Implement CSRF token verification for OAuth example

examples/oauth/src/main.rs

@@ -8,7 +8,7 @@
 //! CLIENT_ID=REPLACE_ME CLIENT_SECRET=REPLACE_ME cargo run -p example-oauth
 //! ```
 
-use anyhow::{Context, Result};
+use anyhow::{anyhow, Context, Result};
 use async_session::{MemoryStore, Session, SessionStore};
 use axum::{
     async_trait,
@@ -142,19 +142,37 @@ async fn index(user: Option<User>) -> impl IntoResponse {
     }
 }
 
-async fn discord_auth(State(client): State<BasicClient>) -> impl IntoResponse {
-    // TODO: this example currently doesn't validate the CSRF token during login attempts. That
-    // makes it vulnerable to cross-site request forgery. If you copy code from this example make
-    // sure to add a check for the CSRF token.
-    //
-    // Issue for adding check to this example https://github.com/tokio-rs/axum/issues/2511
-    let (auth_url, _csrf_token) = client
+async fn discord_auth(
+    State(client): State<BasicClient>,
+    State(store): State<MemoryStore>,
+) -> Result<impl IntoResponse, AppError> {
+    let (auth_url, csrf_token) = client
         .authorize_url(CsrfToken::new_random)
         .add_scope(Scope::new("identify".to_string()))
         .url();
 
-    // Redirect to Discord's oauth service
-    Redirect::to(auth_url.as_ref())
+    // Create session to store csrf_token
+    let mut session = Session::new();
+    session
+        .insert("csrf_token", &csrf_token)
+        .context("failed in inserting CSRF token into session")?;
+
+    // Store the session in MemoryStore and retrieve the session cookie
+    let cookie = store
+        .store_session(session)
+        .await
+        .context("failed to store CSRF token session")?
+        .context("unexpected error retrieving CSRF cookie value")?;
+
+    // Attach the session cookie to the response header
+    let cookie = format!("{COOKIE_NAME}={cookie}; SameSite=Lax; Path=/");
+    let mut headers = HeaderMap::new();
+    headers.insert(
+        SET_COOKIE,
+        cookie.parse().context("failed to parse cookie")?,
+    );
+
+    Ok((headers, Redirect::to(auth_url.as_ref())))
 }
 
 // Valid user session required. If there is none, redirect to the auth page
@@ -195,11 +213,55 @@ struct AuthRequest {
     state: String,
 }
 
+async fn csrf_token_validation_workflow(
+    auth_request: &AuthRequest,
+    cookies: &headers::Cookie,
+    store: &MemoryStore,
+) -> Result<(), AppError> {
+    // Extract the cookie from the request
+    let cookie = cookies
+        .get(COOKIE_NAME)
+        .context("unexpected error getting cookie name")?
+        .to_string();
+
+    // Load the session
+    let session = match store
+        .load_session(cookie)
+        .await
+        .context("failed to load session")?
+    {
+        Some(session) => session,
+        None => return Err(anyhow!("Session not found").into()),
+    };
+
+    // Extract the CSRF token from the session
+    let stored_csrf_token = session
+        .get::<CsrfToken>("csrf_token")
+        .context("CSRF token not found in session")?
+        .to_owned();
+
+    // Cleanup the CSRF token session
+    store
+        .destroy_session(session)
+        .await
+        .context("Failed to destroy old session")?;
+
+    // Validate CSRF token is the same as the one in the auth request
+    if *stored_csrf_token.secret() != auth_request.state {
+        return Err(anyhow!("CSRF token mismatch").into());
+    }
+
+    Ok(())
+}
+
 async fn login_authorized(
     Query(query): Query<AuthRequest>,
     State(store): State<MemoryStore>,
     State(oauth_client): State<BasicClient>,
+    TypedHeader(cookies): TypedHeader<headers::Cookie>,
 ) -> Result<impl IntoResponse, AppError> {
+    csrf_token_validation_workflow(&query, &cookies, &store).await?;
+
     // Get an auth token
     let token = oauth_client
         .exchange_code(AuthorizationCode::new(query.code.clone()))