TLS Fingerprinting
Identifying a client by analysing how it negotiates an encrypted connection, letting servers detect specific tools even before any web request data is exchanged.
Full Definition
TLS fingerprinting is a broad category of client identification techniques based on analysing the TLS ClientHello message. The JA3 hash is the most well-known example. Advanced systems also fingerprint HTTP/2 SETTINGS and HEADERS frames (producing "JA3 + H2" combined fingerprints). Because every HTTP client library and browser produces a characteristic TLS signature, servers can identify Python's requests library, Node.js's https module, or a specific version of Chrome before any HTTP request body is seen. Defeating TLS fingerprinting requires using a TLS library that perfectly mimics a real browser's ClientHello — a challenging technical requirement.
