Balancer has identified a vulnerability in its V2 Composable Stable Pools that led to a recent exploit. The issue arose from the upscale function's rounding logic in the EXACT_OUT path, which introduced a precision error when the scaling factor was non-integer. Attackers exploited this flaw in conjunction with the batchSwap's deferred settlement mechanism, which allows temporary borrowing of assets during transactions. Additionally, the design treating BPT as a regular token enabled bypassing minimum pool share restrictions, reducing pool liquidity to minimal levels. This allowed the manipulation of pool balances and extraction of funds.