Implementing a JA4H-Equivalent Fingerprint with Amazon CloudFront Functions to Mitigate Bot Traffic

JA3 and JA4 fingerprints have been showing up more and more often in discussions about bot mitigation and client identification. Amazon CloudFront exposes TLS-based JA4 as a CloudFront-generated header, but it does not provide the HTTP-request-based JA4H fingerprint. That made me wonder: “Why not implement it ourselves with CloudFront Functions?” So I did. There are a few gotchas and design considerations you need to be aware of, and I’ll cover those as well. In this article, I’ll first demonstrate the limitation through hands-on testing, then work around it with the CloudFront-Viewer-Header-Order header and show how to calculate a JA4H-equivalent fingerprint entirely within CloudFront Functions. What Is JA4? JA4+ is a suite of network fingerprints published by FoxIO and positioned as a successor to JA3. The best-known member is JA4, which is calculated from the TLS ClientHello. It generates a short string that is nearly unique to a particular client implementatio