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cookie

TLS Extension Code: 44

Extension Details

Extension Name
cookie
Extension Code (Decimal)
44
Extension Code (Hex)
0x002C
RFC Reference
RFC 8446

What is cookie?

The Cookie extension in TLS 1.3 is used for stateless handshake retries when the server is under load or needs to verify client reachability (similar to SYN cookies in TCP). When a server receives a Client Hello but is not ready to complete the handshake, it can send a HelloRetryRequest with a cookie. The client must echo this cookie in the next Client Hello. This prevents resource exhaustion attacks and verifies that the client can receive packets at its claimed address. The cookie is opaque to the client and does not affect security.

Role in JA3 Fingerprinting

How This Extension Affects Fingerprinting

The cookie extension is part of the TLS Client Hello packet that JA3 analyzes to create a unique fingerprint of your browser or HTTP client.

JA3 Construction: JA3 concatenates TLS parameters including extensions in a specific format:
TLS_VERSION,CIPHERS,EXTENSIONS,CURVES,POINT_FORMATS

Different browsers and HTTP clients support different sets of TLS extensions, making this a key differentiator in fingerprinting. The presence, absence, or order of extensions like cookie can reveal:

  • Browser type and version (Chrome, Firefox, Safari, Edge)
  • Operating system (Windows, macOS, Linux, Android, iOS)
  • HTTP library (curl, Python requests, Go http.Client, Node.js)
  • Bot detection - automated tools often have distinctive extension sets

Test Your TLS Configuration

See if your browser or HTTP client includes the cookie extension in its TLS handshake:

Test Your JA3 Fingerprint

The JA3 tool will show all TLS extensions your client advertises, including cookie.

Browser & Client Support

Modern Browser Support:

  • Chrome/Edge: Full support for cookie
  • Firefox: Full support for cookie
  • Safari: Full support for cookie

HTTP Client Libraries:

  • curl: Support varies by OpenSSL/LibreSSL version
  • Python requests: Limited TLS extension control
  • curl_cffi: Can mimic browser TLS configurations
  • Scrapfly API: Automatically matches real browser TLS fingerprints

Related TLS Resources

TLS Extensions
Testing Tools

Technical References