JA4 FINGERPRINTING AGAINST AI SCRAPERS: A PRACTICAL GUIDE

Summary from the TLDR InfoSec newsletter

JA4 fingerprinting can be incredibly effective for detecting AI scrapers, as modifying the fingerprint requires recompiling the full TLS stack, unlike other methods, such as behavioral detection, that newer AI agents can mimic. JA4 fingerprints for the ClientHello and HTTP Headers (JA4H) can be combined with traditional systems to build a defense-in-depth detection system, which can then forward logs to a SIEM. This post includes code samples for the detection logic as well as fingerprints for several common libraries and tools.