An apparatus has a number of data holding elements for holding data values which are reset to a reset value in response to a transition of a signal at a reset signal input of the data holding element from a first value to a second value. A reset tree is provided to distribute a reset signal received at root node of the reset tree to the reset signal inputs of the data holding elements. At least one reset attack detection element is provided, with its reset signal input coupled to a given node of the reset tree, to assert an error signal when its reset signal input transitions from the first value to a second value. Reset error clearing circuitry triggers clearing of the error signal, when the reset signal at the root node of the reset tree transitions from the second value to the first value.