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0103-application-access-web-ui-auth-flow.md

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Ryan Clark (ryan.clark@goteleport.com)
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RFD 103 - Application Access Web UI Auth Flow

What

This is an overview of the flow used for setting the relevant cookies when a user logs in to an application through Teleport Web UI.

Why

The method used to set cookies on a different domain requires a few changes to Teleport's default security mechanisms. It's important these changes are documented and understood for future reference.

Details

When a user wants to login to an application, a new application session is created in the auth service. This application session has two values (a name and a bearer token) that need to be provided as cookies when the user navigates to the application's URL.

The flow to be able to set the needed cookies from the application session across different domains is as follows:

sequenceDiagram
    participant User
    participant Proxy UI
    participant Application
    participant Auth Service
    User->>Proxy UI: Launch Application
    Proxy UI->>Auth Service: Create App Session /v1/webapi/sessions/app
    Auth Service->>Proxy UI: Returns two cookie values (app session & subject)
    Proxy UI->>Application: CORS preflight request to appurl.com/x-teleport-auth
    Application->>Proxy UI: Matches Origin against public proxy addresses & allows CORS if a match
    Proxy UI->>Application: POST appurl.com/x-teleport-auth with the cookie values in the header
    Application->>Proxy UI: Sets the cookies on appurl.com if they are valid
    Proxy UI->>User: User is logged in, redirect to appurl.com
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CORS

As we need to enable cross-origin requests from the proxy URL to the application URL, we add middleware before /x-teleport-auth to check the Origin header (this cannot be changed via a fetch request).

We check the origin against the public addresses of the proxy, and if one is a match, we allow a cross-origin request.

The rules around the cross-origin request are strict and the absolute minimum needed:

  • Must be a POST request
  • Only allowed for the /x-teleport-auth endpoint
  • Only allows the headers X-Cookie-Value and X-Subject-Cookie-Value to be set
  • Only allowed from one of the public proxy addresses

Content Security Policy

Teleport's default content security policy disallows cross-origin requests by setting connect-src to 'self' wss:.

To allow for the request to the application's domain, we modify the content security policy for /web/launch URLs only. This changes connect-src to 'self' https://applicationfqdn:*, which allows requests to both proxy as normal, and the application's FQDN.

Cookie Validity Checking

In the handler for /x-teleport-auth, we take the cookie values from the headers and use X-Cookie-Value to look-up the application session created by the auth service.

If this application session exists, we then check the value of the X-Subject-Cookie-Value header against the bearer token stored in the application session.

If these both match, we set the relevant cookies to log the user in once they're redirected to the application.