MSC2398: proposal to allow mxc:// in the "a" tag within messages #2398

eras · 2020-01-02T20:46:36Z

Currently message events (m.room.message) can contain formatted data in its formatted_body field, if the format of the message is org.matrix.custom.html. This is a proposal to allow the mxc:// schema to appear also for a tags in addition to just img tags currently permitted (in fact, required).

Rendered: https://github.com/eras/matrix-doc/blob/MSC2398-mxc-for-a-tags/proposals/2398-allow-mxc-within-message-links.md

SCT:

Implementation: Not required. Feature appears to be self-contained enough to make sense as-is. ~TravisR

~~FCP tickyboxes~~

KitsuneRal

Since mxc:// is effectively a proxy scheme to https://<media-api-path>/download, this looks like a very good way to refer to content residing on Matrix media servers. The alternative, actually, does exist: to use the HTTP(S) /download URL, which is by definition inferior to this proposal.

specification/modules/instant_messaging.rst

turt2live · 2020-01-02T21:30:50Z

@eras also slightly annoying, but the proposal needs to be signed off so we can merge it: https://github.com/matrix-org/matrix-doc/blob/master/CONTRIBUTING.rst#sign-off

Signed-off-by: Erkki Seppälä <flux@modeemi.fi>

eras · 2020-01-03T06:23:08Z

Updated.

Changes:

Added note about alternatives per @KitsuneRal 's comment
Removed changes to instant_messaging.rst per @turt2live 's comment
Added a Signed Off -line to the commit message per @turt2live 's comment

turt2live

Thanks! The proposal itself looks very sane.

eras · 2022-02-13T12:58:57Z

So it came to me that an implementation should be careful when implementing this.

In particular, when a web-based client opens a HTML document from the same domain as the Matrix API, the document can probably use JavaScript to make requests with the user's credentials.

ShadowJonathan · 2022-02-13T13:36:03Z

@eras i believe those thoughts would be best served under the "Security Considerations" header in the proposal 👍

turt2live

non-blocking concerns

turt2live · 2023-11-14T04:19:10Z

proposals/2398-allow-mxc-within-message-links.md

+
+## Proposal
+
+The spec for `a` tags says:


Suggested change

The spec for `a` tags says:

The spec for `a` tags [says](https://spec.matrix.org/v1.8/client-server-api/#mroommessage-msgtypes):

turt2live · 2023-11-14T04:20:06Z

sorry this got stuck :(

@mscbot fcp merge

mscbot · 2023-11-14T04:20:09Z

Team member @mscbot has proposed to merge this. The next step is review by the rest of the tagged people:

Concerns:

~~add explicit text about transforming mxc link to https~~
How do you implement this on web clients with MSC3916

Once at least 75% of reviewers approve (and there are no outstanding concerns), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for information about what commands tagged team members can give me.

dbkr

Going to raise a concern for adding the bit about transforming the mxc link just so there's a clear action point, but just to say that in spite of this and the grammar nitpicks, this is a superbly written and simple, but thorough, MSC, and in general seems like a great change. Thank you!

dbkr · 2023-11-28T11:07:24Z

proposals/2398-allow-mxc-within-message-links.md

+# MSC2398: Proposal to allow mxc:// in the `a` tag within messages
+
+Currently message events (m.room.message) can contain formatted data
+in its formatted_body field, if the format of the message is


Suggested change

in its formatted_body field, if the format of the message is

in their formatted_body field, if the format of the message is

dbkr · 2023-11-28T11:08:43Z

proposals/2398-allow-mxc-within-message-links.md

+
+Currently message events (m.room.message) can contain formatted data
+in its formatted_body field, if the format of the message is
+org.matrix.custom.html. In this subset of HTML `img` tags are permitted


Suggested change

org.matrix.custom.html. In this subset of HTML `img` tags are permitted

org.matrix.custom.html. In this subset of HTML, `img` tags are permitted

dbkr · 2023-11-28T11:10:39Z

proposals/2398-allow-mxc-within-message-links.md

+in its formatted_body field, if the format of the message is
+org.matrix.custom.html. In this subset of HTML `img` tags are permitted
+if they refer to media repository contents with the mxc schema and
+clients are able to resolve these URIs to (ie.) HTTPs to retrieve the


Suggested change

clients are able to resolve these URIs to (ie.) HTTPs to retrieve the

clients are able to resolve these URIs to (eg.) HTTP(s) URLs to retrieve the

Sorry if these feel nitpicky, I'm just having to re-read a couple of times to work out what the MSC is saying so attempting to clarify.

dbkr · 2023-11-28T11:13:37Z

proposals/2398-allow-mxc-within-message-links.md

+-`a`:
+  `name`, `target`, `href` (provided the value is not relative and has a scheme
+  matching one of: `https`, `http`, `ftp`, `mailto`, `magnet`, or `mxc`
+  for `Matrix Content (MXC) URI`)


Much as it's obvious, presumably the spec would also need to explicitly say that the client must transform the mxc link into an appropriate link the browser will understand (ie. what you talk about above with img tags).

This feels like something which is best served by a note rather than requirement. Web clients will probably need to convert it, but other clients might be able to handle a custom scheme for network requests.

I'd suggest that this not be a blocking concern for FCP, as the spec text can provide guidance on how to handle it.

I would really prefer the MSC say explicitly that clients should manage this scheme in one way or another - ultimately a custom handler will have to translate the mxc URL into an HTTP(S) request anyway.

Oh wait but isn't "Clients would be able to handle the mxc schema here in the exact same way they handle img tags" above exactly this description, actually? I personally think that is enough.

What do you think @dbkr?

