BlockSec Phalcon: The recent Balancer Hacker attack method is highly complex.

According to Foresight News, BlockSec Phalcon monitoring has reported that several projects based on Balancer were attacked a few hours ago, resulting in losses exceeding $120 million. This is a highly complex attack method. Preliminary analysis shows that the fundamental cause of the attack was the manipulation of the invariant, which distorted the BPT price calculation, allowing the attacker to profit from a single batch swap in a specific stable pool. For example, in an attack transaction on Arbitrum, the batch swap operation can be divided into three stages: 1. The attacker swaps BPT for the underlying asset to precisely adjust the balance of one token (cbETH) to the edge of the rounding boundary (amount = 9), laying the groundwork for the precision loss in the next step. 2. The attacker then uses a specific amount (=8) to swap between another underlying asset (wstETH) and cbETH. Due to rounding down when scaling the token amount, the calculated Δx slightly decreases (from 8.918 to 8), causing Δy to be underestimated, thereby reducing the invariant (D in Curve's StableSwap model). Since BPT price = D / totalSupply, the BPT price is artificially depressed. 3. The attacker reverses the swap of the underlying asset back to BPT, profiting from the depressed BPT price while restoring the balance.