Security firm: The main reason for the asset theft may be that the Balancer attacker conducted an invariant attack on BPT price calculation

According to ChainCatcher, the security agency BlockSec's on-chain tracking platform BlockSec Phalcon posted on X, stating, "Balancer and several of its forked projects were attacked several hours ago, resulting in losses of over $120 million across multiple chains. This was an extremely sophisticated attack."

Preliminary analysis indicates that the root cause was the attacker's manipulation of the BPT price calculation invariant, which distorted the BPT price calculation and enabled the attacker to profit from specific stablecoin pools through a single batch transaction.

Taking the attack transaction on Arbitrum as an example, the batch swap operation can be broken down into three stages: 1. The attacker exchanges BPT for underlying assets to precisely adjust the balance of one token (cbETH), bringing it close to the rounding boundary (amount = 9). This sets up the conditions for the next step's precision loss; 2. The attacker then uses a pre-constructed amount (= 8) to swap between another underlying token (wstETH) and cbETH. Due to rounding down when scaling token amounts, the calculated Δx is slightly reduced (from 8 0.918 to 8), causing Δy to be underestimated, which in turn reduces the invariant (D) in Curve's StableSwap model. Since BPT price = D / total supply, the BPT price is artificially suppressed; 3. The attacker reverses the underlying assets back to BPT, restoring balance while profiting from the drop in BPT price.