Experimental implementation of unstable MSC4388 for Sign in with QR by hughns · Pull Request #19127 · element-hq/synapse

hughns · 2025-11-03T11:31:33Z

For #19446

This is an implementation of the "insecure rendezvous session" part of MSC4388. This is then used by the 2025 version of MSC4108 which provides a Sign in with QR functionality to allow an existing Matrix client to help authenticate a new Matrix client.

It provides some new Client-Server API endpoints for creating (POST), updating (PUT), fetching (GET) and ending (DELETE) "rendezvous sessions" which are described in depth in the MSCs.

The feature is configured using:

experimental_features:
  msc4388_mode: <mode>

Where mode is one of:

off (default): feature is disabled
authenticated: enabled and requires an authenticated user to create a rendezvous
public: enabled and no authentication is required

The is some more context on these modes in the POST /rendezvous section of the MSC.

n.b. The implementation is independent of the existing experimental MSC4108 support in Synapse which is for the 2024 version.

The experimental feature tracking issue is #19433

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This is an implementation of the rendezvous transport session part of MSC4388. This is part of the 2025 version of MSC4108 The implementation is independent of the existing experimental MSC4108 support in Synapse which is for the 2024 version of MSC4108.

reivilibre

some initial thoughts, seems more or less on track.

rust/src/msc4388_rendezvous/mod.rs

rust/src/msc4388_rendezvous/session.rs

synapse/config/experimental.py

Co-authored-by: Olivier 'reivilibre' <oliverw@element.io>

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reivilibre

As far as I can see this is now correct and sane

rust/src/msc4388_rendezvous/mod.rs

tests/rest/client/test_msc4388_rendezvous.py

This PR contains the following updates: | Package | Update | Change | |---|---|---| | [element-hq/synapse](https://github.com/element-hq/synapse) | minor | `v1.148.0` → `v1.149.0` | --- ### Release Notes <details> <summary>element-hq/synapse (element-hq/synapse)</summary> ### [`v1.149.0`](https://github.com/element-hq/synapse/releases/tag/v1.149.0) [Compare Source](element-hq/synapse@v1.148.0...v1.149.0) ### Synapse 1.149.0 (2026-03-10) No significant changes since 1.149.0rc1. ### Synapse 1.149.0rc1 (2026-03-03) #### Features - Add experimental support for [MSC4388: Secure out-of-band channel for sign in with QR](matrix-org/matrix-spec-proposals#4388). ([#19127](element-hq/synapse#19127)) - Add stable support for [MSC4380](matrix-org/matrix-spec-proposals#4380) invite blocking. ([#19431](element-hq/synapse#19431)) #### Bugfixes - Fix the 'Login as a user' Admin API not checking if the user exists before issuing an access token. ([#18518](element-hq/synapse#18518)) - Fix `/sync` missing membership event in `state_after` (experimental [MSC4222](matrix-org/matrix-spec-proposals#4222) implementation) in some scenarios. ([#19460](element-hq/synapse#19460)) #### Internal Changes - Add log to explain when and why we freeze objects in the garbage collector. ([#19440](element-hq/synapse#19440)) - Better instrument `JoinRoomAliasServlet` with tracing. ([#19461](element-hq/synapse#19461)) - Fix Complement CI not running against the code from our PRs. ([#19475](element-hq/synapse#19475)) - Log `docker system info` in CI so we have a plain record of how GitHub runners evolve over time. ([#19480](element-hq/synapse#19480)) - Rename the `test_disconnect` test helper so that pytest doesn't see it as a test. ([#19486](element-hq/synapse#19486)) - Add a log line when we delete devices. Contributed by [@bradtgmurray](https://github.com/bradtgmurray) @ Beeper. ([#19496](element-hq/synapse#19496)) - Pre-allocate the buffer based on the expected `Content-Length` with the Rust HTTP client. ([#19498](element-hq/synapse#19498)) - Cancel long-running sync requests if the client has gone away. ([#19499](element-hq/synapse#19499)) - Try and reduce reactor tick times when under heavy load. ([#19507](element-hq/synapse#19507)) - Simplify Rust HTTP client response streaming and limiting. ([#19510](element-hq/synapse#19510)) - Replace deprecated collection import locations with current locations. ([#19515](element-hq/synapse#19515)) - Bump most locked Python dependencies to their latest versions. ([#19519](element-hq/synapse#19519)) </details> --- ### Configuration 📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined). 🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied. ♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox. 🔕 **Ignore**: Close this PR and you won't be reminded about this update again. --- - [ ] If you want to rebase/retry this PR, check this box --- This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).  Reviewed-on: https://gitea.alexlebens.dev/alexlebens/infrastructure/pulls/4580 Co-authored-by: Renovate Bot <renovate-bot@alexlebens.net> Co-committed-by: Renovate Bot <renovate-bot@alexlebens.net>

