Blockchain reorganizations, the place networks discard current blocks to comply with an extended chain, have uncovered weaknesses in proof-of-work (PoW) methods, highlighted by Monero’s August 2025 ordeal and earlier disruptions throughout different blockchains.
Decoding Chain Reorgs
A blockchain reorganization, or reorg, occurs when a sequence of blocks is deserted in favor of a competing model with better cumulative proof-of-work (PoW), successfully rewriting a chunk of the ledger. Reorgs roll again transactions in orphaned blocks, sending them again into the mempool for potential inclusion—or omission—later.
This creates openings for double-spending, the place attackers can spend cash on a discarded chain but nonetheless retain them after the reorg. In August 2025, Monero endured repeated reorgs tied to the Qubic mining pool, which amassed a dominant share of hashrate. Qubic publicly described the trouble as an experiment, leveraging its PoW setup to mine Monero blocks and declare rewards.
Picture supply: Vini Barbosa. The X account Vini Barbosa reported that Monero had two 9-block-deep reorgs inside 60 minutes from blocks 3485726 → 3485734 to 3485708 → 3485716.
That power initially enabled a six-block reorg, exhibiting how the ledger might be rewritten. A number of extra adopted, together with a not too long ago reported nine-block reorg that occurred two occasions. Monero’s reorgs stemmed from Qubic’s superior hashrate, permitting non-public mining of an extended chain earlier than revealing it, which pressured nodes to change. The dangers embody double-spending, transaction censorship, and the headache of erased blocks.
Exchanges like Kraken suspended deposits, later demanding 720 confirmations—far past the same old 10—to protect towards losses. The turmoil sparked debate over revamping Monero’s consensus, with proposals starting from merge mining with Bitcoin, to geographically distributed {hardware} to weaken massive swimming pools, to Sprint’s Chainlocks, the place masternodes lock blocks to dam reorgs.
In August 2021, Bitcoin SV confronted the same check when an unknown miner managed over half its hashrate, pulling off a reported a large 100-block reorg. The occasion splintered the chain into three variations, shaking reliability. The trigger was traced to stealth miners constructing hidden chains, resulting in acquainted dangers: double-spends, instability, and shaken confidence.
Reorgs spotlight PoW’s probabilistic finality: Transactions change into safer with further confirmations, however a 51% benefit can override them. Each episodes reveal reorgs as pure correction instruments twisted into assault strategies, fueling requires stronger decentralization and hybrid protections.
Monero and BSV’s experiences reveal the two-sided nature of reorgs—extraordinary in wholesome operation however disruptive when weaponized—pointing to the significance of extensively distributed hashrate to protect a blockchain’s integrity.
Bitcoin ( BTC) is way costlier to assault due to its sheer hashrate dominance in comparison with different PoW blockchains. The community runs on a whole bunch of exahashes per second (EH/s), powered by globally distributed mining farms working specialised ASIC {hardware}.
To reorganize Bitcoin’s chain, an attacker would want to secretly marshal a majority of that hashrate, a feat that requires billions of {dollars} in mining rigs, industrial-scale infrastructure, and big quantities of electrical energy. The extent of funding wanted makes such an try economically irrational.
Monero ( XMR) and Bitcoin SV ( BSV) are far cheaper to assault as a result of their PoW methods function on a fraction of Bitcoin’s hashrate, and the price of entry for mining is drastically decrease.
