open secret

1. N-COUNT If you refer to something as an open secret, you mean that it is supposed to be a secret, but many people know about it. 公開(kāi)的秘密

It's an open secret that the security service bugged telephones. 安全部門(mén)竊聽(tīng)電話(huà)是一個(gè)公開(kāi)的秘密。