OAuth 2.0

Motivation and explanation of OAuth 2.0

Third-Party APIs

Web APIs

• Many apps have APIs that let programs access user data with HTTP requests
  • GET api.github.com/user - The logged-in user's profile
  • GET api.github.com/user/repos - Their repositories, including private ones
  • POST api.github.com/repos/<owner>/<repo>/issues - Create an issue
• Sometimes, you want your app to use this API on behalf of your users
  • A scrum board app that creates GitHub issues when your users' create cards
  • A "Login with GitHub" button (Which you'll implement on HW2)
• How can your users let your app access their GitHub accounts securely?

Bad Ideas: Ask for Their Password

• Have your users give you their GitHub username and password
• Never ask for a password to someone else's site
  • You'd have to store it in plain text to reuse it
  • Your app gets full access to their account. Not just what you need
  • The user can't revoke your access without changing their password
  • It trains users to type their passwords into random sites

Bad Idea: Ask for Their API Key

• Have the user create an API key on GitHub and paste it into your app
• Better than a password, but:
  • The user has to handle a secret. Never trust your users, not even with their own security
  • Requests from your app look exactly like requests from the user
    • No accountability. If your app misbehaves, GitHub can't tell it was you
    • If the key leaks, there's no way to tell who is using it
  • Your usage counts against the user's rate limits (Or bill!)

OAuth 2.0

• OAuth 2.0 (Open Authorization) - The standard way for a user to grant an app limited access to their account on another service
• The user approves your app on GitHub's site
  • Your app never sees their password
• GitHub issues an access token directly to your app
  • The user never handles the access token
  • The token is tied to your app. GitHub knows it's you (Accountability)
  • The token is limited to the permissions (scopes) the user approved
  • The user can revoke your app's access at any time
• The user still has to trust your app with the data they approve

Client Registration

• Before any user can log in, you register your app with GitHub
  • You get/set at least 3 important values during registration
• Client ID - Identifies your app. Public
• Client Secret - Effectively a password for your app
  • Only ever stored on your server (In your .env file). Never in your front end or your git repo
• Redirect URI - Where GitHub sends users after they approve your app
  • http://localhost:8080/authcallback on the HW
  • GitHub only redirects to the URI you registered

OAuth Flows

• OAuth has several flows for different kinds of apps
• We'll use the Authorization Code Flow
  • For apps with a server that can keep a secret
  • The most common flow

The Authorization Code Flow

The Authorization Code Flow

Steps 1-2: Authorization Request

Authorization Request

• The user clicks "Login with GitHub," which sends a GET request to your server
• Your server responds with a 302 redirect to GitHub's authorization endpoint:
  • The location of the redirect is a github url
  • This redirect is a cross-origin request!

Authorization Request - Parameters

• Need to send several parameters in the query string of this redirect
• client_id - Which app is asking
• redirect_uri - Where to send the user back to
• scope - A space-separated list of the permissions you're asking for
• state - XSRF protection (More details later)
• response_type=code - Required by the RFC to request a code (GitHub doesn't require it)
• All values must be percent-encoded (Although none of these values should contain characters that need to be percent-encoded)

At GitHub

• The browser follows the redirect to GitHub
• GitHub authenticates the user
  • With their GitHub auth token cookie, or by asking them to log in
  • This is GitHub's job. Your app never sees any of it
• GitHub shows the user your app's name and the scopes you asked for
  • The user chooses to approve or deny
  • If they already approved your app with these scopes, they skip straight to the redirect

Why Redirects?

