JA4 Fingerprint
What is JA4 Fingerprint?
JA4 FingerprintA 2023 successor to JA3, published by John Althouse at FoxIO, that produces structured, human-readable TLS, HTTP, SSH, and TCP fingerprints designed to remain robust as TLS clients evolve and to be paired across protocols.
JA4 (and the JA4+ family — JA4S, JA4H, JA4X, JA4SSH, JA4T, JA4L) is a 2023 evolution of JA3 published by John Althouse at FoxIO. Where JA3 produced an opaque MD5, JA4 produces a structured string with explicit fields and a small truncated hash, so a fingerprint is both human-readable and trivially groupable by partial match. JA4 covers more parts of the handshake (e.g. signature algorithms, ALPN, the actual TLS version negotiated vs. the offered one) and ignores well-known fields that change for noise reasons, making the resulting fingerprint more stable across TLS-extension shuffling. JA4S fingerprints the server's response, JA4H fingerprints HTTP requests, JA4X fingerprints X.509 certificate issuers, JA4T fingerprints raw TCP options, JA4L estimates light-latency on the path, and JA4SSH fingerprints SSH client handshakes. The license is BSD-3, and integrations exist in Zeek, Wireshark, Suricata, Cloudflare's edge, and several commercial NDR products. JA4-based detections increasingly replace or complement JA3 in modern threat-hunt content.
● Examples
- 01
An NDR product tags a high-confidence Cobalt Strike beacon by matching its JA4 fingerprint plus a JA4H HTTP header pattern.
- 02
A defender writes a Suricata rule that alerts on any TLS client whose JA4 matches a known Go-`net/http` malware family but whose JA4H differs from the legitimate Go SDK signature.
● Frequently asked questions
What is JA4 Fingerprint?
A 2023 successor to JA3, published by John Althouse at FoxIO, that produces structured, human-readable TLS, HTTP, SSH, and TCP fingerprints designed to remain robust as TLS clients evolve and to be paired across protocols. It belongs to the Network Security category of cybersecurity.
What does JA4 Fingerprint mean?
A 2023 successor to JA3, published by John Althouse at FoxIO, that produces structured, human-readable TLS, HTTP, SSH, and TCP fingerprints designed to remain robust as TLS clients evolve and to be paired across protocols.
How does JA4 Fingerprint work?
JA4 (and the JA4+ family — JA4S, JA4H, JA4X, JA4SSH, JA4T, JA4L) is a 2023 evolution of JA3 published by John Althouse at FoxIO. Where JA3 produced an opaque MD5, JA4 produces a structured string with explicit fields and a small truncated hash, so a fingerprint is both human-readable and trivially groupable by partial match. JA4 covers more parts of the handshake (e.g. signature algorithms, ALPN, the actual TLS version negotiated vs. the offered one) and ignores well-known fields that change for noise reasons, making the resulting fingerprint more stable across TLS-extension shuffling. JA4S fingerprints the server's response, JA4H fingerprints HTTP requests, JA4X fingerprints X.509 certificate issuers, JA4T fingerprints raw TCP options, JA4L estimates light-latency on the path, and JA4SSH fingerprints SSH client handshakes. The license is BSD-3, and integrations exist in Zeek, Wireshark, Suricata, Cloudflare's edge, and several commercial NDR products. JA4-based detections increasingly replace or complement JA3 in modern threat-hunt content.
How do you defend against JA4 Fingerprint?
Defences for JA4 Fingerprint typically combine technical controls and operational practices, as detailed in the full definition above.
What are other names for JA4 Fingerprint?
Common alternative names include: JA4+, JA4S, JA4H, JA4X.
● Related terms
- network-security№ 628
JA3 Fingerprint
A TLS client fingerprinting method by John Althouse, Jeff Atkinson, and Josh Atkins (Salesforce, 2017) that hashes the ordered TLS ClientHello parameters into a 32-character MD5 — used to identify and group TLS clients without inspecting payload.
- network-security№ 1280
TLS Handshake
The initial protocol exchange in Transport Layer Security that authenticates the server (and optionally the client) and derives the symmetric keys used to encrypt the rest of the session.
- network-security№ 1279
TLS (Transport Layer Security)
The IETF-standardized cryptographic protocol that provides confidentiality, integrity, and authentication for traffic between two networked applications.
- defense-ops№ 338
Detection Engineering
The discipline of designing, testing, deploying, and maintaining security detections as code, with measurable coverage of adversary techniques.
- network-security№ 326
Deep Packet Inspection (DPI)
An inspection technique that examines the full payload of network packets — not just headers — to identify applications, content, and threats.
- network-security№ 609
Intrusion Detection System (IDS)
A passive security control that monitors network or host activity for malicious behaviour and raises alerts without blocking traffic.