Qubic vs Monero: Selfish-Mining Timeline and What Changed After

From mid-2025 into spring 2026, Monero lived through the most serious test of its mining security in years. A single project, Qubic, built up a large share of the network’s hashrate, claimed “51%”, and was linked to the deepest chain reorganization in Monero’s history. Then it left for Dogecoin. This is a dated, sourced recap of what happened, what was and wasn’t proven, and what the episode means for miners and merchants now that the dust has settled.

What Qubic is, and why it mined Monero

Qubic describes itself as a layer-1 network with “useful proof of work” aimed at AI computation. Its founder, Sergey Ivancheglo (known online as Come-from-Beyond), steered the network toward mining Monero’s RandomX algorithm with the CPU power of Qubic’s “computors”. Qubic paid those miners its own token, which, as several outlets reported, could make mining XMR through Qubic more profitable than mining XMR directly.

That economic angle matters. Monero relies on CPU mining with RandomX to stay decentralized, and its security budget is far smaller than Bitcoin’s. Anyone willing to subsidize miners can attract hashrate quickly.

Timeline of the episode

August 2025: the “51%” claim

On Monday 11 August 2025 Qubic published a blog post saying it had completed its attempt to dominate the Monero network and reached 51% of hashrate after a month-long push. The same day, Monero’s consensus monitors recorded a six-block-deep reorganization that discarded around 60 previously valid blocks, Cointelegraph reported the next day.

Reactions split sharply:

• Ledger CTO Charles Guillemet said Monero appeared to be in the middle of a successful 51% attack. He estimated that keeping such dominance could cost about $75 million per day, The Block reported on 12 August.
• Luke Parker, lead developer of SeraiDEX, argued that a six-block reorg with orphaned blocks didn’t prove a successful 51% attack. He said the figures “don’t add up”, and that Qubic might have had 51% of estimated blocks but less than 51% of total hashrate.
• SlowMist’s co-founder wrote that the attack “seems to have succeeded” but questioned whether it made economic sense.

Ivancheglo said Qubic planned to demonstrate majority control between 2 and 31 August. He also said Qubic would temporarily stop reporting its pool hashrate, which made independent verification harder. XMR dropped about 8% to around $248 during the turmoil.

The selfish-mining angle

Most technical observers described Qubic’s behaviour as selfish mining. A pool that withholds blocks it has found and releases them strategically can orphan honest miners’ blocks and earn more than its fair share of rewards, even with well under 50% of hashrate. That explains how a pool can cause reorgs and win “most blocks” over a window without holding a true majority of hashpower.

A preprint cited in a May 2026 retrospective estimated Qubic’s real peak share at about 22–34%, not a sustained majority. The claim of a full “51% takeover” is still contested.

Exchange and community reactions

In mid-August Kraken temporarily paused XMR deposits, while keeping withdrawals and trading open. It said a single mining pool had gained more than 50% of the network’s hashing power, which posed a potential risk to network integrity, Cointelegraph reported on 17 August 2025.

Within the Monero community, responses included:

• Moving hashrate. Miners were urged to switch away from large pools, and to decentralized options such as P2Pool, to dilute any single operator.
• Disputing the narrative. Many contributors stressed that Monero’s privacy and cryptography were untouched.
• Fighting back. Ivancheglo said a DDoS around 4 August had cut Qubic’s pool hashrate from 2.6 GH/s to 0.8 GH/s before it recovered.

September 2025: the 18-block reorg

The more damaging event came a month later. Between 14 and 15 September 2025, Monero nodes recorded an 18-block reorganization starting at height 3,499,659, Decrypt and The Block reported on 15 September. Roughly 117–118 transactions sat on the orphaned branch for close to 40 minutes before being invalidated. Decrypt described it as erasing about 36 minutes of transaction history and called it the deepest rollback in Monero’s 12-year history.

Why it mattered:

• The 10-block lock didn’t cover it. Monero wallets make received outputs spendable after 10 blocks, and many services treated about 10 confirmations as final. An 18-block reorg went past that rule of thumb.
• Attribution wasn’t proven. The Block noted that the pool behind the reorg was unknown and that responsibility hadn’t been established. Community podcaster Xenu suggested Qubic had used a selfish-mining variant, and Ivancheglo rejected that.
• Researchers responded. The Monero Research Lab group advised users to wait for more than 10 confirmations before accepting payments. It suggested developers consider temporarily rolling DNS checkpoints, which are signed records that tell nodes which blocks to treat as final.

DNS checkpoints were debated openly because they trade decentralization for safety. They can be deployed quickly, but they add trust in whoever controls the DNS records. Other countermeasures in public discussion included consensus algorithm changes, merge-mining, and ChainLocks-style finality.

Counter-intuitively, XMR rose more than 5% on the day of the reorg news and was up nearly 12% for the week, at about $302, according to Decrypt’s CoinGecko figures.

Software hardening that followed

Not every release in late 2025 was a direct response to Qubic, but node robustness was a clear theme:

• 0.18.4.2 (26 August 2025): fixed a privacy leak when using a malicious remote node.
• 0.18.4.3 (9 October 2025): a “highly recommended” release that adds /24 subnet deduplication in peer selection to disadvantage spy nodes, Cointelegraph reported on 10 October 2025.
• 0.18.5.4 (October 2026): fixed DNSSEC validation used for checkpoints, blocklists and update checks, according to the GitHub release notes.

For a full rundown of these releases, see our coverage in the News section.

April 2026: Qubic moves to Dogecoin

Qubic’s interest moved on. In August 2025 its community voted in an X poll, with Dogecoin winning more than 300 votes, ahead of Kaspa and Zcash, as the next proof-of-work chain to target, Cointelegraph reported on 17 August 2025.

Qubic’s own recap of its 2 April 2026 all-hands call says Dogecoin mining went live on 1 April 2026, starting an estimated four-week transition from Monero. A retrospective published on 29 May 2026 dates the completed migration to 30 April 2026. That piece also points out that Dogecoin’s much larger merge-mined hashrate makes a similar disruption there far less practical.

Note: We could not find a reliable public source quantifying Monero’s hashrate immediately after Qubic left, so we don’t cite a figure. Check live pool distribution on a hashrate tracker before drawing conclusions.

What the episode taught Monero

1. It was an economic-security problem, not a cryptographic one

No privacy property was broken. The ring signatures, stealth addresses and amounts stayed private throughout. The risk came from block ordering and finality, which depend on how hashrate is distributed.

2. Subsidized hashrate is a real threat to smaller PoW chains

A project that pays miners in its own token can rent a big slice of a chain’s security budget. Monero’s RandomX keeps mining open to ordinary CPUs, but it can’t stop an outside network from paying those CPUs more.

3. Confirmation policy matters

4. Decentralized pools are the community’s main lever

Moving hashrate to smaller pools and P2Pool spreads block production across many operators and makes it harder for any single entity to withhold blocks. For a practical guide, see our Mining category.

Tip: If you run a business that accepts XMR, write down your confirmation policy and set it by value. A few extra minutes of waiting is cheap insurance against a deep reorg.

Bottom line

The Qubic saga ran from a June 2025 announcement to an April 2026 exit. In between, it produced a six-block reorg, an exchange deposit pause, and an 18-block reorg that broke the 10-confirmation rule of thumb. Whether Qubic ever held a true majority is still disputed. The broader lesson isn’t: Monero’s privacy held, but its consensus security depends on hashrate staying spread out. Keeping it that way is up to miners and pool choices.