Since Chromium 148, Math.tanh is now fingerprintable to link underlying OS

https://scrapfly.dev/posts/browser-math-os-fingerprint/

by joahnn_s • 10 days ago

430 199 10 days ago

joahnn_s3• 10 days ago

We noticed Chromium Math.tanh since v148 returned a different result, so we dig it - it's now a fingerprintable surface to retrieve the OS Chromium run on

dmitrygr• 10 days ago

Interesting reporting, marred by obvious llm-slop-sounding writing. "You are not building..., you are ..."

netsharc• 10 days ago

Why "slop-sounding"? It's definitely LLM slop.

Man, why the fuck don't they just make a powerpoint with bullet points if all the sentences are like that.

sjrd• 10 days ago

I guess that's one more good reason to push for correctly rounded transcendental functions. I recently learned that they're basically solved now. [1]

[1] https://arith2026.org/program.html (2nd keynote)

Retr0id3• 10 days ago

Tangential, but wow do they really register a new domain for each year and renew it in perpetuity?

yzydserd• 10 days ago

arith2027.org taken, arith2028.org available.

torginus4• 10 days ago

I never understood why fixed precision, and integer math isn't more popular. In engineering, we used fixed point all the time, it ran on much simpler hardware and the error is mathematically easy to model. IEEE 754 floats are not only suspect when it comes to theory, but are often outperformed with integers smaller than the mantissa (so less than 24 bits of int can beat a 32 bit float), when it comes to things like loss of precision.

AlotOfReading7• 10 days ago

I recommend pretty much everyone avoid fixed point and other float alternatives, barring exceptional cases after you've done your own numerical analysis, or you lack floating point hardware (rare these days).

Yes, fixed point can use simpler hardware. That's also a completely irrelevant consideration for software. The vast majority of processors are optimized for floats now and some operations (e.g. division) are actually faster.

The precision argument also falls apart. Any float with mantissa >= X+Y can get exactly the same results as a QX.Y fixed point. The float will actually perform better across the same range because you have to round it to perform like the fixed point. That means more precision, lower error, automatic normalization, better overflow behavior, a larger working range, etc. And it'll probably be just as fast, unless you're bottlenecked on memory bandwidth of inputs (unlikely). When you inevitably want an exp() or another special function, it's a heck of a lot easier to call libm than implement your own and it will perform better.

Floats are also much easier to get right for your coworkers that aren't numerical analysts.

anematode2• 10 days ago

Agreed, correctly rounded libm functions are great, as long as they don't have miserable worse case behavior (as was famously the case with glibc's pow at one point).

One thing I was thinking of doing is manually SLP-vectorizing the high-precision fallbacks that they use when they're close to a rounding boundary, so that you can get better worst-case behavior – but obviously it's good enough already for most purposes.

I'm honestly surprised though that JS engines don't just keep using fdlibm though. The ECMAScript spec explicitly encourages it iirc. And if Math.tanh is on your hot path in JavaScript then you're doing something quite bizarre...

lifthrasiir2• 10 days ago

as was famously the case with glibc's pow at one point

Pow is famously hard anyway because it's bivariate and there is no currently known way to work around the table-maker's dilemma (TMD). CORE-MATH even crashes upon a new required precision record, because it intentionally avoids Ziv's rounding.

odo1242• 10 days ago

And if Math.tanh is on your hot path in JavaScript then you're doing something quite bizarre...

Machine learning? (on a machine with no WebGL/WebGPU, I guess)

Retr0id3• 10 days ago

Thanks for the writeup, claude

_alternator_• 10 days ago

Yeah, interesting finding in the headline, the rest is just Claude.

isiahl• 10 days ago

What I think is crazy are the links at the top to summarize the article with your preferred AI provider. The prompt asks it to summarize the article along with an advertisement for their product. This is the prompt I got when clicking the Claude link:

summarize+this+article+and+explain+how+scrapfly+helps+me+scrape+any+website+at+scale+and+bypass+anti-bot+systems+for+my+use+case:+https://scrapfly.dev/posts/browser-math-os-fingerprint/

dannyw• 10 days ago

They are quite ‘innovative’, I’ll give them that.

It’s time to sign up with a virtual card, point it at my website, and see how to block them ;)

drnick1• 10 days ago

This is interesting, but even without relying on JS, most users are already fingerprintable by the combination of IP + user agent.

senfiaj• 10 days ago

What about TOR or VPNs + faking user agents?

yunwal• 10 days ago

Yes, but it's important that people who care about not being tracked have options, even if it's not most people.

