Or mine where the entire browser profile is on a ramdisk.
lima•Jun 1, 2026
Higher IO latencies in HDs might actually make this attack easier - more contention means more bits of data.
tosti•Jun 1, 2026
I laugh at your spying attempts from my cURL e-mail service that I use instead of a web browser.
tovve•May 29, 2026
Still don't really understand how it works - I put the reddit logo into your local storage and it only took 20ms to take it out again instead of 50ms so therefore you have reddit open in another tab?
nostrademons•May 31, 2026
I assume it's something like this:
Attacking website periodically makes random reads from a large file in localStorage. Other tabs and websites open have Javascript running that periodically performs operations that will result in SSD traffic. For example, GMail has a certain polling interval to check for new mail, and each request is going to result in a cache write that makes the SSD busy and delays other conflicting IO operations. Reddit checks for new chat messages. Large memory-heavy websites get paged out of RAM.
The pattern of IO operations that a website makes creates a fingerprint of interference with the IO ops that the attacking website is doing, showing up as differing amounts of latency as the SSD is periodically busy. This fingerprint can then be reconstructed to a specific website by training a CNN on it, basically using a neural net to classify a certain pattern of delays to the IO ops that other websites are doing.
In theory it makes sense, but it seems very noisy. Anything that makes absolutely zero requests or IO operations in the background (like say HN, or most old-school text sites) wouldn't show up, and would be indistinguishable from any other zero-request site. And having other sources of IOps on the same computer - say you're running an Ethereum client that's perpetually updating the blockchain, or you're downloading a bunch of torrents, or you've got DropBox and it's syncing your directory - would introduce noise that throws off the classifier.
puppycodes•May 31, 2026
Thats a good explination. It does seem extremely noisy and not at all practical for fingerprinting a user compared to other methods. If you have javascript enabled assume you can be fingerprinted.
doodlebugging•May 31, 2026
That's interesting. Thanks for the explanation. If I read this right this isn't as effective against spinning HD-based systems and there is a dependence on the user maintaining more than one tab as they browse?
If that's the case then my system which is still HD-based is not threatened and since I tend to close tabs and windows and just spin up a new private window for each site while clearing cookies, etc on exit then maybe this is a non-issue for me. Or maybe just block javascript too.
nostrademons•Jun 1, 2026
It'd have some effectiveness against spinning HDDs because it's really just measuring contention for the I/O subsystem, but it'd likely be less because the kernel usually buffers writes to HDDs internally. But then, the kernel also usually buffers writes to SSDs, just with lower latency between the call and the data being written.
I don't think too highly about this particular threat vector - it seems like the kind of attack where you could perhaps get a working proof of concept going in the lab to write a paper and demonstrate some results, but actually using it to attack people at scale seems prohibitively noisy. People that close all their tabs when not at use are not at risk (and the data I had was that most people don't actually use browser tabs, they're very much a power-user feature). People who have disk-intensive other processes like Bittorrent or various file-syncing services aren't really at risk, because those other processes inject similar noise into the data stream. The signal in general seems weak because of buffering and differing SSD latency and so on.
maverwa•May 31, 2026
That’s timing the cache, that’s old stuff by know. As I understand, this writes a relatively large file („Gigabytes“) using this OPFS api, which is different from the „localStorage“ api.
This seems to use actual filesystem storage on the client, instead of living completely in memory (which may be reasonable given the size of files supported). This allows to actually time SSD IOPS latency by doing random reads.
Collected enough of these samples, together with the information of what else runs on the host, put that in the ML-Blender and the result will be able to tell you, with some accuracy, from a given set of samples, what’s running on the host.
I am sure i misunderstood some things because there are so many caches and unknowns in that setup that I struggle to understand how there could be any correlation, but that’s my understanding so far.
Wait, wait, wait: browsers allow websites to store junk on my drive? They take up gigabytes of memory and still write to disk on top of this? Without even asking whether the site can use local storage?
Years and years back when laptops still had HDDs, I had a script to put the Firefox profile &c on a ramdisk and sync it on reboots so that it didn't spin up the drive constantly. I guess I should have kept doing it.
I thought the whole point of cookies, local storage, session storage, and indexed DB were to avoid what origin private file system is doing.
