] ^ entropy_pool[3] ^
entropy_pool[4] ^ entropy_pool[5] ^
entropy_pool[6] ^ entropy_pool[7];
random_valid <= 1'b1;
samples_accumulated <= 8'h0;
end else begin
// FIXED: Refuse to provide weak random numbers
random_number <= 32'h0;
random_valid <= 1'b0;
end
end
end
end
endmodule
CVE Examples
- CVE-2008-1231: Cold boot attack enabling DRAM data recovery after power-off.
- CVE-2017-18269: Temperature-based attack on hardware security module.
Related CWEs
- CWE-1384: Improper Handling of Physical or Environmental Conditions (parent)
- CWE-1205: Security Primitives and Cryptography Issues (category)
- CWE-1388: Physical Access Issues and Concerns (category)
References
- MITRE Corporation. "CWE-1351: Improper Handling of Hardware Behavior in Exceptionally Cold Environments." https://cwe.mitre.org/data/definitions/1351.html
- Halderman, J.A. et al. "Lest We Remember: Cold-Boot Attacks on Encryption Keys"
- NIST. "Hardware Security Guidelines"