According to CoinMeta, an assessment by Hacken found that approximately $91.3 billion of USDT tokens, which account for nearly half of the total USDT circulation, are controlled by a multi-signature mechanism involving 2 or more parties. These tokens lack built-in mechanisms for delay, revocation, or rollback. If attackers obtain two of the signature keys, they could change the contract owner and perform management operations such as issuing additional tokens, freezing addresses, and suspending transfers. Hacken emphasizes that there is currently no evidence indicating that these keys have been compromised or that any security incidents have occurred. After conducting a technical risk assessment, Bluechip has upgraded the rating of these tokens from D to C.