You mean I could have just saved stuff as a file this whole time instead of serializing it to a string? Why didn't we just do this from the start?
nostrademons•May 31, 2026
It's still sandboxed and deleted when the user clears private data for the website.
The main advantage it has over things like cookies, local storage, etc. is that it provides a byte-oriented, random access API and as a result, you can use third-party libraries like SQLite that expect a file API. Which is more important now that we have tools like Emscripten and WebAssembly that let you use existing C libraries on the web. At the same time it has security guarantees such that webpages cannot write arbitrary files that will be viewed and executed by the user.
Also, in theory you could use this side-channel attack on localStorage and sessionStorage. Its only requirement is that it needs an API that writes to disk where you can measure the latency of a synchronous call, since the fingerprinting is just measuring the interference pattern between disk accesses the attacking website does vs. disk accesses that other websites do.
sheept•May 31, 2026
Is this surprising? Websites have long been silently writing to disk, for cache, cookies, and blobs. OPFS just provides a file-system-like API for ultimately the same functionality
runako•Jun 1, 2026
Yes? From the paper:
"On Chrome and Safari, OPFS supports very large files, up to 60 % of disk space, which is more than sufficient to avoid the page cache on most typical systems, as even a small disk size of 64 GB would allow us to create a 38.4 GB OPFS file."
I am indeed surprised to learn that a random website can write a file that takes up 60% of my disk. Is this obviously a capability of Web browsers?
ElProlactin•Jun 1, 2026
Ten movies streaming across that, that Internet, and what happens to your own personal Internet? I just the other day got... an Internet [email] was sent by my staff at 10 o'clock in the morning on Friday. I got it yesterday [Tuesday]. Why? Because it got tangled up with all these things going on the Internet commercially. [...] They want to deliver vast amounts of information over the Internet. And again, the Internet is not something that you just dump something on. It's not a big truck. It's a series of tubes. And if you don't understand, those tubes can be filled and if they are filled, when you put your message in, it gets in line and it's going to be delayed by anyone that puts into that tube enormous amounts of material, enormous amounts of material.
hulitu•Jun 1, 2026
> Is this obviously a capability of Web browsers?
The main capability is RCE, but it seems that they need a way to store the payload.
cinntaile•Jun 1, 2026
There's a whole trend with websites not uploading anything to their servers due to privacy and whatnot, where do you suppose the data is being saved for repeat visits...
runako•Jun 1, 2026
What your'e describing I would expect to be measured in kilobytes, not tens of gigabytes.
There is no reason for any person to think that a website needs to store data sized in the "full Ubuntu install" range to facilitate repeat visits.
Rohansi•Jun 1, 2026
> There is no reason for any person to think that a website needs to store data sized in the "full Ubuntu install" range to facilitate repeat visits.
Do you think people expect that for apps they've installed? Should those also be limited to a few MB?
cinntaile•Jun 1, 2026
You make a reasonable point, while kilobytes might be too little it probably shouldn't be 30gb. 5gb might be ok. In the settings it should be possible for the user to set their own limit. I am not familiar with browser storage but there is hopefully a mechanism to inform the user that their limit might not be enough.
rendaw•Jun 1, 2026
Not only that, but they don't even provide any visibility into what's being stored. Firefox developer tools doesn't even have OPFS browser functionality. IIRC I even saw some stuff about going out of the way to make it inaccessible by the user.
kccqzy•May 31, 2026
> Without even asking whether the site can use local storage?
Where did you see this in the article? I had some recollection that Firefox at least did require asking the user.
nozzlegear•May 31, 2026
Firefox doesn't ask permission just to use localstorage, no modern browser does this. The closest thing you get is when a site wants to persist storage with "navigator.storage.persist()", which should prompt you for permission. But localstorage data usually persists anyway, and only gets deleted if the browser's storage is "under pressure", so I've never personally worked on a site or web app that had to use that API.
anygivnthursday•Jun 1, 2026
You mean by default or it cannot be configured that way? I believe, I had Chrome configured to not allow storage by default, only for sites I added to an exclusion list. I cant remember now, but isnt there also an option to change the default on Firefox to deny or always ask for permission?
nozzlegear•Jun 1, 2026
Just by default - I didn't know you could configure your browser to disallow storage by default.
atoav•Jun 1, 2026
Btw. as per EU law (GDPR) website owners are required to aquire informed consent for any kind of client side storage if it contains information that is personal. And it has been ruled that any information that can be used to identify returning users is such.
