A Reddio-linked Ethereum vault suffered a loss of approximately 9.25 ETH after an attacker exploited a cross-vault asset double-counting flaw involving stETH. Existing transaction analysis identified RedSonic Vault, where an unrestricted registerErc20 function allowed a second stETH share class, rsvstETH, to reference collateral already backing rsvETH. The attacker used a 1,139 WETH Balancer flash loan, deposited 9.34 stETH without minting corresponding rsvETH shares, inflated the raw-balance-based pricing, and redeemed overlapping claims on the pooled collateral. The recovered stETH was exchanged for ETH on Curve and the loan was repaid in the same transaction. The incident was highlighted by @SlowMist_Team and has renewed attention on vault security, audit practices, liquidity and market confidence in stETH; trading volume for stETH was reported as unavailable after the exploit notification.