Balancer V1

An attacker first used swaps to reduce the Balancer V1 BPool’s WBTC reserve to near-dust levels, then exploited rounding in joinswapPoolAmountOut. When the function reverse-calculated the WBTC required to mint a caller-specified amount of BPT, 18-decimal fixed-point arithmetic rounded the required input down to 1 satoshi while still minting the full BPT amount.

8 BPT through these underpriced joins and exited the pool proportionally to drain DPI, USDC, WETH, and WBTC.