Token Leakage in Buffalo with Bearer Tokens

Token leakage in Buffalo applications that rely on Bearer Tokens occurs when access tokens are exposed beyond their intended scope or persistence layer. Because Bearer Tokens are typically simple strings that authorize requests, any accidental exposure—such as logging, serialization, or misconfigured headers—can lead to unauthorized access. In Buffalo, this often surfaces in request handling pipelines where tokens are read from headers and then passed to downstream services or stored unintentionally in logs.

The framework’s convention-driven behavior can inadvertently contribute to leakage if developers bind token values to structs or logs without sanitization. For example, when request parameters or headers containing Authorization: Bearer <token> are mirrored into response metadata or debug output, the token becomes accessible to unintended recipients. Similarly, if token values are included within HTML templates or serialized into JSON responses through model binding, they may be exfiltrated via cross-site scripting (XSS) or insecure direct object references (IDOR).

Cross-referencing an OpenAPI specification with runtime findings is a key strength of middleBrick in this context. The scanner can detect whether an endpoint expects an Authorization header but also reflects header values in the response body, which is a leakage pattern. A common real-world vulnerability tied to this is CVE-2021-29425, where session tokens were echoed in headers, enabling session hijacking. By running a black-box scan, middleBrick checks unauthenticated attack surfaces and flags cases where Bearer Tokens appear in logs, error messages, or unintended data channels, providing mappings to OWASP API Top 10 and PCI-DSS requirements.

Another vector involves improper caching or session storage. If Buffalo controllers store Bearer Tokens in session cookies without the HttpOnly and Secure flags, or if tokens are embedded in URLs via query parameters, they can be leaked through browser histories or referrer headers. The scanner’s checks for Data Exposure and Unsafe Consumption help identify these patterns by correlating spec definitions—such as required security schemes—with runtime responses that expose tokens in URLs or insecure cookies.

Moreover, SSRF findings can compound token leakage risks. An endpoint that accepts user-supplied URLs and makes outbound requests with Bearer Tokens stored in server-side headers might inadvertently send tokens to malicious services if input validation is weak. middleBrick’s SSRF check examines whether outbound requests can be influenced via crafted inputs, ensuring tokens are not forwarded to arbitrary endpoints. Together, these checks provide a clear remediation path by highlighting where token handling diverges from secure defaults.

To remediate token leakage in Buffalo, focus on ensuring Bearer Tokens are handled strictly as opaque credentials, never echoed, logged, or stored in mutable contexts. Use middleware to extract and validate tokens without binding them to request contexts that might be serialized or exposed. The following code examples illustrate secure patterns.

First, define a secure middleware that reads the Bearer Token from the Authorization header and validates it without logging or reflecting the token value:

// In middleware/bearer_token.ex
defmodule MyAppWeb.BearerTokenMiddleware do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "authorization") do
      ["Bearer " <> token] ->
        # Validate token via external auth service; do NOT store token in conn.assigns
        if valid_token?(token) do
          conn
        else
          send_resp(conn, 401, "Unauthorized")
          |> halt()
        end
      _ ->:
        send_resp(conn, 401, "Missing token")
        |> halt()
    end
  end

  defp valid_token?(token) do
    # Implement token verification, e.g., JWT validation or introspection
    true
  end
end

Second, ensure that token values are never included in logs or error messages. Configure your logger to filter sensitive headers and avoid inspecting conn.req_headers in debug output. In your endpoint definitions, avoid binding token-containing parameters to models or assigns:

# In controller
plug MyAppWeb.BearerTokenMiddleware

def show(conn, %{"id" => id}) do
  # Do NOT retrieve token from conn.private or assigns for logging
  resource = MyApp.get_resource!(id)
  render(conn, "show.json", resource: resource)
end

Third, when making outbound HTTP requests, do not inject Bearer Tokens via query parameters or mutable structs. Use environment variables or secure vaults for configuration, and set headers explicitly in the request without exposing them in logs:

# In a client module
headers = [{"Authorization", "Bearer #{System.get_env("API_TOKEN")}"}]
# Use HTTP client that does not log request details
HTTPoison.get!("https://api.example.com/data", headers)

Lastly, leverage middleBrick’s scans to validate that your endpoints do not reflect tokens in responses or headers. The CLI tool can be integrated into development workflows to catch regressions early:

# Scan your Buffalo API endpoint
middlebrick scan https://api.example.com/openapi.json

By combining secure token handling patterns with automated scanning, you reduce the risk of token leakage while maintaining compatibility with Bearer Token authentication flows.