People think the GDPR is just about cookies, but it is agnostic of the technology used.
Maximum fines: €20 million, or 4% of the company's total worldwide annual turnover of the preceding financial year — whichever is higher.
And informed consent means they need to know what data you collect/store for which purposes and there needs to be an equally easy to select No-Option.
runako•Jun 1, 2026
This doesn't really address the issue here. The condition here is that a site might decide that it needs to store (say) a copy of the Red Hat server installation package on each user's local machine (20GB) to facilitate repeat visits.
The stored data is not related to the user at all. The problem is that the website gets to silently write 20GB to the user's disk.
Vinnl•Jun 1, 2026
I don't think LocalStorage allows you to store gigs of data though, and IIRC this method depended on the Origin-Private File System API.
HeartStrings•Jun 1, 2026
Hostile LLMs? In my browser? At this time of the year?
Khaine•Jun 1, 2026
And Web Developers want more and more OS features built into the browser. This is why I'm against it. Features are only ever abused.
noirscape•Jun 1, 2026
Browsers have an absolute insane level of relatively unchecked permissions to do whatever they want on a client.
There's a lot of effort by browser developers to scope creep the browser into essentially being an OS-agnostic tech stack (one where, conveniently, code can be shipped across the network "as necessary", removing a lot of user agency for the software being ran); Chrome being the biggest driver of this, while Firefox has an extremely weak spine in trying to limit it.
It's fairly dire and I wouldn't be surprised if there's a lot more of these side channel attacks in a lot of web APIs.
noelwelsh•Jun 1, 2026
It's also the technology that will allow software to run without a continuous connection to the server. If you want to break out of a world where companies own your data it's the tech that is needed.
Tangurena2•Jun 1, 2026
Flash ended up getting blocked/banned by all browsers because it turned into a giant gaping security hole.
> By January 2021, all major browsers were blocking all Flash content unconditionally.
It looks like we-the-users need to be blocking any and every one of these parasites.
I have a feeling they may have pushed for that more because it was controlled by a third party, and not the browser developers themselves.
rayiner•Jun 1, 2026
Now that we have AI, can we go back to real apps and native tech stacks? And revert the browser to a text-display interface?
VMG•Jun 1, 2026
Unfortunately, real apps and native tech stacks can not only write data to your SSD, they can usually write data to the user directory however they want and they can read it as well!
Browsers are at least somewhat sandboxed
hollerith•Jun 1, 2026
This is a Linux-centric take. It does not apply for example to iPadOS or to AluminiumOS (coming soon to a Googlebook near you). It applies less and less over time to MacOS.
Yes, if one is committed to the standard Linux desktop, then one must hope that any proprietary apps one might need will continue to be available through the browser, but I'm ready to let the standard Linux desktop go (not right now, but eventually).
rayiner•Jun 1, 2026
But you need relatively few native apps with those capabilities, in contrast to visiting many different websites for content consumption.
MisterTea•Jun 1, 2026
Why not sandbox the process then?
freedomben•Jun 1, 2026
> can we go back to real apps and native tech stacks
Please God, no. If you're worried about the invasiveness of browser-based apps, native is out of the frying pan and into the fire
veunes•Jun 1, 2026
The uncomfortable part is that each step is usually justified by a real use case
DanielHB•Jun 1, 2026
If you open an incognito window in chromium it is profile on ram
Gormo•Jun 1, 2026
My shortcut for launching "clean" Chromium session is `chromium --user-data-dir=$(mktemp -d)` -- each launch creates a new transient profile directory under /tmp, which is itself a RAM disk. Persistent settings are achieved by setting system-wide defaults in /etc/chromium, including using system-wide managed policy JSON.
data-ottawa•Jun 1, 2026
Does this maintain your browser extensions (and their settings)?
Gormo•Jun 1, 2026
Yes, extensions can be installed into the system-wide config via entries in the manged policy JSON. Settings configured in a specific browser session naturally won't persist, though, but defaults set in an initial preferences config will be present.
binyu•Jun 1, 2026
> Wait, wait, wait: browsers allow websites to store junk on my drive?
