SSL tampering via non-200 responses to proxy CONNECT requests — Mozilla

Microsoft security researchers Shuo Chen, Ziqing Mao, Yi-Min Wang, and Ming Zhang reported that when a CONNECT request is sent to a proxy server and a non-200 response is returned, then the body of the response is incorrectly rendered within the context of the request Host: header. An active network attacker could use this vulnerability to intercept a CONNECT request and reply with a non-200 response containing malicious code which would be executed within the context of the victim’s requested SSL-protected domain. Since this attack requires the victim to have a proxy configured, the severity of this issue was determined to be high.