MAS/OAuth login on iOS stops after MAS login page loads; no token or device created

[baumar73]

Steps to reproduce

1. Use Element X iOS 26.05.3 with a self-hosted Matrix server using MAS/OAuth.
2. Homeserver: <redacted self-hosted homeserver>
3. Server stack:
   • Synapse 1.153.0
   • Matrix Authentication Service 1.17.0
   • Element Web behind the same domain
4. In Element X, choose the homeserver and continue with the MAS/OAuth login flow.
5. The web authentication sheet loads the MAS login page, but the login does not complete and Element X shows the same generic error as before.

Outcome

What did you expect?

Element X should complete the MAS/OAuth login flow, exchange the authorization code, and create a new Matrix device for the iPhone.

What happened instead?

From the server side, the Element X login starts correctly but never progresses past the MAS login page. No iPhone access token or Synapse device is created.

Element Web on the same Mac/browser and same account works successfully against the same MAS/Synapse server, including the consent step and /oauth2/token exchange.

Server-side evidence

Relevant MAS/Synapse events from the failed Element X attempt:

2026-06-01T00:12:25Z GET /_matrix/client/v3/login 200
User-Agent: Element X/26.05.3 (iPhone 16 Pro Max; iOS 26.5; Scale/3.00)

2026-06-01T00:12:27Z POST /oauth2/registration 201
Client registration:
  redirect_uris: ["https://element.io/oauth/ios/io.element.elementx"]
  application_type: "native"
  client_name: "Element X"

2026-06-01T00:12:29Z GET /authorize 303
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 18_7 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.5 Mobile/15E148 Safari/604.1

2026-06-01T00:12:29Z GET /login 200
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 18_7 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/26.5 Mobile/15E148 Safari/604.1

After that there is no iPhone POST /login, no /consent, no /oauth2/token, and no new Synapse device for the account.

The MAS authorization grant row for this attempt remains unfulfilled and unexchanged:

client_name:      Element X
application_type: native
redirect_uri:     https://element.io/oauth/ios/io.element.elementx
fulfilled_at:     NULL
exchanged_at:     NULL

For comparison, a successful Element Web login immediately before/after the iPhone attempt creates the expected MAS session, token exchange and Synapse device.

Server checks already verified

• /.well-known/matrix/client advertises MAS via org.matrix.msc2965.authentication
• /.well-known/openid-configuration returns 200
• /_matrix/client/versions returns 200 and includes sliding sync support
• /_matrix/client/v3/login returns password + OAuth-aware flows
• MAS doctor reports the client well-known and homeserver checks as valid
• Password login and Element Web OAuth login work for the same account

Suspected area

This looks related to the iOS ASWebAuthenticationSession / MAS web-auth flow rather than the homeserver itself. Element X starts OAuth and MAS serves /login, but the app never reaches the point where credentials are posted or a callback/token exchange is completed.

Relevant code paths appear to be:

• AppSettings.oAuthRedirectURL
• OAuthAuthenticationPresenter.authenticate(using:)
• ASWebAuthenticationSession(url:callback:)

At minimum, it would help if Element X surfaced the exact ASWebAuthenticationSession error/cancellation reason instead of a generic error, because the server side only shows the flow stopping after GET /login.

Phone model

iPhone 16 Pro Max

Operating system version

iOS 26.5 according to Element X user agent

Application version

Element X 26.05.3

Homeserver

Will you send logs?

Server-side MAS/Synapse logs are included above. App-side logs can be provided if there are instructions for extracting them from Element X after this failure.

[pixlwave]

App-side logs can be provided if there are instructions for extracting them from Element X after this failure.

If you tap on the app version number at the bottom of the start screen 7 times it will open the Feedback screen for you to send us the logs. Please include https://github.com/element-hq/element-x-ios/issues/5669 in the report so that it is linked against this issue.