Cross Site Request Forgery in Buffalo with Api Keys

Cross Site Request Forgery (CSRF) in Buffalo combined with the use of API keys can expose state-changing endpoints when API keys are treated as equivalent to session cookies and are automatically included by browsers in requests initiated by third-party origins. Buffalo leverages HTTP cookies for authentication and relies on same-site cookie attributes and CSRF tokens to prevent unauthorized commands. When API keys are stored in cookies (e.g., a cookie named X-API-Key) and lack SameSite=Strict or Lax, or are accepted via custom headers but the origin check is not enforced consistently, a malicious site can craft forms or scripts that trigger authenticated actions with the victim’s API key included automatically.

Consider a Buffalo API that accepts an API key in a header (X-API-Key) but does not verify the request origin. If the browser automatically sends cookies (including the API key cookie) with a POST initiated from an external site, the endpoint may execute privileged operations such as changing email or password because it trusts the key. This mirrors classic CSRF against cookie-based sessions, but with keys that may have broader or longer-lived permissions. Additionally, if API keys are embedded in JavaScript (e.g., in a SPA build or via a global config), a reflected XSS flaw can trivially harvest them, turning CSRF into account takeover. Even if CORS is permissive, browsers enforce same-origin policies for credentials unless credentials: include is explicitly used; however, misconfigured CORS with wildcard origins and exposed headers can weaken defenses.

For example, a handler that only checks the presence of an API key in a header and does not validate the Origin or Referer header may be bypassed via a crafted form on attacker.com. The request will carry the cookie with the API key, and Buffalo will authorize the action based solely on the key. This is especially risky for unsafe methods (POST, PUT, DELETE) that change state without requiring a CSRF token or a same-site cookie policy. The risk is compounded when API keys are long-lived and lack scope restrictions, allowing a forged request to perform actions far beyond what the user intended.

Proper CSRF mitigation in Buffalo with API keys requires consistent origin validation, strict same-site cookie attributes, anti-CSRF tokens for state-changing routes, and avoiding automatic sending of API key cookies on cross-site requests. Security checks should verify both the key and the request context, including method, origin, and referer, to ensure that a key issued to a browser session is not usable in forged cross-site requests.

Remediation focuses on ensuring API keys are not automatically sent cross-site and that each request is validated for origin and method. Use SameSite=Strict or Lax on cookies, require explicit anti-CSRF tokens for state-changing operations, and validate Origin/Referer headers for key-bearing requests. The following examples assume you are using Buffalo’s middleware and routing features to enforce these controls.

1) Set SameSite cookies and secure flags:

// In your application configuration (e.g., actions/app.go)
app := buffalo.New(buffalo.Options{
    Env:         ENV,
    SessionStore: &cache.SessionStore{},
})
// Ensure cookies are SameSite and Secure in production
sessionMW := sessionmiddleware.New(sessionStore)
app.Use(sessionMW.SetSessionCookieOptions(&http.Cookie{
    HttpOnly:   true,
    Secure:     true,
    SameSite:   http.SameSiteStrictMode,
    Path:       "/",
    MaxAge:     3600,
}))
app.Use(sessionMW)

2) Validate Origin/Referer for API key requests and reject cross-origin calls:

func validateOrigin(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        allowedOrigin := "https://your-frontend.com"
        origin := r.Header.Get("Origin")
        referer := r.Header.Get("Referer")
        if origin != allowedOrigin && referer != allowedOrigin {
            http.Error(w, "invalid origin", http.StatusForbidden)
            return
        }
        next.ServeHTTP(w, r)
    })
}

// Apply to sensitive routes
app.Post("/account/change-email", validateOrigin, func(c buffalo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    if !isValidKey(apiKey) {
        return c.Error(401, errors.New("unauthorized"))
    }
    // proceed with change logic
    return nil
})

3) Use anti-CSRF tokens for state-changing actions and require them alongside API keys:

// Generate and validate CSRF tokens per session
func generateCSRFToken(session *session.Session) (string, error) {
    token := uuid.NewString()
    session.Set("csrf_token", token)
    return token, session.Save()
}

func validateCSRFToken(session *session.Session, token string) bool {
    expected, ok := session.Get("csrf_token").(string)
    if !ok || expected != token {
        return false
    }
    return true
}

// Example form handler
app.Post("/transfer", func(c buffalo.Context) error {
    session, _ := sessionStore.Get(c.Request(), sessionName)
    if !validateCSRFToken(session, c.Param("csrf_token")) {
        return c.Error(403, errors.New("invalid csrf token"))
    }
    apiKey := c.Request().Header.Get("X-API-Key")
    if !isValidKey(apiKey) {
        return c.Error(401, errors.New("unauthorized"))
    }
    // perform transfer
    return nil
})

4) Avoid storing API keys in cookies when possible; prefer Authorization: Bearer and strict CORS:

// Configure CORS to restrict origins and not allow credentials universally
corsMW := corsmiddleware.New(cors.Options{
    AllowedOrigins:   []string{"https://your-frontend.com"},
    AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
    AllowedHeaders:   []string{"Authorization", "X-API-Key", "Content-Type"},
    ExposedHeaders:   []string{"Content-Type"},
    AllowCredentials: false, // do not allow credentials from wildcard origins
    MaxAge:           3600,
})
app.Use(corsMW)

These steps ensure that API keys used in Buffalo applications are not automatically exposed to cross-site contexts and that each sensitive request is verified for origin, method, and token validity, reducing the impact of CSRF and key leakage.