Nomic nBTC Double-Spend Exploit

On September 9, 2026, Nomic — the Bitcoin bridge that mints nBTC for the Cosmos and Osmosis ecosystems — disclosed a double-spend exploit that left roughly 36% of Osmosis's alloyed Bitcoin (allBTC) without reserves, a backing shortfall worth about $3.1 million.

What happened

Nomic's custom IBC forwarding mechanism failed to verify account ownership when it processed Bitcoin deposits. By chaining two separate bugs, an attacker was able to double-spend nBTC and deliver false transfer vouchers to Osmosis over the Inter-Blockchain Communication protocol. In total the attacker minted 39.84 nBTC — roughly 40.65 BTC of nominal value — with no Bitcoin behind it, worth about $3.09 million at the ~$77,700 BTC price of the time. The invalid tokens had been created earlier and went undetected for an extended period; the discrepancy surfaced only after Nomic stopped producing Bitcoin checkpoints around September 6–7. Osmosis published an official statement at 09:48 UTC on September 9 and immediately froze deposits, withdrawals, and redemptions for allBTC.

Aftermath

An emergency chain upgrade quarantined 22.65 BTC tied to the attacker's address, leaving a gap of roughly 17 BTC to fully restore allBTC's backing. Osmosis stressed that neither Osmosis itself nor IBC was compromised — the flaw lived entirely in the Nomic chain. A complete post-mortem had not been published as of late September 2026, and because the quarantined funds cover only part of the shortfall, the status here is partially-recovered.

Why it matters

Wrapped and "alloyed" assets inherit the trust assumptions of every bridge that mints them: allBTC was only as sound as Nomic's weakest verification check. As with the same-week Symbiosis fake-syBTC mint and the earlier Allbridge forged-CCTP exploit, the recurring lesson is that a bridge must independently prove that real collateral exists before it credits a mint — a valid-looking message or voucher is not proof of funds. That the exploit sat unnoticed for weeks also shows how thin monitoring can be around cross-chain reserve accounting.