http://y7yea4pmqqtznb33qiugvysyn2bob5v62e4pvoadoibrwkq3tsddjeyd.onion/zh-hans/glossary/man-in-the-middle-attack
Even though you think you are communicating securely with Bahram, you are, in fact, only communicating securely with the spy, who is also communicating securely to Bahram! This is the man-in-the-middle attack.