Balancer V1 (BPool rounding) hack — August 2026

Legacy Balancer V1 pools lost about $1.39m from 31 August 2026 to a rounding error in their fixed-point arithmetic. The first exploit transaction landed at 02:23 UTC in block 25,872,249 and took about $234,000 from a single pool: the attacker used flash loans to push the pool's wrapped bitcoin reserve close to zero, at which point the 18-decimal maths in the single-sided join path rounded so badly that one satoshi of input minted 4,408.8 pool tokens, which were then exited against the pool's reserves. Copycats replicated the technique across the long tail of remaining V1 pools, and Balancer's own accounting puts the total drained from 120 pools at $1,393,694.67 at attack-time prices.

SlowMist noted that the pool lacked the safeguards that would have stopped this: a minimum effective input, a minimum pool balance, and validation of relative error. The contract did carry a minimum-balance check, but it was enforced only when assets were bound or rebound to the pool, not during ordinary swaps.

Part of it came back. White hats front-ran some of the copycat extractions, winning the race with block-builder payments that consumed about $169,000, and between 8 and 16 September 296.401711 ETH was returned to the Balancer DAO multisig: 120 ETH from the main exploiter, whom Balancer describes as a greyhat, 168.23 ETH from three anonymous white hats after each kept a 10% bounty, and 8.17 ETH from the block builder Ultrasound.money. That is about $715,000 at 31 August prices, roughly half the loss, and it is recorded as recovered. A proposal posted on 18 September would split it across the 120 pools in proportion to each pool's attack-time loss and then among liquidity providers by their holdings at block 25,872,248; it was still unnumbered and had not gone to a vote on 24 September, and no claim window is open.

The reason the entry matters beyond its size is that the code is unmaintained and widely forked. Balancer Labs wound down as a company earlier in 2026 following the much larger V2 exploit recorded elsewhere in this registry, the V1 contracts are immutable with no pause mechanism, and on 14 September a proposal for an orderly wind-down of the Balancer DAO itself was posted. Balancer warned users that the legacy contracts put liquidity provider funds at risk. Forks running the same code remain exposed, and this entry exists partly so that the second occurrence of a bug family is on the record alongside the first.