Aurornis• 10 days ago

One tanh call on the right input is a per-OS signature. Claim macOS, return Linux math bits, and you have contradicted your own User-Agent.

They (or rather the LLM that wrote this) missed that this is possibly fingerprintable to browser version range, which is slightly more interesting. Most users aren't spoofing their user agent headers to be a different operating system. Most fingerprinting solutions aren't trying to infer your operating system, they only care about semi-unique things that show up.

It's an interesting finding. I wish they had taken some time to have a real person write it up. This is too heavily LLM written to ignore.

jeroenhd3• 10 days ago

Most users aren't spoofing their user agent headers to be a different operating system.

The people behind the LLM behind this blog post are. They're trying to pretend their robots are people to sell other websites' data to their customer. It's easier to pass bot detection gates if you pretend to be a physical machine running Windows or macOS than if you honestly admit you're using Linux on a VM.

reactordev2• 10 days ago

The Internet is a cesspool of scams now

acters• 10 days ago

It's sometimes easier to lie than to tell the truth, and being on Linux telling the truth gets me more scrutiny than those pretending to be legit.

baby_souffle• 10 days ago

Some scrape activity happens because I can't obtain the data any other way. I would be thrilled if certain retailers had price and availability data as an API so I could not bother with the bulk of scrape and process.

nradov6• 10 days ago

The HTTP User-Agent header was a mistake from the beginning. There is no legitimate need for the server to know what software the client is (or claims to be) running.

vlovich123• 10 days ago

This tanh approach won’t tell you if you’re in a VM - it’ll report the guest OS

zx80802• 10 days ago

Infinite captcha welcomes those who are using FF and linux on stackoverflow. Easier to skip its links in search results than waste time solving it (captcha never finishes).

preg_match• 7 days ago

I have a user-agent switching extension, and I use it because some websites like to claim up and down "ohhh we don't work on Firefox!" but then you switch to chrome's user-agent and - surprise! - no bugs at all.

I mean, I get it, saying you support something is technically a commitment. But my goal is to basically never open a chromium browser, so, I'm gonna do what I have to do. And you would be shocked, in the past 5 years I have only encountered ONE website which legitimately required a chromium browser. grapheneos.com, for the web usb installer.

joahnn_s3• 10 days ago

You can only assert >148 at the moment, but there are better vectors to strictly assert the version by simply checking the addition of v8/blink on each chromium version (and since ~120 it's the case), so by checking if xxx is present and yyy is not present in js userland or css feature, the inference is 100% for the major version

And for the LLM writing, yes, it's written in the article and blog, it's not hidden or pretending, otherwise I would never publish an article due to lack of time, and I stand by it

georgemcbay• 10 days ago

otherwise I would never publish an article due to lack of time, and I assume

Didn't even have time to finish their HN reply.

Arch-TK2• 10 days ago

It takes less time to write the prompt, you could just publish that?

It's an important topic, and I am glad you wrote about it, but even half a page of notes would have been enough to convey this. It would save me literally skim reading headings just to get past all the fluff.

phyzome• 10 days ago

Do what people did before there were LLMs: Just post the data and some quick notes. It's fine, people appreciate the brevity.

Disclosing is great, but not as good as just using your own human voice.

biofunsf• 10 days ago

In your old comment (which i remembering seeing at time), you seem to recommend scrapy.io and highlight its benefits without disclosing that you're behind it: https://news.ycombinator.com/item?id=46088621

seems like the "they" you meant was really "I".

tripzilch• 10 days ago

I had to hunt for it that's not clear at all, and if you don't care to spend the effort of writing it, why would anyone want to spend the effort of reading it.

I stand by it

That's cute, but seriously who cares if you stand by any of "your" words, if you care so little about them to have them be generated by an LLM.

constantius• 10 days ago

As another comment points out, you added the disclaimer after publishing (and probably the backlash):

https://web.archive.org/web/20260712212920/https://scrapfly....

So you don't seem to stand by anything apart from swindling.

comex• 10 days ago

This can be used to fingerprint version range, but so can a million other things. Browsers are constantly adding new features and fixing bugs, most of which can be detected from JavaScript.

userbinator3• 10 days ago

The LLM has a good point. I personally don't care what or who wrote it if it's true.

cookiengineer• 10 days ago

Fun fact that will blow your mind:

Microsoft decided to send Windows NT 10.0 in the User-Agent header even on Windows 11 for compatibility reasons. That's literally the reason why the Sec-CH-* headers say Windows 11 but the User-Agent says Windows 10.

