- Different OSes (Linux, macOS, Windows) cause browsers to produce slightly different floating-point results for certain math functions like Math.tanh due to differing underlying C math libraries (glibc, libsystem_m, UCRT).
- This creates a fingerprinting vector, as anti-bot systems can detect OS by examining last-bit discrepancies in results from Math.tanh, CSS trig functions, and Web Audio compressor operations.
- Chrome 148+ changed Math.tanh to use the host OS's std::tanh instead of V8's bundled fdlibm, introducing this leak; earlier versions do not have it.
- Spoofing math correctly requires reproducing exact bit patterns per OS, considering library variations (e.g., Apple's scalar vs. Accelerate frameworks) and ensuring deterministic compilation without compiler optimizations affecting fused multiply-add.
- Scrapfly's browser implements precise reproductions or lifts original libraries (e.g., mapping UCRT on Linux) to match claimed OS bit-for-bit, validated against real devices and browsers.