Uninitialized Value on Reset for Registers Holding Security Settings
Security-critical logic is not set to a known value on reset.
When the device is first brought out of reset, the state of registers will be indeterminate if they have not been initialized by the logic. Before the registers are initialized, there will be a window during which the device is in an insecure state and may be vulnerable to attack.
The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.
Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.
Shown below is a positive clock edge triggered flip-flop used to implement a lock bit for test and debug interface. When the circuit is first brought out of reset, the state of the flip-flop will be unknown until the enable input and D-input signals update the flip-flop state. In this example, an attacker can reset the device until the test and debug interface is unlocked and access the test interface until the lock signal is driven to a known state by the logic.
The flip-flop can be set to a known value (0 or 1) on reset, but requires that the logic explicitly update the output of the flip-flop if the reset signal is active.
Weaknesses in this category are related to system power, voltage, current, temperature, clocks, system state saving/restoring, and resets at the platform and SoC level.
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