How Google achieves seamless SSO across multiple domains like Gmail and Youtube?

Software Engineer

2024-10-13 18:14:25 +0700 · 2 mins read

Have you ever wondered how logging into Gmail can automatically sign you into YouTube, Google Drive, and all other Google services? This behind-the-scenes technique is called Single Sign-On (SSO). In this blog, I will show you some techniques that Google used.

The Challenge of Cross-Domain Authentication

Web browsers enforce strict security policies that prevent one domain from accessing cookies set by another domain. This policy, known as the Same-Origin Policy, is essential for protecting user data but poses a challenge for implementing SSO across multiple domains (e.g., google.com and youtube.com).

To overcome this, Google employs a combination of cookies, tokens, and background server requests to synchronize authentication sessions across its services.

Step-by-Step Login Flow

1. Initial Login Request
When a user navigates to a Google service (e.g., Gmail at https://mail.google.com) and they are not logged in, Gmail redirects the user to the central authentication server at https://accounts.google.com.

At https://accounts.google.com, the user is prompted to enter their credentials. Upon successful authentication, the server sets a session cookie for the .google.com domain.

// language: bash
HTTP/1.1 200 OK
Set-Cookie: SID=SESSION_COOKIE; Domain=.google.com; Secure; HttpOnly

2. Generating Cross-Domain Session Identifiers
Google's authentication server also generates a session identifier or token. This token is used to establish sessions on other Google-owned domains.

The authentication server redirects to other Google-owned domains to establish sessions on those domains using the session identifier.

Redirect to YouTube:

// language: bash
GET /accounts/SetSID?sidt=UNIQUE_SESSION_ID&continue=https://mail.google.com/mail&<other_params>
Host: accounts.youtube.com

Response from YouTube:

// language: bash
Set-Cookie: SID=YOUTUBE_SESSION_COOKIE; Domain=.youtube.com; Secure; HttpOnly

YouTube redirects to Localized Domains (e.g., Vietnam):

// language: bash
GET /accounts/SetSID?sidt=UNIQUE_SESSION_ID&continue=https://mail.google.com/mail&<other_params>
Host: accounts.google.com.vn

Response from Localized Domains:

// language: bash
Set-Cookie: SID=LOCALIZED_SESSION_COOKIE; Domain=.google.com.vn; Secure; HttpOnly

When accounts.youtube.com and other localized domains receive the request with the session identifier, they verify the identifier with the central authentication server. If valid, they set their own session cookies for their respective domains.

3. Final Redirection
Once all necessary cookies are set, the user is redirected back to the original service which extracted from the continue param (&continue=https://mail.google.com/mail)

When the user navigates to another Google service (e.g., YouTube at https://www.youtube.com), the .youtube.com cookies are automatically sent with the request, allowing the user to be logged in without re-entering credentials.

Step-by-Step Logout Flow

The browser sends background requests to other domains to clear their cookies.

Youtube:

// language: bash
GET /ClearSID?sidt=UNIQUE_SESSION_ID HTTP/1.1
Host: accounts.youtube.com

Localized Domains:

// language: bash
GET /ClearSID?sidt=UNIQUE_SESSION_ID HTTP/1.1
Host: accounts.google.com.vn

Each domain clears its cookies and invalidates the session

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