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What a server knows about you before it reads a single header

Last week I sent the same HTTPS request to the 500 most popular websites from eight countries, and a fifth of them refused it everywhere.

  • Servers can identify clients using TLS ClientHello message before HTTP headers
  • Chrome and Python present TLS library capabilities differently, creating unique fingerprints
  • JA3 and JA4 fingerprints are more stable than user agent for client identification

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