Organizations that have performed a risk assessment and determined that HTTPS inspection is a requirement should ensure their HTTPS inspection products are performing correct transport layer security (TLS) certificate validation. Products that do not properly ensure secure TLS communications and do not convey error messages to the user may further weaken the end-to-end protections that HTTPS aims to provide.

TLS and its predecessor, Secure Sockets Layer (SSL), are important Internet protocols that encrypt communications over the Internet between the client and server. These protocols (and protocols that make use of TLS and SSL, such as HTTPS) use certificates to establish an identity chain showing that the connection is with a legitimate server verified by a trusted third-party certificate authority.

HTTPS inspection works by intercepting the HTTPS network traffic and performing a man-in-the-middle (MiTM) attack on the connection. In MiTM attacks, sensitive client data can be transmitted to a malicious party spoofing the intended server. In order to perform HTTPS inspection without presenting client warnings, administrators must install trusted certificates on client devices. Browsers and other client applications use this certificate to validate encrypted connections created by the HTTPS inspection product. In addition to the problem of not being able to verify a web server’s certificate, the protocols and ciphers that an HTTPS inspection product negotiates with web servers may also be invisible to a client. The problem with this architecture is that the client systems have no way of independently validating the HTTPS connection. The client can only verify the connection between itself and the HTTPS interception product. Clients must rely on the HTTPS validation performed by the HTTPS interception product.

A recent report, The Security Impact of HTTPS Interception [2], highlighted several security concerns with HTTPS inspection products and outlined survey results of these issues. Many HTTPS inspection products do not properly verify the certificate chain of the server before re-encrypting and forwarding client data, allowing the possibility of a MiTM attack. Furthermore, certificate-chain verification errors are infrequently forwarded to the client, leading a client to believe that operations were performed as intended with the correct server. This report provided a method to allow servers to detect clients that are having their traffic manipulated by HTTPS inspection products. The website badssl.com [3] is a resource where clients can verify whether their HTTPS inspection products are properly verifying certificate chains. Clients can also use this site to verify whether their HTTPS inspection products are enabling connections to websites that a browser or other client would otherwise reject. For example, an HTTPS inspection product may allow deprecated protocol versions or weak ciphers to be used between itself and a web server. Because client systems may connect to the HTTPS inspection product using strong cryptography, the user will be unaware of any weakness on the other side of the HTTPS inspection.