[kondbg]

The authors of the original paper [1] identified that the set of client cipher suites advertised by each browser can be used to fingerprint and identify a browser.

Caddy records the cipher suite advertised by the client during the TLS handshake and then later examines the client's user agent. Using the fingerprinting techniques mentioned in the paper, Caddy then determines whether or not the advertised user-agent is compatible with the user-agent that it inferred through the client cipher suites.

TLS interception proxies establish their own TLS connection to the server. Depending on what underlying TLS library the proxy uses, it also has its own unique fingerprint. When the TLS proxy forwards the user's request, Caddy detects the mismatch and flags it as a MITM.

[1] https://jhalderm.com/pub/papers/interception-ndss17.pdf