Security agency: Balancer attackers' invariant attack on BPT price calculation may be the main reason for asset theft.

BlockSec Phalcon, the on-chain tracing platform under the security firm BlockSec, published an article on the X platform stating that "Balancer and several of its forks were attacked several hours ago, resulting in more than $120 million in losses across multiple chains. This was an extremely complex attack. Preliminary analysis indicates that the root cause was that the attacker manipulated the invariant calculation of BPT price, thereby distorting the BPT price calculation and allowing the attacker to profit from a specific stablecoin pool through a single batch transaction. Taking the attack transaction against Arbitrum as an example, the batch exchange operation can be broken down into three stages: 1. The attacker exchanges BPT for the underlying asset to precisely adjust the balance of a token (cbETH) to be close to the rounding boundary (amount = 9). This sets the stage for the subsequent loss of precision; 2. The attacker then uses a pre-constructed amount (= 8) to exchange between another underlying token (wstETH) and cbETH. Due to the rounding down when scaling token amounts, the calculated Δx is slightly reduced (8 0.918 to 8), resulting in Δy The price of BPT is undervalued, thus reducing the invariant (D) in Curve's StableSwap model. Since BPT price = D / total supply, the price of BPT is artificially suppressed; 3. The attacker reverses the underlying asset back to BPT, restoring balance and profiting from the decline in BPT price.