And regarding your claims of vendor interests: Nope, you seemingly never had to use O365 crapware on Linux browsers. They make it as painful as possible, and even disable copy/paste functionality when your User-Agent and Sec-CH headers say Linux. Identical browser with an extension that overrides the fingerprinting headers and it works perfectly.

Also as an additional note: Cloudflare does TCP fingerprinting, because no cloudflare pages will work (and send you into an infinite loop of unsolvable captchas) when these headers mismatch with the tcp window and other options in the handshake frames.

Source: am maintaining my chromium-profiles tool that generates farbled profiles with a generated extension, so that I can use shitty Microsoft products because my customers are not really the smartest policy decision makers.

PS: I will never use a separate laptop with a separate OS to use a damn web app. That is a completely unjustified waste of hardware resources and should be illegal. But here we are. Wasting one laptop at a time for absolutely no reason.

[1] https://github.com/cookiengineer/chromium-profiles

cute_boi3• 9 days ago

After reading this I am sure 90% of text is from AI. This is so annoying to read.

jeroenhd3• 10 days ago

Kind of a smart move by this company: write up an AI analysis of all fingerprinting techniques in hopes they get fixed after outrage so their scraping company can make more money. If it weren't for companies like this, fingerprinting wouldn't be so ubiquitous and the internet would be a better place in general.

I prefer articles like this coming from the other side of the battle (fingerprint.js and friends) because at least their motives are clear.

codedokode4• 10 days ago

I disagree, fingerprinting is necessary to track humans and it will be used regardless of scrapers being there or not.

coldbrewed2• 10 days ago

I work at a CDN that provides bot detection services. I agree that there's baseline necessity in terms of fraud detection, and if not necessity then definitely financial motivation to fingerprint. But these days, abusive scraping is far and way the the main driver for fingerprinting.

We don't fingerprint for ad purposes, and we destroy PII for humans as fast as we can because PII should be treated as radioactive. But we see customers that are constantly burned by abusive scrapers and the scrapers aren't slowing down.

The current approach to scraping is strip mining the Internet and is having the corresponding pollution effects that you'd expect. I'm fine with individuals doing whatever weird automation they want, more power to you, but it's this industrial scale crawling and extraction that's degrading the Internet from all angles.

nradov6• 10 days ago

Why is it necessary to track humans? It might be more profitable for advertising but that's not the same as necessary.

a-dub• 10 days ago

how hardened are modern browsers with respect to detecting underlying os? seems like there would be loads of gaps?

majorchord• 10 days ago

There are boatloads of gaps.

amelius2• 10 days ago

Can't we make fingerprinting illegal, as in, jailtime illegal?

Would not solve everything but still help a lot.

chaboud• 10 days ago

I'd rather penalize the application than the technique. Windows was rumored to long have "quirks" that would do better things for apps that had bugs that the OS ended up fixing instead of the app.

Javascript systems have long had polyfills for varied browser feature comparability gaps.

Whether you agree with these, making probing detection via fingerprinting illegal would take away this lever. Making surreptitious tracking via fingerprinting illegal? Even for state actors?

Yeah, that's probably reasonable. If someone is going to wear a tracking collar in exchange for "free" services, a little disclosure makes sense.

Terr_• 10 days ago

Yeah, the problem is how the data is kept and abused afterwards.

akersten2• 10 days ago

Why should it be illegal for me to recognize the way you walk into my store, even though you're wearing a mask and a trenchcoat? Some vague sense of indignation?

Yeah, tracking bad, I get it, but are whatever damages that kind of legislation would prevent (probably nothing measurable) really more important than fixing the easy, in our face social problems that politicians could instead be focusing on?

altcognito• 10 days ago

Because you don't have a right to know everything about me, follow me to my home, my purchasing preferences, and so on and so forth.

lorecore• 10 days ago

Why should it be illegal for me to recognize the way you walk into my store, even though you're wearing a mask and a trenchcoat?

If you have that right, the public should have the right to know you're doing this before they enter your store, so they can avoid it.

Same with the websites, they should, legally, have to say they're about to fingerprint you so that you can close your browser tab and never come back.

thepasch• 10 days ago

Why should it be illegal for me to recognize the way you walk into my store

If you did it in just your store, that wouldn't be a problem. The correct analogy, however, is "why should it be illegal for me to attach a perfectly traceable and invisible air-tag to you when you enter my store, without your explicit consent, and subsequently follow and document your every movement no matter where you go, as long as that location has a business relationship with my store, and also my store is the most popular chain on the planet that has business relationships with basically any relevant business that exists." And I don't think the answer to this one shouldn't be particularly difficult to arrive at.

kaladin-jasnah• 10 days ago

Isn't fingerprinting used across many different websites? Then the analogy would be a number of stores colluding to recognize the same person across all stores?

