The combination of BCH's rapid difficulty adjustment algorithm and Bitcoin ABC's rolling checkpoints introduces a novel fragmentation attack strategy. By generating a low-PoW alternate chain from a chosen point in the BCH blockchain, an attacker can rapidly reduce the difficulty to extend it. No node which has observed the legitimate chain at higher difficulty will accept the attacker's chain, however, under a temporary eclipse attack new nodes could be led to follow it.
Normally, this attack could only be sustained while the targets remain eclipsed from the legitimate network. Rolling checkpoints provide a window of opportunity during which the attacker can rapidly extend their alternate chain, triggering finalization. Should a target observe the legitimate chain after that point it will be regarded as an excessive reorg and treated as invalid.
The scope of this attack is limited by a number of factors:
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The attacker must perform enough work on their chain to discover blocks at rate which will eventually cause the difficulty to drop. They must then sustain that chain with minimal PoW to keep the network time within a usable range.
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Targets must be susceptible to an eclipse attack.
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Fork/release-based checkpoints will eventually invalidate the attacker's alternate chain.
These are certainly steep obstacles for the attacker, however, the most expensive issue (1) can benefit from reuse of work. Only one attack chain needs to be created that reduces the difficulty, after which any number of additional branches can be spawned from that parent. These branches can be kept up to date using minimal work to prevent the difficulty from rising before application in an attack.
Issue (3) somewhat mitigates the efficiency gain through rework. As the attacker's chains conflict with new release-based checkpoints, another difficulty-lowering chain would need to be created to attack new client versions.
Overall the practicality and potential impact of this strategy is low, but it is within reach of a moderately funded attacker. It is an example of attacks that clients may be exposed to when defecting from the "honest" strategy of accepting the longest observed chain.
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source https://www.reddit.com/r/btc/comments/aweqpp/slow_burn_fragmentation_attack/
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