1. [JA3 Fingerprint](https://scrapfly.io/web-scraping-tools/ja3-fingerprint)
2. [ TLS Extensions ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extensions "Browse all TLS Extensions")
3. extended\_master\_secret
 
   [  Browse All TLS Extensions ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extensions) 

  # extended\_master\_secret

TLS Extension Code: `23`

 

  ### Extension Details

 

Extension Name

`extended_master_secret`

 

Extension Code (Decimal)

`23`

 

Extension Code (Hex)

`0x0017`

 

RFC Reference

 [ RFC 7627  ](https://datatracker.ietf.org/doc/html/rfc7627) 

 

 

 

  ### What is extended\_master\_secret?

 

The Extended Master Secret extension addresses the "Triple Handshake Attack" vulnerability in TLS 1.2 and earlier by binding the master secret to the full handshake context. Without this extension, an attacker performing a man-in-the-middle attack could potentially force the client and server to derive the same master secret for different handshakes. The extended master secret is computed using a hash of the entire handshake, preventing this attack. Defined in RFC 7627, this extension significantly improves TLS security and is widely supported by modern clients.

 

 

  ### Role in JA3 Fingerprinting

 

  How This Extension Affects Fingerprinting 

 The **extended\_master\_secret** 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 `extended_master_secret` 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 `extended_master_secret` extension in its TLS handshake:

 [  Test Your JA3 Fingerprint ](https://scrapfly.io/web-scraping-tools/ja3-fingerprint) 

 The JA3 tool will show all TLS extensions your client advertises, including extended\_master\_secret.

 

  ### Browser & Client Support

 

**Modern Browser Support:**

- **Chrome/Edge:** Full support for extended\_master\_secret
- **Firefox:** Full support for extended\_master\_secret
- **Safari:** Full support for extended\_master\_secret
 
**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

- [server\_name (SNI)](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/server-name)
- [supported\_groups](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/supported-groups)
- [ALPN](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/application-layer-protocol-negotiation)
- [supported\_versions](https://scrapfly.io/web-scraping-tools/ja3-fingerprint/extension/supported-versions)
 
 

##### Testing Tools

- [JA3 Fingerprint Tool](https://scrapfly.io/web-scraping-tools/ja3-fingerprint)
- [Browser Fingerprint Test](https://scrapfly.io/web-scraping-tools/browser-fingerprint)
 
 

 

 

  ### Technical References

 

- **RFC Specification:** [ RFC 7627 - TLS Extensions  ](https://datatracker.ietf.org/doc/html/rfc7627)
- **IANA Registry:** [ TLS ExtensionType Values  ](https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml)
- **JA3 Project:** [ JA3 - TLS Client Fingerprinting  ](https://github.com/salesforce/ja3)