Technically even a cookie is junk on your drive
> Without even asking whether the site can use local storage?
Would it be practical to ask permission for every site you visit? It would be better to periodically check the size of your home folder (where the browser profiles normally reside)
veunes•Jun 1, 2026
The funny part is that "put your browser profile on a ramdisk" used to sound like an obsessive performance tweak, and now it starts to look like a privacy mitigation
opengrass•May 31, 2026
{first.last}@tugraz.at
Aurornis•May 31, 2026
I’m skeptical of these side channel attacks that rely on training a neural network on specific controlled scenarios on controlled hardware. I believe that with enough time and effort and the perfect circumstances where the user is only visiting their website and doing one other thing that the network was trained on it can match.
It does not seem useful as a general purpose side channel vector.
hansvm•May 31, 2026
It depends what you mean by "general purpose." First, these things generalize more often than you'd expect. Second, even in the absence of generalization they're still useful for, e.g., fingerprinting activities to manufacture a unique ID where non previously existed.
Aurornis•Jun 1, 2026
The paper isn’t describing a unique ID fingerprint. It’s looking for specific activity patterns to match against training data of running specific commands on specific hardware.
ramenat2am•May 31, 2026
That's basically just a research, theoretical attack vector. It doesn't mean it's viable for general purpose old school mass privacy invasion
1970-01-01•Jun 1, 2026
Publish or perish. It worked once, in a controlled lab, mostly (80-90% guess). Good enough for more millions in funding..
It's really not surprising that letting websites run arbitrary code on your machine, even in a sandbox, would lead to things like this.
sigmoid10•May 31, 2026
There's no such thing as a sandbox "on your machine" when you really think about it. The code still runs on the same hardware and there are tons of ways to fiddle with said hardware that could be exploited (like rowhammer). The only "real" sandbox is fully dedicated hardware down to bare metal with zero connections to sensitive systems.
matheusmoreira•May 31, 2026
And now that Google's web environment integrity is getting repackaged into captchas, it seems we won't even be able to try to block such things in the future...
ptek•Jun 1, 2026
Ahhh Arstechnica, I wonder if the technical article is by Dan Goodin. (It is)
I enjoyed his C Programming books for dummies series.
francoi8•Jun 1, 2026
It should be fairly easy to mitigate no? Simply add random access times. Localstorage doesn't need to be that fast. More generally I find it very annoying how much browsers allow by default (javascript, localstorage, gpu access etc.) - there's only a very limited amount of websites I want to be able to run gpu accelerated shaders.
ars•Jun 1, 2026
> Simply add random access times.
That doesn't work. Because the random times are uniformly distributed it's possible to remove it from the data by additional sampling. You do make it harder because you need a lot more data, but it's still possible to extract the signal, because the noise is uniform.
progval•Jun 1, 2026
The random times don't have to be uniformly distributed. Though it's enough for attackers to know the distribution to de-noisify it.
eterm•Jun 1, 2026
The interesting mitigation would be snapping I/O to a course clock.
You could then set it to hold the result until the next tick.
E.g. An I/O tick of 20ms, and it would only return on 20ms boundaries, then almost every SSD would look the same.
It would slow down the API a bit, but privacy has tradeoffs.
danbruc•Jun 1, 2026
Probably still does not work. Assume a request takes X ms and let us look at what you will observe depending on where within a tick period it arrives.
If it arrives anywhere from 0 ms to (20 - X) ms after a tick, it will complete before the next tick, so the measured duration will be between X ms and 20 ms. If it arrives later in the tick period, it will miss the next tick and have to wait an additional tick period, so the measured duration will be between 20 ms and (20 + X) ms.
If you make N repetitions, you would normally see a spike of density 1 at X. With the 20 ms tick wait, you will see a uniform distribution of density 1/20 between X and (20 + X).
You would have to perform each request and then return the result exactly 20 ms after it was received in order to mask the request duration. But that just creates a new target, your timers and queues to delay the response. Or making the load so high, that requests take more than 20 ms.
exabrial•Jun 1, 2026
correction, websites have a way to spy on visitors: Javascript.
mr-pink•Jun 1, 2026
sounds like nonsense. i guess this works on some test environment but not in real life. you would never know that I am running tetris, for example
dwedge•Jun 1, 2026
> Even Meta and Yandex were recently caught joining in the privacy-invasive free-for-all.
