Back

In other words, it's not great that this is possible, but nowhere near an immediate and present danger to anyone except a very very small group of people doing very very specific things.

If you're in that group, you'd already known you are. You'd have someone to ask about this. And you'd almost certainly be using some other tools to anonymize yourself anyway.

If that's not the case, then this is almost certainly not something to lose sleep over. Signal remains a safe choice of a secure IM.

If you are still worried about this, my read of it is that these things might make the attack more difficult:

turn off automatic downloading of media files

This makes this attack rely on you clicking the image to download it, making it very difficult for the attacker to know when to check for the cached status of the resource.

This is important, because for each attachment the attacker can only ask this question once per the period Cloudflare caches these resources (not sure exactly).

You can also:

turn off push notifications – this makes the attack rely on you clicking the chat to download the image

turn off read receipts – again, this makes it more difficult for the attacker to know when to ask the question they can only ask once per a specific period of time

use Signal over Tor or a VPN to obscure your actual location – the attacker would get the rough location of the exit node

Technical details tl;dr:

- Signal (and other communication platforms) uses Cloudflare with caching enabled for media

- one can check on which Cloudflare endpoints a given attachment URL got cached (one can use a VPN for this), giving them the ability to roughly geolocate users whose Signal downloaded the file

- a doctored version of Signal (or whatever app) allows the attacker to send the message with an image, and extract the attachment URL to know what URL to check for having been cached

@rysiek Thank you for this summary.

BTW, does using a trusted proxy in Signal help to mitigate this issue?

@agturcz I am not sure, I don't know enough about trusted proxies.

@rysiek You can set a proxy to be used by Signal. I would expect that in this case request to download the attachment from CDN goes through the proxy. And the best the attacker will get is the ip address of the proxy.

However, I will reveal my ip to the proxy. That's why trusted.

@rysiek I'll give you a week to #TouchGrass and come back...

I sent my grandma the app store link for @signal and she was able to install it and communicate with the family without having to guide her through anything. They've really made it that simple. I wouldn't be able to do that with any other app.

@contrapunctus @agturcz yes, I am aware of all these. I am also aware of Simplex, Briar, and whole slew of completely decentralized IMs. And I made a long ranty talk about shortcomings of Signal that one time, got pretty popular on media.ccc.de.

And I still react badly to unnecessarily alarmist hot takes that can lead regular folks to make bad technological decisions.

@rysiek @agturcz Then, I confess to being confused about what you mean.

Why did you find it to be "a pain to use"?

Some clients don't have end-to-end encryption enabled by default - I hope that will change some day, but I never found that to be a dealbreaker. If someone sends cleartext, me and my friends immediately ask them to enable OMEMO.

Still, no feature or convenience is worth using a centralized silo. Reddit, Twitter, and Meta are proof enough.

@contrapunctus @agturcz first of all, please don't explain centralization to me, I was talking about it before it was cool:
https://media.ccc.de/v/30C3_-_5319_-_en_-_saal_g_-_201312282330_-_technomonopolies_-_rysiek

Secondly, "some clients don't support X" is a deal breaker. Because now regular folks need to track and think about whether or not their contact's server supports a safety feature they rely on.

Third, "if someone sends a cleartext…" is not anywhere near being acceptable for a communication tool like that. Sending cleartext should not be *possible*.

@contrapunctus @agturcz I had worked with people reporting on Panama Papers, I had worked with people working with sources whose threat model included men with guns who were trained and willing to use them.

This kind of "no biggie if someone sends cleartext, we can ask them to enable OMEMO" stuff is what can get people killed. Advocating for tools like that is putting real people in real danger.

I am glad XMPP is improving, but it is simply nowhere near a Signal replacement yet.

@rysiek @agturcz Sounds like @snikket_im is your best bet, then. All Snikket clients have OMEMO enabled by default. And this way you actually can actually trust the operator, i.e. yourself, and control exactly what cloud services are used (including "none").

And Signal is seemingly not the perfect solution it's being made out to be, either.

https://troet.cafe/@pixelschubsi/113808514523533577

https://troet.cafe/@pixelschubsi/113808528593247949

@contrapunctus you seem to be ignoring what I and others are telling you about how dangerous what you're doing – promoting XMPP into a space it has no business being in in its current state – is.

I am done with this conversation.

@agturcz @snikket_im

@rysiek @contrapunctus @agturcz @snikket_im I still have to disagree that Signal is better than XMPP in general. Signal has these terrible qualities of using a phone number for registration and being a centralized service. It's also not (afaik) really free software, and they might not be telling us everything. I do think it's nice that Signal proxies cleverly hide traffic using HTTPS. Ultimately XMPP is the wiser choice, and who sends messages in plaintext? All traffic to the server is encrypted with TLS.

@rysiek @contrapunctus @agturcz @snikket_im In any case, whether it's Signal, XMPP, or Delta Chat, you would have to take some configuration steps to hide the fact that you are using them. Delta Chat is pretty easy to hide and sneak through firewalls because e-mail is generally treated as innocuous. Choosing the right server with Delta Chat probably your safest route.

@claudius @rysiek @agturcz You'll face it with every open standard - email, IRC, ActivityPub (!!), Matrix...but somehow, only XMPP gets singled out as "unusable" (even though plenty of people - like me - use it without any issues.)

It's a bloody stupid excuse to use a centralized system for the whole world's communication, and you'll never convince me otherwise.

Have fun silo-hopping. The sane people will be on XMPP as they've always been, pointing and laughing at the silo-lovers.

@contrapunctus @rysiek @agturcz the XEP process is simply broken and clients don't keep up. I did not have nearly as many problems with any of the things you listed. Yes, email is a horribly old protocol that is held together by spit and duct tape, but at least it does not get in my way. XMPP never worked frictionless for me. And, yes, I really tried. And, no, I am not stupid.

@viq @rysiek @agturcz Let's not post FUD?

Here's a response to it.
https://www.moparisthebest.com/against-silos-signal/

OMEMO author Tim Henke also made this fairly civil comment on the post, which was - somewhat surprisingly - deleted by Soatok. https://www.moparisthebest.com/tim-henkes-omemo-response.txt