  Cryptocurrency achieves distributed consensus using proof of work (PoW).
Prior research in blockchain security identified financially incentivized
attacks based on withholding blocks which have the attacker compromise a victim
pool and pose as a PoW contributor by submitting the shares (earning credit for
mining) but withholding the blocks (no actual contributions to the pool). We
advance such threats to generate greater reward advantage to the attackers
while undermining the other miners and introduce the share withholding attack
(SWH). SWH withholds shares to increase the attacker's reward payout within the
pool, in contrast to the prior threats withholding blocks, and rather builds on
the block-withholding threats in order to exploit the information about the
impending block submission timing, challenging the popularly established
assumption that the block submission time is completely random and unknown to
miners. We analyze SWH's incentive compatibility and the vulnerability scope by
identifying the critical systems and environmental parameters which determine
the attack's impact. Our results show that SWH in conjunction with block
withholding yield unfair reward advantage at the expense of the
protocol-complying victim miners and that a rational miner will selfishly
launch SWH to maximize its reward profit. We inform the blockchain and
cryptocurrency research of the novel SWH threat and include the potential
countermeasure directions to facilitate such research and development.
