Web browsers and operating systems making a HTTPS request via a proxy server are vulnerable to man-in-the-middle (MITM) attacks against HTTP CONNECT requests and proxy response messages. HTTP CONNECT requests are made in clear text over HTTP, meaning an attacker in the position to modify proxy traffic may force the use of 407 Proxy Authentication Required responses to phish for credentials.

WebKit-based clients are vulnerable to additional vectors due to the fact that HTML markup and JavaScript are rendered by the client Document Object Model (DOM) in the context of the originally requested HTTPS domain.

For more information, refer to the FalseCONNECT website.

An attacker in the position to control HTTP CONNECT requests and proxy responses can conduct MITM attacks, which may include credential phishing and, where vulnerable WebKit-based clients are involved, arbitrary HTML and JavaScript injection.

Apply an update

Check with affected software vendors and apply an update, if available. Those unable or unwilling to apply an update should consider the following workarounds.

Avoid untrusted networks

Avoid using proxy-configured clients while connected to untrusted networks, including public WiFi. Using a proxy-configured client on an untrusted network increases the chance of falling victim to a MITM attack.

Disable proxy configuration settings

If use of proxy auto-configuration (PAC) or web proxy auto-discovery (WPAD) is not required, consider disabling them.