Damn, even Meta have joined the dark side?
meindnoch•Jun 1, 2026
Sic transit gloria mundi :'(
dbg31415•Jun 1, 2026
Maybe don’t let Google decide what its browser can and can’t do on my computer…
Why do browsers need to do this? Feels like an edge case need, at best, that was likely just a cover for some power Google wanted to exploit.
gblargg•Jun 1, 2026
I'm surprised their 1GB file wasn't cached entirely in RAM during the attack, eliminating the SSD from any timing. Do people keep their machines that heavily loaded that a file being constantly read from doesn't stay in the cache?
basilikum•Jun 1, 2026
I think what would be more interesting is using this as a side channel for communication between different sandboxed contexts.
zeafoamrun•Jun 1, 2026
That's it, I'm turning off JavaScript for everything non essential from here on out.
Because every time they open up, the site gives them the F̶R̶O̶S̶T̶ cold shoulder.
bzmrgonz•Jun 1, 2026
I wonder if mozilla's container tabs blocks this type of tracking?
pixel_popping•Jun 1, 2026
Why would it be a new way? Tracking via timing have always existed, you can also know browsing history of someone with some DNS trick, nothing really new, article is misleading with "new way", it's literally possible since a decade.
20 Comments
Attacking website periodically makes random reads from a large file in localStorage. Other tabs and websites open have Javascript running that periodically performs operations that will result in SSD traffic. For example, GMail has a certain polling interval to check for new mail, and each request is going to result in a cache write that makes the SSD busy and delays other conflicting IO operations. Reddit checks for new chat messages. Large memory-heavy websites get paged out of RAM.
The pattern of IO operations that a website makes creates a fingerprint of interference with the IO ops that the attacking website is doing, showing up as differing amounts of latency as the SSD is periodically busy. This fingerprint can then be reconstructed to a specific website by training a CNN on it, basically using a neural net to classify a certain pattern of delays to the IO ops that other websites are doing.
In theory it makes sense, but it seems very noisy. Anything that makes absolutely zero requests or IO operations in the background (like say HN, or most old-school text sites) wouldn't show up, and would be indistinguishable from any other zero-request site. And having other sources of IOps on the same computer - say you're running an Ethereum client that's perpetually updating the blockchain, or you're downloading a bunch of torrents, or you've got DropBox and it's syncing your directory - would introduce noise that throws off the classifier.
If that's the case then my system which is still HD-based is not threatened and since I tend to close tabs and windows and just spin up a new private window for each site while clearing cookies, etc on exit then maybe this is a non-issue for me. Or maybe just block javascript too.
I don't think too highly about this particular threat vector - it seems like the kind of attack where you could perhaps get a working proof of concept going in the lab to write a paper and demonstrate some results, but actually using it to attack people at scale seems prohibitively noisy. People that close all their tabs when not at use are not at risk (and the data I had was that most people don't actually use browser tabs, they're very much a power-user feature). People who have disk-intensive other processes like Bittorrent or various file-syncing services aren't really at risk, because those other processes inject similar noise into the data stream. The signal in general seems weak because of buffering and differing SSD latency and so on.
Collected enough of these samples, together with the information of what else runs on the host, put that in the ML-Blender and the result will be able to tell you, with some accuracy, from a given set of samples, what’s running on the host.
I am sure i misunderstood some things because there are so many caches and unknowns in that setup that I struggle to understand how there could be any correlation, but that’s my understanding so far.
Years and years back when laptops still had HDDs, I had a script to put the Firefox profile &c on a ramdisk and sync it on reboots so that it didn't spin up the drive constantly. I guess I should have kept doing it.
It's a sad day when Arch users are right (again) https://wiki.archlinux.org/title/Firefox/Profile_on_RAM
I thought the whole point of cookies, local storage, session storage, and indexed DB were to avoid what origin private file system is doing.
You mean I could have just saved stuff as a file this whole time instead of serializing it to a string? Why didn't we just do this from the start?
