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supported_versions

TLS Extension Code: 43

Extension Details

Extension Name
supported_versions
Extension Code (Decimal)
43
Extension Code (Hex)
0x002B
RFC Reference
RFC 8446

What is supported_versions?

The Supported Versions extension allows clients to indicate which TLS versions they support. In TLS 1.3, this extension is mandatory and replaces the legacy_version field in the Client Hello. Clients typically advertise support for TLS 1.3, TLS 1.2, and sometimes TLS 1.1. The server selects the highest mutually supported version. This extension was introduced to avoid compatibility issues with middleboxes that rejected unknown TLS versions in the legacy_version field. Different browsers and clients advertise different version sets, making this a key component of JA3 fingerprinting.

Role in JA3 Fingerprinting

How This Extension Affects Fingerprinting

The supported_versions 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 supported_versions 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 supported_versions extension in its TLS handshake:

Test Your JA3 Fingerprint

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

Browser & Client Support

Modern Browser Support:

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

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