[dodgerdan]

To paraphrase another comment on this; if I have to trust the server for Matrix E2EE to work, why don’t I just use Slack? The whole point of E2EE is so you don’t have to trust the server.

And a “warning” that says something like “your group chat has been compromised by someone with access to the server or it might just be bob bought a new phone” is laughable as a status quo.

[Arathorn]

The point is that if your threat model cares about interception, you have to verify the identity of users out of band - same as Signal or WhatsApp or whatever.

Then, if you've done that, and an unverified device is added to the conversation, the warning shows up and explicitly means "your group chat has been compromised" - just like a big red TLS warning on a webpage means "this certificate is bad". It does *NOT* mean that "it might just be bob bought a new phone"; if bob bought a new phone, he'll have verified it when he logged in, and so the warning doesn't show up.

Anyone who actually uses Matrix will be familiar with the fact that it's pretty rare to see a big red X on a user who you've verified, and when it happens, you go ping them to ask wtf is going on - much like you'd ping the webmaster of a site whose TLS certificate is bad. This is why the Matrix community is not up in arms about this "vulnerability" - it is literally following the intended design, right from the point that we announced verification back in https://element.io/blog/e2e-encryption-by-default-cross-sign....

Now, this design can absolutely be improved on - at the least, we could go back to freezing the room whenever there's an unverified device, forcing the users to decide how to flush out the old device. Or we could switch to TOFU, and block unverified devices by default. But we've chosen to prioritise migrating the clients onto a single E2EE codebase (matrix-rust-sdk-crypto) first, so we do that work in a single place, rather than in quadruplicate across matrix-{js,ios,android,rust}-sdk.

[dodgerdan]

> The point is that if your threat model cares about interception, you have to verify the identity of users out of band - same as Signal or WhatsApp or whatever.

I’ve read your blog posts and this comment, congratulations it seems to have satisfied most people. Howerver it is NOT the same as Signal. Signal servers cannot just add a device to a group chat.

I would challenge you to get one reputable cryptographer to back what you’re claiming about these vulnerabilities and your proposed fixes.

To me Matrix isn’t secure, the organizations responses to these disclosures has been poor and the “fixes” weak.

[martinralbrecht]

A quick comment on this, we did address this in our paper:

"In environments where cross-signing and verification are enabled, adding a new unverified user adds a warning to the room to indicate that unverified devices are present. However, it is possible for a homeserver to add a verified user to rooms without changing the security properties of the room. This allows a colluding homeserver and verified user to eavesdrop on rooms not intended for them. In other words, the warning regarding unverified devices is independent to whether the device is intended to participate in the specific room. Finally we note that users may, of course, simply ignore warnings." https://nebuchadnezzar-megolm.github.io/static/paper.pdf

[DyslexicAtheist]

the paper and this design decision by Matrix teams has recently been discussed at length in an SCW podcast (paging Thomas H. Ptacek). One of my fav episodes yet. Very sobering unfortunately.

https://securitycryptographywhatever.buzzsprout.com/1822302/...

[Arathorn]

All the pointing-and-laughing derision on the podcast seemed pretty toxic, tbh. Rather than all the "oh look how stupid Matrix are" schtick, it might have been even more interesting and informative (and less obnoxious) to get the other viewpoint and understand why we don't consider server-controlled group membership to be catastrophic in practice... as per my explanations elsewhere on the thread.

Honestly, this whole thing shows the infosec community at its worst: optimising for sensationalism/drama/press-coverage over constructive criticism and research. And it's particularly grating when the target is a mission-driven non-profit FOSS project, rather than a bungling megacorp vendor or whatever. Meanwhile things like OpenSSL RCEs or even Signal RCEs like https://thehackerblog.com/i-too-like-to-live-dangerously-acc... fly past without the researchers doing the podcast circuit(!)