The main advantage it has over things like cookies, local storage, etc. is that it provides a byte-oriented, random access API and as a result, you can use third-party libraries like SQLite that expect a file API. Which is more important now that we have tools like Emscripten and WebAssembly that let you use existing C libraries on the web. At the same time it has security guarantees such that webpages cannot write arbitrary files that will be viewed and executed by the user.
Also, in theory you could use this side-channel attack on localStorage and sessionStorage. Its only requirement is that it needs an API that writes to disk where you can measure the latency of a synchronous call, since the fingerprinting is just measuring the interference pattern between disk accesses the attacking website does vs. disk accesses that other websites do.
"On Chrome and Safari, OPFS supports very large files, up to 60 % of disk space, which is more than sufficient to avoid the page cache on most typical systems, as even a small disk size of 64 GB would allow us to create a 38.4 GB OPFS file."
I am indeed surprised to learn that a random website can write a file that takes up 60% of my disk. Is this obviously a capability of Web browsers?
The main capability is RCE, but it seems that they need a way to store the payload.
There is no reason for any person to think that a website needs to store data sized in the "full Ubuntu install" range to facilitate repeat visits.
Do you think people expect that for apps they've installed? Should those also be limited to a few MB?
Where did you see this in the article? I had some recollection that Firefox at least did require asking the user.
People think the GDPR is just about cookies, but it is agnostic of the technology used.
Maximum fines: €20 million, or 4% of the company's total worldwide annual turnover of the preceding financial year — whichever is higher.
And informed consent means they need to know what data you collect/store for which purposes and there needs to be an equally easy to select No-Option.
The stored data is not related to the user at all. The problem is that the website gets to silently write 20GB to the user's disk.
There's a lot of effort by browser developers to scope creep the browser into essentially being an OS-agnostic tech stack (one where, conveniently, code can be shipped across the network "as necessary", removing a lot of user agency for the software being ran); Chrome being the biggest driver of this, while Firefox has an extremely weak spine in trying to limit it.
It's fairly dire and I wouldn't be surprised if there's a lot more of these side channel attacks in a lot of web APIs.
> By January 2021, all major browsers were blocking all Flash content unconditionally.
It looks like we-the-users need to be blocking any and every one of these parasites.
https://en.wikipedia.org/wiki/Adobe_Flash
Browsers are at least somewhat sandboxed
Yes, if one is committed to the standard Linux desktop, then one must hope that any proprietary apps one might need will continue to be available through the browser, but I'm ready to let the standard Linux desktop go (not right now, but eventually).
Please God, no. If you're worried about the invasiveness of browser-based apps, native is out of the frying pan and into the fire
Technically even a cookie is junk on your drive
> Without even asking whether the site can use local storage?
Would it be practical to ask permission for every site you visit? It would be better to periodically check the size of your home folder (where the browser profiles normally reside)
It does not seem useful as a general purpose side channel vector.
Not really joking here.
https://hannesweissteiner.com/
https://hannesweissteiner.com/publications/frost/
I enjoyed his C Programming books for dummies series.
That doesn't work. Because the random times are uniformly distributed it's possible to remove it from the data by additional sampling. You do make it harder because you need a lot more data, but it's still possible to extract the signal, because the noise is uniform.
You could then set it to hold the result until the next tick.
E.g. An I/O tick of 20ms, and it would only return on 20ms boundaries, then almost every SSD would look the same.
It would slow down the API a bit, but privacy has tradeoffs.
If it arrives anywhere from 0 ms to (20 - X) ms after a tick, it will complete before the next tick, so the measured duration will be between X ms and 20 ms. If it arrives later in the tick period, it will miss the next tick and have to wait an additional tick period, so the measured duration will be between 20 ms and (20 + X) ms.
If you make N repetitions, you would normally see a spike of density 1 at X. With the 20 ms tick wait, you will see a uniform distribution of density 1/20 between X and (20 + X).
You would have to perform each request and then return the result exactly 20 ms after it was received in order to mask the request duration. But that just creates a new target, your timers and queues to delay the response. Or making the load so high, that requests take more than 20 ms.
Damn, even Meta have joined the dark side?
Why do browsers need to do this? Feels like an edge case need, at best, that was likely just a cover for some power Google wanted to exploit.
Because every time they open up, the site gives them the F̶R̶O̶S̶T̶ cold shoulder.