• In steps 1-4, your app and GitHub communicate through the user's browser
• These requests need to have lax cookies attached
  • Must be GET requests that navigate to the other site
• Need to send information in these requests
  • Use a cross-site redirect with a query string

Steps 3-4: Authorization Grant

Authorization Grant

• If the user approves, GitHub responds with a 302 redirect to your redirect URI
• The redirect url will contain a code in a query string
  • It will also contain the state value (Discussed later)
• This code is a one-time value that your server can trade for an access token
• High entropy (The RFC requires at least 128 bits)
• Short-lived (GitHub codes expire after 10 minutes)
• Assume the code has been compromised
  • It traveled through the browser in a URL: history, logs, extensions
  • The user handled it (Should you trust them?)
• So the code alone must not be enough to get an access token

Steps 5-6: Token Exchange

Token Exchange

• Your server sends a request directly to GitHub (You can use libraries to send request to GitHub on your HW)
  • Since this is direct communication, we can use any HTTP method and format (Enter JSON)
• You will "trade in" the code for an access token
• GitHub must verify that it's really your app redeeming the code
• Your app authenticates with its client secret
  • The RFC recommends HTTP Basic authentication:
    • Authorization: Basic base64(client_id:client_secret)
  • It also allows them in the body of the POST request
  • Never in the URL
• APIs vary in these details. Read their documentation

Token Response

• The access token went straight from GitHub to your server
  • It never touched the user's browser
  • The user does not know the value of this token
• No need to trust the user!

Steps 7-8: API Access

API Access

• Send the access token in the Authorization header with type Bearer
  • "Bearer" - Whoever holds (bears) the token has access
• Why not a cookie?
  • Your server is sending requests on behalf of many different users
  • Cookies are how browsers store tokens. A header is simpler for a server

Login With GitHub

• You can now use the GitHub API to access the user's account information
  • Find a suitable unique id and use it to identify this user
• First login: Create an account linked to their GitHub ID (No password)
• Later logins: Find their existing account by GitHub ID
• Then log them in exactly like a password login
  • Issue your own auth token in a cookie
  • Never use GitHub's access token as your auth token
• If you're only using OAuth for login, no need to even store the access token
  • Requires a new access token for each login though (Tradeoff)

State

Recall: Login XSRF

• Login XSRF: The attacker logs the victim into the attacker's account
• Your /authcallback endpoint is a login endpoint!
• The usual XSRF defenses don't help here:
  • /authcallback is designed to receive cross-site requests (A redirect from github.com)
  • It's a GET navigation, so SameSite=Lax cookies are sent
  • GitHub builds the request, not your front end. There's no chance to add a XSRF token

The Attack

• The attacker starts "Login with GitHub" on your app using their own GitHub account
• They stop before following the final redirect and keep the unused code
• The attacker gets the victim to open a link to your callback with the attacker's code
• To your app, this looks exactly like the end of a normal login
• Your app redeems the code with its own client secret. Everything checks out
• The victim is now logged in to the attacker's account!
  • Everything they save goes to an account the attacker controls

The State Parameter

• Before redirecting to GitHub (Step 1):
  • Generate a random, unguessable state value
  • Bind it to this browser
  • Add it to the authorization request
• GitHub sends the same state back with the code (Step 4)
• At /authcallback:
  • Check that the state matches the one bound to this browser
  • If it's missing or doesn't match: 400 Bad Request and stop. Never redeem the code
  • Delete the state after checking. Each state is single use
• The state is a XSRF token for your OAuth callback

Binding State to the Browser

• The user usually isn't logged in yet, so there's no session to attach the state to
• One approach: Store it in a cookie before the redirect
  • Set-Cookie: oauth_state=Xk3fR9...; HttpOnly; Max-Age=600; SameSite=Lax
  • At the callback, compare the query string state to the cookie
• Attacker cannot set a cookie for our site
• Attacker can't read this cookie since our site never redirects to the attack site
• SOP will block responses so they can't fake a request and read the set-cookie header
• You have some freedom in how you implement this on the HW, as long as it stops the attack

Refresh Tokens

Refresh Tokens

• Many APIs make access tokens expire quickly (e.g. 1 hour)
• Along with the access token, they issue a long-lived refresh token
• When the access token expires, use the refresh token to obtain a new one:
  • You get a new access token (And sometimes a new refresh token)
• The user doesn't have to do anything
• Why not just have the access tokens be long-lived?
  • More secure when the auth server and resource server are not the same server
    • More on this much later when we discus JWTs

Further Reading

• RFC 6749 - OAuth 2.0
• RFC 6750 - OAuth 2.0 Tokens