ja4plus 0.4.1

JA4+ network fingerprinting library for TLS, TCP, HTTP, SSH, and X.509 analysis

A Python library and CLI for JA4+ network fingerprinting. Implements all eight JA4+ methods for identifying and classifying network traffic based on TLS, TCP, HTTP, SSH, and X.509 characteristics. Supports QUIC, IPv4/IPv6, and multi-segment TCP reassembly.

JA4+ is a set of network fingerprinting standards created by FoxIO. This library is an independent Python implementation of the published specification. For the original spec, see the FoxIO JA4+ repository.

Supported Fingerprint Types

Type	Protocol	Description
JA4	TLS/QUIC	Client fingerprint from ClientHello messages
JA4S	TLS/QUIC	Server fingerprint from ServerHello messages
JA4H	HTTP	Client fingerprint from request headers and cookies
JA4T	TCP	Client OS fingerprint from SYN packets
JA4TS	TCP	Server fingerprint from SYN-ACK packets
JA4L	TCP/QUIC	Light distance and latency estimation
JA4X	X.509	Certificate structure fingerprint from OID sequences
JA4SSH	SSH	Session type classification from traffic patterns

QUIC Initial packets (RFC 9001/9369) are automatically decrypted to extract TLS ClientHellos. IPv4 and IPv6 are both supported across all fingerprinters.

Installation

pip install ja4plus

For fingerprint identification (browsers, malware, C2 frameworks):

pip install ja4plus[lookup]

CLI

The ja4plus command is available after installation:

# Analyze a PCAP file
ja4plus analyze capture.pcap

# JSON output for SIEM ingestion
ja4plus --format json analyze capture.pcap

# Only specific fingerprint types
ja4plus --types ja4,ja4t analyze capture.pcap

# Live capture (requires root)
sudo ja4plus live eth0

# Fingerprint a certificate
ja4plus cert server.der

# Identify known fingerprints
ja4plus --lookup analyze capture.pcap

Output formats: --format table (default), json (JSONL), csv

Fingerprint Lookup

ja4plus includes a bundled database of known JA4+ fingerprints from FoxIO's ja4plus-mapping.csv. Identifies Chrome, Firefox, Safari, Python, Cobalt Strike, Sliver, IcedID, and more.

from ja4plus.ja4db import lookup

result = lookup("t13d1516h2_8daaf6152771_02713d6af862")
# {"application": "Chromium Browser", "type": "ja4", "notes": ""}

Python API

Quick Start

from scapy.all import rdpcap
from ja4plus import JA4Fingerprinter

packets = rdpcap("capture.pcap")

fp = JA4Fingerprinter()
for packet in packets:
    result = fp.process_packet(packet)
    if result:
        print(f"JA4: {result}")

All Fingerprinters

from ja4plus import (
    JA4Fingerprinter,      # TLS Client
    JA4SFingerprinter,     # TLS Server
    JA4HFingerprinter,     # HTTP
    JA4TFingerprinter,     # TCP Client (SYN)
    JA4TSFingerprinter,    # TCP Server (SYN-ACK)
    JA4LFingerprinter,     # Latency
    JA4XFingerprinter,     # X.509 Certificate
    JA4SSHFingerprinter,   # SSH
)

All fingerprinters share a common interface:

Method	Description
process_packet(pkt)	Process a packet, returns fingerprint string or None
get_fingerprints()	Returns list of all collected fingerprint dicts
reset()	Clears all collected state

Function-Based API

For one-shot fingerprinting without maintaining state:

from ja4plus import generate_ja4, generate_ja4s, generate_ja4h

fingerprint = generate_ja4(packet)

See docs/usage.md for detailed usage of each fingerprinter and docs/api_reference.md for the full API.

Fingerprint Formats

Type	Format	Example
JA4	{proto}{ver}{sni}{ciphers}{exts}{alpn}_{hash}_{hash}	t13d1516h2_8daaf6152771_e5627efa2ab1
JA4S	{proto}{ver}{exts}{alpn}_{cipher}_{hash}	t130200_1301_a56c5b993250
JA4H	{method}{ver}{cookie}{ref}{cnt}{lang}_{h}_{h}_{h}	ge11cr0800_edb4461d7a83_...
JA4T	{window}_{options}_{mss}_{wscale}	65535_2-4-8-1-3_1460_7
JA4TS	{window}_{options}_{mss}_{wscale}	14600_2-4-8-1-3_1460_0
JA4L	JA4L-{C|S}={latency_us}_{ttl}	JA4L-S=2500_56
JA4X	{issuer}_{subject}_{extensions}	a37f49ba31e2_a37f49ba31e2_dd4f1a0ef8b2
JA4SSH	c{mode}s{mode}_c{pkts}s{pkts}_c{acks}s{acks}	c36s36_c51s80_c69s0

Spec Validation

ja4plus is validated against FoxIO's official test vectors:

python tests/download_test_vectors.py
pytest -m spec_validation -v

Development

git clone https://github.com/Crank-Git/ja4plus.git
cd ja4plus
pip install -e ".[dev]"
pytest tests/ -v

Requirements

• Python 3.8+
• scapy >= 2.4.0
• cryptography >= 42.0.0

License

This library is released under the BSD 3-Clause License.

The JA4+ fingerprinting specifications were created by FoxIO. JA4 (TLS Client) is open source under BSD-3-Clause per FoxIO. Other JA4+ methods (JA4S, JA4H, JA4T, JA4TS, JA4L, JA4X, JA4SSH) implement FoxIO's specifications under the FoxIO License 1.1, which is permissive for academic, internal business, and security research use.

See LICENSE for full details.

Acknowledgments

JA4+ was created by John Althouse at FoxIO. This library is an independent implementation of the published specification. For the original spec and reference implementation, see github.com/FoxIO-LLC/ja4.