A legacy Balancer V1 liquidity pool was drained of approximately $234,000 on August 31 after an attacker exploited a rounding weakness in its single-asset deposit calculations. Security firm SlowMist traced the incident to the joinswapPoolAmountOut function, where extreme manipulation of the pool’s WBTC balance caused the required deposit to round down to just one satoshi while still issuing thousands of Balancer Pool Tokens.
The affected Ethereum pool contained WBTC, WETH, USDC and DeFi Pulse Index tokens. The attacker used nested flash liquidity involving Spark or Aave, Morpho and Uniswap V3 to push WBTC reserves toward zero before exploiting the calculation. The attack ultimately minted about 4,408.8 BPT for negligible WBTC inputs and then redeemed those pool tokens for real assets held by the contract.
Rounding Bug Turned Dust Into Pool Tokens
Balancer V1 performs parts of its pool mathematics using 18-decimal fixed-point arithmetic, while WBTC itself uses eight decimals. Once the attacker reduced WBTC reserves to a dust level, the reverse calculation determining how much WBTC was needed for a requested BPT output lost enough precision to round the input down dramatically. The vulnerability was therefore not simply that WBTC has fewer decimals, but that the contract’s rounding behavior became exploitable under an extreme reserve state.
SlowMist identified additional missing safeguards, including no minimum effective input, no minimum reserve requirement on the vulnerable join path and no relative-error check. A MIN_BALANCE protection existed elsewhere in the V1 contracts but applied to bind and rebind, not the exploited transaction path. The incident shows how a mathematically valid calculation can still become economically unsafe when boundary conditions are insufficiently constrained.
That broader software risk is not unique to DeFi. The U.S. National Vulnerability Database documented another 2026 fixed-point arithmetic vulnerability in which incorrect rounding direction produced unsafe results in smart-contract math. Precision and rounding are security properties when financial software uses integer arithmetic to determine how much value users can deposit, withdraw or receive.
Legacy Contracts Leave Few Recovery Options
The exploited V1 contracts date to Balancer’s earliest architecture and are immutable, meaning developers cannot simply patch deployed pools or centrally pause them. The episode comes after Balancer began winding down Balancer Labs as a corporate entity following the much larger 2025 V2 exploit. An official Balancer governance proposal confirms that operations were subsequently consolidated under Balancer OpCo Limited as an agent of the DAO. The corporate restructuring did not remove old V1 contracts from Ethereum, leaving residual liquidity exposed to code that can no longer be upgraded.
The latest exploit is separate from the 2025 Balancer V2 incident, although both involved calculation and rounding weaknesses. For remaining V1 liquidity providers, the practical risk is particularly stark: immutable legacy contracts can continue operating indefinitely even after active development has moved elsewhere. With no automatic migration capable of removing positions from those pools, users retaining capital in deprecated deployments must assess and withdraw that liquidity themselves.
