renderer string vs render

Your renderer string says NVIDIA. Your pixels say software.

A story about a detection flag we chased in the wrong direction for a while, and the general lesson underneath it, which is the most expensive one in this area:

you can spoof what the browser says about the GPU, and you cannot spoof what the GPU draws.

What happened

A commercial fingerprinting service was returning tampering = True on our realness gate. The seed under test reported an NVIDIA GTX 980, so the first hypothesis was obvious: something about that GPU value is wrong, probably the renderer string.

That hypothesis was wrong, and three measurements killed it.

A seed sweep. Ten different seeds on the same build, each reporting a different GPU. Ten out of ten flagged. Not value-specific, then. Whatever it was applied to every identity equally.

An A/B across two builds. The previous engine release and the current one, same rate. Not a regression introduced by the newer build.

The same build on a different operating system. On Windows, the same two seeds came back tampering = False, with the underlying model score around 0.15 to 0.17, well under the threshold that raises the flag.

Same code, same seeds, same detector. Clean on one host, flagged on the other.

The actual cause

The flag only appeared when the browser ran headless on Linux, where there is no GPU and rendering falls to a software rasterizer.

The renderer string still claimed a GTX 980, because that is what the profile said to claim. The pixels coming out of the canvas and WebGL pipeline were produced by a software rasterizer, which does not draw the same thing a real GTX 980 draws. Antialiasing edges differ, floating-point rounding differs, and the small inconsistencies are exactly what a model trained on real hardware output learns to notice.

So the tell was not "this fingerprint is unusual". It was "the hardware this browser claims and the hardware that drew this image are not the same hardware".

Why this is the expensive class of problem

Everything else in a fingerprint is a value you can set. This one is an output.

A renderer string is a string. A canvas hash is the result of running a rasteriser over a shape, and to change it consistently you have to change the rasteriser or the machine. That is why:

• Spoofing the string alone is not enough, and is arguably worse than leaving it honest, because it creates a claim the pixels contradict.
• The mismatch is invisible in every check that only reads values. Our own release gates read values for a long time before this surfaced, which is why they now assert presence rather than absence.
• It is worst exactly where automation usually runs: servers, containers and CI, none of which have a GPU.

If you deploy browser automation to a cloud machine and it behaves differently from your laptop, this is high on the list of reasons, and it will not show up in any property-level audit.

What the options actually are

There is no clean fix, and pretending otherwise would be dishonest. There are three imperfect ones:

1. Give the machine a real GPU. A cloud instance with hardware acceleration, and a browser configured to use it. Expensive, and not available everywhere.
2. Make the claim match the reality. If the render is software, do not claim a discrete card. This is unattractive, because a software renderer is itself a strong server signal, but a consistent weak story beats a contradictory strong one.
3. Tune the software path so its output is closer to the claimed hardware. The most work, the least certain, and the direction with the most headroom.

The one thing that does not work is claiming a better GPU harder.

The other half of the story, which is about testing

The same fingerprinting reports that carried the tampering flag also carried a second flag: high_activity.

It was not the product. It was us, running the same detector demo repeatedly through the same small set of exit addresses over an afternoon. The service was measuring velocity from that address and telling us so.

Two things follow, and they cost more time than the actual bug:

• A test harness can manufacture the signal it is measuring. If you hammer a scoring endpoint from one address, you are creating the condition you are trying to observe.
• Read every flag before believing the one you were looking for. We nearly attributed a velocity artefact to the browser build.

Space the runs out, rotate the exit, and check whether the flag moves with the browser or with the harness.

What to take from it

If you are debugging something like this, the order that would have saved us time:

1. Sweep identities before blaming one. Ten seeds took an hour and eliminated the entire first hypothesis.
2. Change one axis at a time, and include the host operating system as an axis. The answer here was the host, which was not on the list of suspects at all.
3. Read all the flags, not the one you came for.
4. Ask whether the signal is a value or an output. Values you can set. Outputs you have to actually produce, and that is a different and much harder job.

Short answers to the questions that lead here

Why am I flagged as tampering when every value looks right? Because one of the signals is not a value, it is an output. A renderer string can be set. The pixels a rasterizer draws cannot be, and the two can contradict each other.

Does this only happen on Linux? It happens wherever there is no GPU, which is most servers and containers. It showed up for us on headless Linux and came back clean on Windows with a real GPU.

Will a better GPU string fix it? No. Claiming a better card harder makes the contradiction larger, not smaller.

What are my options? A machine with a real GPU, a claim that matches the software render, or work on making the software output resemble the claimed hardware. All three are imperfect and the first is the only easy one.

How do I test for it? Sweep several identities before blaming one, change one axis at a time, and include the host operating system as an axis. That is what took us from the wrong answer to the right one.

See also: WebGL renderer strings, for what those strings encode and why a software rasterizer is a giveaway, and why headless renders different fonts, which is the same value-versus-output problem on a different surface.

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From the notes of invisible_playwright. The measurements above are ours: ten seeds, two engine builds, and two host operating systems. The wrong first hypothesis is ours too.