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.
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
"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?
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.
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.
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?
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.
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.
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.
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.
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?
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.
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.
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.
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.
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.
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.
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...
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
"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?
Where did you see this in the article? I had some recollection that Firefox at least did require asking the user.
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.
It does not seem useful as a general purpose side channel vector.
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.
I enjoyed his C Programming books for dummies series.