Tested on NetBSD 10 amd64 in a mostly 2025Q2 environment. # Synapse 1.149.1 (2026-03-11) # Synapse 1.149.0 (2026-03-10) ## Features - Add experimental support for [MSC4388: Secure out-of-band channel for sign in with QR](matrix-org/matrix-spec-proposals#4388). ([\#19127](element-hq/synapse#19127)) - Add stable support for [MSC4380](matrix-org/matrix-spec-proposals#4380) invite blocking. ([\#19431](element-hq/synapse#19431))

hughns mentioned this pull request Nov 3, 2025

MSC4108: Mechanism to allow OAuth 2.0 API sign in and E2EE set up via QR code matrix-org/matrix-spec-proposals#4108

Open

21 tasks

hughns force-pushed the hughns/msc4108-2025 branch 4 times, most recently from 9b3bd15 to 7a06a34 Compare January 21, 2026 16:51

hughns force-pushed the hughns/msc4108-2025 branch from 7a06a34 to a1305e2 Compare January 21, 2026 17:12

hughns changed the title ~~Proof of MSC implementation of MSC4108 version 2025~~ Implementation of unstable MSC4388 Jan 23, 2026

hughns mentioned this pull request Jan 23, 2026

MSC4388: Secure out-of-band channel for sign in with QR matrix-org/matrix-spec-proposals#4388

Draft

12 tasks

hughns requested review from reivilibre and sandhose February 4, 2026 11:23

hughns added 2 commits February 4, 2026 11:44

Changelog

5c44b2d

Links to experiemental feature tracking issues

0cb3764

hughns marked this pull request as ready for review February 4, 2026 13:06

hughns requested a review from a team as a code owner February 4, 2026 13:06

hughns changed the title ~~Implementation of unstable MSC4388~~ Experimental implementation of unstable MSC4388 Feb 4, 2026

hughns changed the title ~~Experimental implementation of unstable MSC4388~~ Experimental implementation of unstable MSC4388 for Sign in with QR Feb 4, 2026

hughns mentioned this pull request Feb 4, 2026

MSC4388: Secure out-of-band channel for sign in with QR - 2025 version of rendezvous #19433

Open

reivilibre reviewed Feb 4, 2026

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reivilibre self-requested a review February 4, 2026 18:27

hughns mentioned this pull request Feb 9, 2026

Tracking issue for supporting Sign in with QR #19446

Open

6 tasks

hughns and others added 7 commits February 9, 2026 15:08

Refactor to use Python response encdoing

aecd128

Use structs for request types

a4c6a46

Define soft and hard limits

ce9d561

Suggestion from code review

e7076d6

Co-authored-by: Olivier 'reivilibre' <oliverw@element.io>

Include last modified time in the sequence token

029ff36

Fix inverted condition

3942572

Add Sec-Fetch-* checking in GET requests

a66af7b

hughns force-pushed the hughns/msc4108-2025 branch from 5e98d02 to a66af7b Compare February 11, 2026 10:41

Add extra comment about return type being JSON and so unlikely to be …

651b766

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Merge branch 'develop' into hughns/msc4108-2025

3aaecd0

reivilibre approved these changes Feb 16, 2026

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rust/src/msc4388_rendezvous/mod.rs Outdated Show resolved Hide resolved

MadLittleMods added the A-Login label Feb 16, 2026

sandhose approved these changes Feb 19, 2026

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tests/rest/client/test_msc4388_rendezvous.py Outdated Show resolved Hide resolved

hughns added 3 commits February 25, 2026 16:36

Remove authlib check

23907b1

Remove unnecessary sequence_token check

9eb6212

Merge branch 'develop' into hughns/msc4108-2025

2274e50

hughns enabled auto-merge (squash) February 25, 2026 16:47

hughns merged commit f78d011 into develop Feb 25, 2026
79 of 81 checks passed

hughns deleted the hughns/msc4108-2025 branch February 25, 2026 17:41

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Experimental implementation of unstable MSC4388 for Sign in with QR#19127

Experimental implementation of unstable MSC4388 for Sign in with QR#19127
hughns merged 15 commits intodevelopfrom
hughns/msc4108-2025

hughns commented Nov 3, 2025 •

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hughns commented Nov 3, 2025 •

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