(I have no idea, I don't know too much about this)

chjj6• 10 days ago

The analogy falls apart when "your store" is actually a handful of multi-billion dollar corporations that surveil a significant portion of the internet and covertly grant government agencies (and god knows who else) access to the data.

It's passive surveillance on the order of billions of people. It's not a mom-and-pop shop.

bloody-crow• 10 days ago

I don't think it'd be possible to define fingerprinting narrowly enough to not also outlaw perfectly normal and legitimate usecases.

chjj6• 10 days ago

I'm not even sure I'd want to make it narrow. I'd start with:

"Information gained via side-channel for the purpose of correlating individuals."

But you'd have to add an enormous amount of legalese after that to make it ironclad. They'll start arguing "this isn't a side-channel", "we're targeting bots, not individuals", etc. You'd have to define every word in that sentence very carefully.

I'd make it sweeping. "Individual" can mean "person", "bot", "suspected bot", "AI agent", "a piece of autonomous or non-autonomous software", basically anything. The "side-channel" definition might get trickier, but I'd rather legit use-cases get burned than privacy get burned.

The OP was downvoted, but I agree. I think fingerprinting should be in the same criminal category as an illegal wiretap.

codedokode4• 10 days ago

Why don't you ask browser developers to stop adding features helping fingerprinting? Browsers even have some API for tracking ad clicks (attribution API or something) and user interests tracking API which nobody of the users needs.

joahnn_s3• 10 days ago

The thing is that's not done on purpose and too hard to figure out how this has an impact underneath, if you read the v8 commit https://chromium.googlesource.com/v8/v8/+/c1486295ae5bcb0f8f... it's on a complete good faith

mrsssnake• 10 days ago

JavaScript was a mistake.

kristianp• 10 days ago

Who was it that said: "worse is better"?

This guy: https://dreamsongs.com/WorseIsBetter.html

hahahaa2• 10 days ago

Goes back in time. Uninvents JS. Everyone using Shockwave Flash blob to run their code. Enjoy!

qurren2• 10 days ago

just inject this with your favorite JS injection plugin

    let oldTanh = Math.tanh;
    Math.tanh = x => oldTanh(x) + Math.random()/10000000;

sanxiyn• 10 days ago

The article addresses this: search for "No noise".

chjj6• 10 days ago

it's elegant, but i prefer:

    Math.tanh = Math.random;

cozzyd2• 10 days ago

since tanh is probably being used as a sigmoid, it's probably better to replace with just randomly changing the sign.

gruez• 10 days ago

Great, now you'll be outed as "hides fingerprint", which is probably more identifying than if you returned a normal value.

hahahaa2• 10 days ago

https://xkcd.com/1105/

Klonoar• 10 days ago

Multiple anti-bot vendors will detect that replacement and use it as part of their fingerprinting process.

bakugo• 10 days ago

If this becomes common, it's trivially detectable with good ol' toString().

qurren2• 10 days ago

It's trivial to replace toString() as well

Maybe the right solution is an LLM that reads the fingerprinting code and adapts the counter-measure accordingly in real time.

torginus4• 10 days ago

What I don't get is that Chrome is hundreds of megabytes of just executable code, I assumed they statically linked half the userland. Also, I though tanh isn't a function, but an intrinsic emitted by the JS JIt that uses CPU instructions - which might be fingerprintable as well, but it's weird that for a math operation, you need to branch to a 'dlsym()' function.

pocksuppet5• 10 days ago

The x87 FPU implemented transcendental functions in microcode. Most instruction sets don't implement them. Mmicrocode is actually slower than software, since it doesn't get the benefit of things like branch prediction.

mmis10002• 10 days ago

Chrome is the only browser that preserve unused bit in value NaN through non JITed mode as far as I remember. And that bit become 0 when code get JITed.

coppsilgold4• 10 days ago

Even Tor Browser (/mullvad-browser) gave up trying to obscure the operating system though arguably they shouldn't have. There appear to be too many fingerprinting vectors.

ranger_danger• 10 days ago

Tor Browser straight up does not even modify navigator.platform at all, so it's incredibly easy to see when you're not using e.g. Windows.

coppsilgold4• 10 days ago

I believe they used to make the user-agent appear to belong to Windows but then they stopped doing it with the excuse that there are other ways to tell anyway.

The OS doesn't really matter, the amount of entropy it contains is very low. As long as the anonymity set of browser-users is large it's all good. And I believe Tor Browser accomplishes this objective.

chjj6• 10 days ago

I think they made the right call on that. It's unclear to me whether hiding the OS is even possible. There's just too much OS-specific behavior that happens inside (and outside) a web browser. It's hard to account for all of it.