I guess it just seemed odd to me that an explicit change to the spec text was given but then additional behaviour specified in the MSC. I suppose it's all there though so it doesn't really matter.

dbkr · 2023-11-28T11:19:31Z

@mscbot concern add explicit text about transforming mxc link to https

dbkr · 2024-01-09T15:25:35Z

@mscbot resolve add explicit text about transforming mxc link to https

ara4n · 2024-01-12T19:33:21Z

proposals/2398-allow-mxc-within-message-links.md

+retrieve resources linked this way. Some clients may already
+accidentally support this, though, due to consistent handling of URIs.
+
+## Security considerations ##


It's probably worth spelling out explicitly here that the security risks of linking directly to content in the repository is not just HTML, but any format which can execute code and potentially XSS needs to be mitigated by a valid CSP - e.g. JS, PDF, SWF, SVG. This isn't anything new, but it's still a security consideration. I can't think of a world where a HTTPS url derived from an MXC URL wouldn't be subject to CSP when a plain HTTPS url to the same content would, but still worth spelling it out at least in the MSC, if not the final spec.

mscbot · 2024-01-12T19:33:37Z

🔔 This is now entering its final comment period, as per the review above. 🔔

erikjohnston · 2024-01-12T19:44:55Z

proposals/2398-allow-mxc-within-message-links.md

@@ -0,0 +1,55 @@
+# MSC2398: Proposal to allow mxc:// in the `a` tag within messages


How does this proposal interact with MSC3916, which basically adds authentication to the media API? I assume if we can make it work for img tags we can make it work for a, but I think that needs to be verified.

The media ID would be attributed to a single event somewhere, and clients would be translating mxc:// to https:// for the user. This then behaves no differently than if a user were to get linked to the download endpoint directly - authentication would be required, and if that user can't see the original event then the link is broken for that user.

This then behaves no differently than if a user were to get linked to the download endpoint directly - authentication would be required, and if that user can't see the original event then the link is broken for that user.

But getting linked directly to the download endpoint won't work, as there is no way for a user to authenticate AIUI? I thought for media web clients would potentially need to download the media in service workers or whatever to make the authentication work, in a similar manner as today to make encrypted attachments work. Is that method viable here?

A service worker would be required, which should be workable. The client can essentially iframe it and append the appropriate headers. This then ties into Matthew's comment about security considerations: #2398 (comment)

I'm tempted to suggest a blocking concern on this as it feels valuable to test/implement, particularly while MSC3916 and friends are being experimented with. It's akin to custom emoji/stickers where I think we want to avoid making the problem worse short-term if we can avoid it.

erikjohnston · 2024-01-15T12:44:50Z

@mscbot concern How do you implement this on web clients with MSC3916

Sorry, but sounds like we need to double check the above before we can land this. Hopefully someone can just confirm its possible. c.f. #2398 (comment)

mscbot · 2024-01-17T19:33:48Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

turt2live · 2024-01-18T15:40:27Z

(FCP is not complete because there are outstanding concerns - see matrix-org/mscbot-python#3 )

See matrix-org/matrix-spec-proposals#2398

turt2live · 2024-03-26T02:45:45Z

The outstanding concern on this does not appear easily resolved - I'm cancelling FCP or returning this MSC back to its pre-FCP state (whichever the bot lets me do) pending further work in the area of media linking. It feels like changes may be required that would warrant a new FCP call anyways.

@mscbot fcp cancel

mscbot · 2024-03-26T02:45:47Z

This proposal is not in FCP nor has had FCP proposed.

KitsuneRal approved these changes Jan 2, 2020

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specification/modules/instant_messaging.rst Outdated Show resolved Hide resolved

eras force-pushed the MSC2398-mxc-for-a-tags branch from f320313 to 3ce4ef8 Compare January 3, 2020 06:12

MSC2398: proposal to allow mxc:// in the "a" tag within messages

85eb356

Signed-off-by: Erkki Seppälä <flux@modeemi.fi>

eras force-pushed the MSC2398-mxc-for-a-tags branch from 3ce4ef8 to 85eb356 Compare January 3, 2020 06:20

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Sorunome mentioned this pull request Jan 27, 2020

MSC2427: Proposal for JSON-based message formatting #2427

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turt2live added the kind:maintenance MSC which clarifies/updates existing spec label Apr 20, 2020

deepbluev7 mentioned this pull request Nov 27, 2020

MSC2881: Message Attachments #2881

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events: Add methods to sanitize messages ruma/ruma#1161

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Add mxc to allowed schemes in hyperlinks

915a2af

See matrix-org/matrix-spec-proposals#2398

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MSC2398: proposal to allow mxc:// in the "a" tag within messages #2398

MSC2398: proposal to allow mxc:// in the "a" tag within messages #2398

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	The spec for `a` tags says:
	The spec for `a` tags [says](https://spec.matrix.org/v1.8/client-server-api/#mroommessage-msgtypes):

	in its formatted_body field, if the format of the message is
	in their formatted_body field, if the format of the message is

	org.matrix.custom.html. In this subset of HTML `img` tags are permitted
	org.matrix.custom.html. In this subset of HTML, `img` tags are permitted

	clients are able to resolve these URIs to (ie.) HTTPs to retrieve the
	clients are able to resolve these URIs to (eg.) HTTP(s) URLs to retrieve the

		@@ -0,0 +1,55 @@
		# MSC2398: Proposal to allow mxc:// in the `a` tag within messages

MSC2398: proposal to allow mxc:// in the "a" tag within messages #2398

Are you sure you want to change the base?

MSC2398: proposal to allow mxc:// in the "a" tag within messages #2398

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