Cross Site Request Forgery in Buffalo with Bearer Tokens

Cross Site Request Forgery (CSRF) in a Buffalo application becomes more nuanced when Bearer Tokens are used for authentication. While traditional CSRF defenses often rely on same-site cookies and synchronizer tokens, Bearer Tokens are typically stored in headers and not automatically sent by browsers in cross-origin requests. However, if a Buffalo app embeds a Bearer Token in JavaScript (for example to call an API from frontend code), a malicious site can trigger authenticated requests via XMLHttpRequest or fetch to an endpoint that relies solely on the token in a header. Importantly, browsers enforce same-origin policies for cookies and do not automatically include credentials for cross-origin requests unless credentials are explicitly allowed. When developers misuse CORS or expose tokens to client-side code, CSRF-like behavior can occur via trusted origins being abused or via token leakage.

Another attack surface involves APIs that accept Bearer Tokens via query parameters or fragments for convenience or legacy reasons. If a Buffalo endpoint accepts a token in a query string, a malicious site can embed an image or script with a URL containing the token, leading to unauthorized actions. This mirrors classic CSRF where the browser automatically includes cookies; here, the browser includes the token if it is part of the URL. Additionally, if CORS is misconfigured to allow credentials or overly broad origins, a compromised frontend script can make authenticated requests on behalf of the user. The risk is compounded when the token has broad scopes, lacks short lifetimes, or is not bound to a strict referrer or origin check. The scanner’s checks for BOLA/IDOR and BFLA/Privilege Escalation will surface endpoints where token handling and authorization are weak, increasing the likelihood of abuse.

Consider an endpoint that performs a sensitive state change without verifying the request origin and relies only on a Bearer Token provided in the Authorization header. If an attacker can trick a victim’s browser into making a request to that endpoint (for example via an img or form submission that references a full URL with the token), the browser will not include the Authorization header unless the request is explicitly configured with credentials. However, if the Buffalo application uses a permissive CORS policy and exposes the token to JavaScript, an attacker’s script can call the endpoint directly from the victim’s browser, effectively bypassing CSRF protections that assume same-site cookie behavior. This is less about the browser automatically sending the token and more about flawed trust in the origin when tokens are handled inappropriately. The scanner’s CORS and input validation checks help identify overly permissive configurations that enable such abuse.

Real-world examples include endpoints that modify user settings or initiate financial actions based on a Bearer Token without origin verification. For instance, a request to /api/v1/transfer that accepts Authorization: Bearer and lacks a proper SameSite cookie policy or CSRF-safe design can be abused if the token is inadvertently exposed. The scanner’s inventory management and unsafe consumption checks can detect endpoints that accept tokens in non-standard locations or lack referrer and origin validation. Proper remediation involves ensuring tokens are never embedded in JavaScript, using strict CORS rules, and coupling Bearer Token authentication with anti-CSRF measures such as same-site cookies or custom headers that cannot be set cross-origin.

Ultimately, the combination of CSRF and Bearer Tokens in Buffalo highlights the importance of defense-in-depth. Even though browsers do not automatically include Authorization headers in cross-origin requests, application-level trust in origins, token placement, and CORS settings can create scenarios where unauthorized commands are executed. By scanning with middleBrick, teams can identify endpoints where token handling, CORS, and authorization intersect with CSRF risks, enabling targeted fixes that align with OWASP API Top 10 and common compliance frameworks.

To mitigate CSRF risks when using Bearer Tokens in Buffalo, focus on secure token handling, strict CORS policies, and server-side origin verification. Avoid exposing tokens to client-side JavaScript and never accept tokens in query parameters. Below are concrete code examples that demonstrate secure patterns.

1. Use Authorization header with strict CORS and no wildcard credentials:

// In your Buffalo app, set CORS middleware to be strict
app.Use(CorsMiddleware({
    AllowedOrigins:   []string{"https://app.yourdomain.com"},
    AllowedMethods:   []string{"GET", "POST", "PUT", "DELETE"},
    AllowedHeaders:   []string{"Authorization", "Content-Type"},
    ExposeHeaders:    []string{}, // do not expose sensitive headers
    AllowCredentials: true, // only true if you must support cookies; ensure origins are not *
}))

// Example handler requiring Bearer Token in Authorization header
func TransferHandler(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" {
        return c.Error(401, errors.New("unauthorized"))
    }
    const prefix = "Bearer "
    if !strings.HasPrefix(auth, prefix) {
        return c.Error(401, errors.New("invalid authorization header"))
    }
    token := strings.TrimPrefix(auth, prefix)
    if !isValidToken(token) { // implement token validation (introspection/jwks)
        return c.Error(403, errors.New("forbidden"))
    }
    // proceed with action, ensuring token scopes permit the operation
    return c.Render(200, r.JSON(Map{"status": "ok"}))
}

2. Avoid tokens in URLs and query parameters:

// BAD: Token in query string — easily leaked in logs and referrer headers
// GET /api/v1/action?token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

// GOOD: Token only in Authorization header
curl -X POST https://api.yourdomain.com/api/v1/action \
  -H "Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." \
  -H "Content-Type: application/json" \
  -d '{"amount":100}'

3. Enforce same-site cookies if using session cookies alongside tokens:

// Set session cookie with SameSite and Secure flags
session.Options = &securecookie.Options{
    Path:     "/",
    MaxAge:   3600,
    HttpOnly: true,
    Secure:   true,
    SameSite: http.SameSiteStrictMode,
}
session.Set(c.Session(), "user_id", userID)

4. Validate Origin and Referer headers for sensitive actions as an additional layer:

func ValidateOrigin(r *http.Request) bool {
    allowedOrigin := "https://app.yourdomain.com"
    origin := r.Header.Get("Origin")
    referer := r.Header.Get("Referer")
    return origin == allowedOrigin || referer == allowedOrigin
}

func SensitiveActionHandler(c buffalo.Context) error {
    if !ValidateOrigin(c.Request()) {
        return c.Error(403, errors.New("invalid origin"))
    }
    // proceed with token validation and business logic
    return nil
}

5. Use short-lived tokens and refresh token rotation to reduce the impact of token leakage:

// Example token validation that checks expiry and scope
func isValidToken(token string) bool {
    parsed, err := jwt.Parse(token, func(t *jwt.Token) (interface{}, error) {
        if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
            return nil, fmt.Errorf("unexpected signing method")
        }
        return []byte(os.Getenv("JWT_SECRET")), nil
    })
    if err != nil || !parsed.Valid {
        return false
    }
    claims, ok := parsed.Claims.(jwt.MapClaims)
    if !ok {
        return false
    }
    // Check expiry and required scopes
    if claims["exp"] == nil || int64(claims["exp"].(float64)) < time.Now().Unix() {
        return false
    }
    requiredScope := "transfer:write"
    if scopes, ok := claims["scope"].(string); !ok || !strings.Contains(scopes, requiredScope) {
        return false
    }
    return true
}

By combining strict CORS, header-only token transmission, origin validation, and short-lived scoped tokens, Buffalo applications can effectively defend against CSRF while maintaining secure Bearer Token usage. Regular scans with middleBrick help verify that these controls are correctly implemented and that no endpoints inadvertently expose tokens or accept them in unsafe contexts.