What it measures
The signal performs a single small floating-point operation on a typed-array buffer and reads a single byte of the result. The byte's value differs deterministically between x86/x86-64 FPUs and ARM/ARM64 FPUs because the two CPU families use different conventions when representing certain edge-case results. The signal hashes that single byte.
The exact byte index, the specific marker values for each CPU family, and the floating-point operation that produces them are part of the internal recipe and not part of the public contract.
Why hardware-bound
The observed byte is determined by the host CPU's FPU and emitted by native floating-point instructions that the V8, SpiderMonkey, and JavaScriptCore JITs all generate without rewriting. On the same physical machine, Chrome, Safari, Firefox, and Brave all return the same byte, so the signal contributes to the cross-browser hardware-only fingerprint.
Privacy browsers cannot reasonably farble the underlying operation without breaking every page that does floating-point math. Brave Standard Shields, Firefox resistFingerprinting, and Tor Browser all leave it alone in practice.
Confidence rules
| Confidence | Trigger |
|---|---|
| normal | Typed arrays available and the byte read succeeded |
| absent | Typed arrays unavailable (non-browser runtime) or an unexpected exception |
Things worth knowing
- Rosetta (x86 emulation on Apple Silicon) and Windows ARM x86-translation expose the underlying physical CPU's convention rather than the emulated one. The JIT runs natively on the host FPU regardless of the emulated user-space architecture.
- Web Workers observe the same byte. The FPU is shared across worker contexts.
- Big-endian platforms would require a different read; no modern browser-capable CPU is big-endian, so the little-endian assumption holds in practice.
- The signal exists in some form in several other public fingerprinting libraries. Benny's specific encoding and integration into the hardware-only fingerprint are not necessarily portable across implementations.
Last reviewed 2026-06-04
