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This post knows where you're viewing it from (Lemmy doesn't proxy external images) [ARCHIVED]
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this post was submitted on 11 Aug 2023
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Privacy
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This is possible because Lemmy doesn't proxy external images but instead loads them directly. While not all that bad, this could be used for Spy pixels by nefarious posters and commenters.
Note, that the only thing that I willingly log is the "hit count" visible in the image, and I have no intention to misuse the data.
Nice example!
I think proxying everything through lemmy would have a pretty big bandwidth/scalability impact. I expect the lemmy clients dont send any unique user info on these image requests so not sure how useful it would be as a spy pixel? Maybe I'm missing something :-)
It would be interesting to see just how much info is shared when lemmy requests the image. If there is [potentially] sensitive info being shared, the devs might be interested in working on it too (I have no idea how to check such a thing, this comment is just so I can find the post later when more people have shared their wisdom on it)
None (by Lemmy), as Lemmy doesn't actually request the image (that would be proxying). Your browser requests the image directly by URL. Lemmy, technically, doesn't even know an image exists. It just provides the HTML and lets your browser do the work.
Exactly. The text of this post is simply :
I get the same result when I browse directly to the link.
So, if OP links a malcious website we have a problem ... (?).
Oh dangit, it's simpler than I thought. So the only data being sent is...just whatever is sent in your average GET request.
Yes. It's also a pretty standard way of serving images. A lot of Email clients do that too.
That's also how these services that show you when a email is read work.
Not really that huge of a problem. When making requests you also usually send a header which includes the user agent.
The program just logs how many times the image has been requested and it reads the user agent data. No Javascript is actually executed.
Well it might be possible to have a XSS somehow but I haven't really done much research into this possibility.
In general it's a pretty standard way of handling embedded images. Email does this too. That's how you have these services that can check if someone read a mail
Yup. And to add, your browser will send things like:
Your IP address. Technically this is sent by the OS doing networking and is unavoidable. At best, a VPN can hide this, because the VPN sits in the middle.
Various basic request headers, which most notably contains user agent (identifies browser) and language headers, both which you can fake if you want to.
Cookies for that domain (if you have any). Those can track you across multiple requests and thus build up a profile of you.
That's why you should use a native app, which won't send any of that identifying info (except for IP but there's nothing you can do on that)
Notably, this allows remote parties to associate your IP address with your interests, as revealed by the Lemmy communities that you browse.
One way is for the image host to use the HTTP Referer field. (Standards-respecting web browsers pass the URL of the web page being viewed to the server hosting the image.)
Another way is by posting an image with a unique URL.
Even if Referer is withheld and the image is not unique, the image host can still do basic fingerprinting of your client's request header and your OS's TCP quirks, and associate that fingerprint with your IP address.
An option for Lemmy to proxy media would be very helpful. Small instances could perhaps disable it, although they might not need to, since the additional load would scale with the number of users on that instance.
I suspect with a coordinated pool of posts or multiple comments on the same post, you could narrow that IP address down to an actual user account.
When a new comment is posted by a user, store, against their username, all IP addresses that visited since the last comment in that thread (by anyone). When a second comment is posted by a user, remove any IP addresses that don't appear in both lists.
I suspect you would have a very short list after two comments, and a single address after 3. It would also be extremely easy to both lure someone into viewing an image and bait them into multiple replies. Geolocate that IP and you know know vaguely where that user lives.
Time to make sure you're always on a VPN I guess.
You could also send the image through a DM if you want to find a particular user
Oh yeah, that'd be much less effort.
Even without that, once your Lemmy interests are sold/shared by IP address, they can be associated with your real identity as soon as you log in to a service that knows who you are.
Were you expecting otherwise? Loading an external image is no different than loading an external website with images. Lemmy and reddit are link aggregators, not proxies. Having to proxy everything would run a significant bandwidth for instance admin who are often paying out of pocket for hosting.
Any chance that's why this account is posting the same image and gibberish? @googa
From what I remember, that image was hosted on hexbear.net, so I don't think so.
How do you get an image to run code? I guess I somehow missed something important in website development.
Edit: I saw that you said you're using Pillow to actually render the image from code. That's neat! ...and scary
Proxying external images means that instead of the image being downloaded from the original link, your Lemmy server would download it and serve it for you. The Lemmy server acts as a proxy.
But it means performing a lot of extra traffic. And realistically you'd want to cache the image because otherwise your server will likely get banned for the high volume of requests you send. But caching the images requires more storage and can have potential for legal issues.
And images are one thing, but literally any content is the problem. Images are just the most obvious because they often load without even having to click on the image and thus you'll get far higher volume of user data. Literally anything you link to has this issue and you cannot proxy all of it.
Share source code? I'm curious
It's just a simple Flask server. I parse the user-agent using the
user_agentsPython library, apply some conditionals upon the result, render the image using Pillow and send it to the user.