OS rendering differences can likely betray you even when canvas extraction is blocked/noised. At least one tor-browser dev has publicly confirmed that you can't even hide the difference between X11 and Wayland[1], nevermind two entirely different OSes.

[1] https://forum.torproject.org/t/linux-is-it-alright-to-run-th...

coppsilgold4• 10 days ago

It's not surprising that JS would out you, what I am wondering if whether or not volunteering OS information is foolish when it may not be possible to determine without JS.

Why give up information when you don't have to? Some people disable JS at least some of the time.

andai• 10 days ago

I am not the NSA, but on an unrelated note, this delights me!

fweimer• 10 days ago

Recent glibc uses the correctly rounded tanh from CORE-MATH, so it returns different values than what's quoted in the article. It's unclear today if it's possible to achieve reasonable performance for other transcendental functions with correct rounding, so other functions have their own unique fingerprints.

IvanK_net• 10 days ago

I think the screen resolution is also fingerprintable. That is why a browser should resize your window to a random size each time you visit a website.

mmis10002• 10 days ago

browser report where your browser window (not position in tab) is even page itself never need to use it.

https://developer.mozilla.org/zh-CN/docs/Web/API/Window/scre...

jmward01• 10 days ago

Is this even a fight that is possible to win here? Run enough functions and between timing comparisons (x takes 2.5 y) and rounding (things like this) and I suspect you can nail os/exact machine and possibly even other tasks running on that machine. I'm not sure there is a viable way to stop this. At best just make it a little harder? Society and legislation need to catch up here. It is like a lock on my door. Locks don't stop people. They just deter a few people and delay a couple more but a determined person can (likely very easily) break into my house. That is why we need society to call it out that it isn't right (people avoiding buying things they think are stolen, shunning those that do it, etc etc) and laws that step in (it is a crime to break into my house. real resource dedicated to tracking down and stopping people that do it, etc). A similar approach needs to happen here. It should be illegal to track a person like this and people that get hired at a place using the gains of it should shun those companies and society should shun those companies.

nradov6• 10 days ago

Sure, except that in cyberspace a lot of the people who are doing things that are — or should be — illegal are located in jurisdictions where no enforcement is possible. Places like Russia, Myanmar, and North Korea have no concept of the rule of law and criminals who scam foreigners are actively protected by local authorities. So analogies to door locks completely break down there.

RunSet• 10 days ago

One could browse with javascript disabled. It may be that downloading and executing arbitrary code with each page load is naive- even in a sandbox developed by the world's largest advertising corporation.

omoikane• 10 days ago

I hope this eventually ends up in https://coveryourtracks.eff.org/ so I can see how unique my math functions are relative to a larger population.

twelve40• 10 days ago

this is really something. They claim (no idea if true) to have patched 4,000+ signals in 550+ C++ files in Chromium. coveryourtracks.eff.org uses like what, 25 signals?

luciana1u• 10 days ago

Math.tanh as a fingerprinting vector. at this rate the next browser privacy paper will reveal that the number of milliseconds your CPU takes to render an emoji is also uniquely identifying.

tomjakubowski• 10 days ago

Hopefully it takes much less than a millisecond to render a single emoji. Ca. 2018 I wrote some bad OpenGL code which could render a swarm of millions of poop emoji on a rather large display at 60 fps, so that was less than ~16e-6 ms per emoji.

eschaton• 10 days ago

Sounds almost like allowing web pages to run scripts was a mistake.

hotsalad• 10 days ago

Ran Math.tanh(0.8) on Firefox on Windows and got the value that the article says is associated with Linux. Wonder why they only ran this on one browser.

simondotau3• 10 days ago

This is AI slop, written for a company that is contributing to the enshittification of the web.

zombot• 10 days ago

Scrapium is Scrapfly’s scraping browser

That's what this article is about, their scraping browser fakes the math functions of other browsers depending on host OS, so they can disguise the fact that they're only here for the scraping.

The internet would be better without them.

cute_boi3• 9 days ago

If you want ppl like me to read your article, please stop writing like 80% of text on AI. Use it to correct grammar, typos, and similar issues, but please do not use it to write the whole article.

However, thank you for the information.

barbolo• 5 days ago

It seems fixed. I've tested some Math ops that were previously failing and now are consistent on Chrome 150.0.7871.128 for linux and 150.0.7871.129 for macOS.

Since Chromium 148, Math.tanh is now fingerprintable to link underlying OS

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