Bleichenbacher Attack in Phoenix with Hmac Signatures

A Bleichenbacher attack is a cryptographic padding oracle technique originally described against PKCS#1 v1.5–style RSA encryption. The same adaptive chosen-ciphertext pattern can be observed when HMAC-based signature verification in Phoenix is implemented in a way that reveals whether a signature is valid before checking other security properties. In such a setup, the server may return different timing or status responses depending on whether the HMAC matches, effectively turning verification into an oracle.

Consider a Phoenix endpoint that signs a JSON payload with HMAC-SHA256 using a shared secret and expects a client-supplied signature in a header. If verification proceeds by computing HMAC over the raw body and comparing it in a non-constant-time fashion, an attacker can iteratively modify the payload and observe subtle timing differences or HTTP status nuances. Over many requests, the attacker can recover information about the expected HMAC or forge valid signatures without knowing the secret, especially when the endpoint also exposes other behaviors (e.g., error messages) that vary by verification outcome.

When combined with broader API risks observable by middleBrick—such as unauthenticated endpoints, missing rate limiting, or inconsistent error handling—the attack surface expands. For example, an endpoint that does not enforce strict input validation or lacks proper rate limiting may allow an attacker to submit many crafted requests quickly, accelerating the adaptive padding-oracle–style process. middleBrick’s checks for Input Validation, Rate Limiting, and Authentication help identify whether an API unintentionally exposes timing or behavioral differences that facilitate such adaptive attacks.

In a real-world scenario, an API might accept a message and an Hmac-Signature header, then proceed to verify using a comparison that leaks information through early exit. This becomes critical when the same endpoint also handles sensitive operations or when responses include stack traces or distinct error messages. middleBrick’s Data Exposure and Authentication checks can surface inconsistent responses or missing integrity safeguards, highlighting patterns that resemble a Bleichenbacher-style oracle in the context of HMAC verification.

Because middleBrick scans the unauthenticated attack surface and runs checks in parallel, it can detect indicators such as variable response codes, timing anomalies inferred from multiple probes, and missing countermeasures like constant-time comparison. While middleBrick reports findings and provides remediation guidance, it does not patch or block; instead, it informs developers that the combination of Hmac Signatures and endpoint behavior may enable adaptive forgery or information leakage.

To prevent Bleichenbacher-like adaptive attacks on HMAC verification in Phoenix, ensure that signature validation always uses constant-time comparison and that the endpoint behaves uniformly regardless of signature validity. Below are concrete, realistic examples using the Phoenix framework with Hmac Signatures-style verification.

Example 1: Constant-time HMAC verification in a Phoenix controller

defmodule MyAppWeb.ApiController do
  use MyAppWeb, :controller

  @secret System.get_env("HMAC_SECRET") |> Base.decode16!()

  def process(conn, %{"payload" => payload, "hmac" => received_hmac}) do
    computed_hmac = :crypto.mac(:hmac, :sha256, @secret, payload)

    if secure_compare(computed_hmac, Base.decode16!(received_hmac)) do
      # proceed with trusted request handling
      json(conn, %{status: "ok"})
    else
      # Return a uniform error response without leaking which part failed
      conn
      |> put_status(:unauthorized)
      |> json(%{error: "invalid_request"})
    end
  end

  defp secure_compare(a, b) when byte_size(a) == byte_size(b) do
    # Constant-time comparison to avoid timing leaks
    do_secure_compare(a, b, 0, true)
  end

  defp secure_compare(_a, _b, _len, _acc) when byte_size(_a) != byte_size(_b) do
    # Always return false and keep timing similar regardless of length mismatch
    do_secure_compare(<<0>>, <<1>>, 1, false)
  end

  defp do_secure_compare(<<>>, <<>>, _pos, acc), do: acc

  defp do_secure_compare(<>, <>, pos, acc) do
    do_secure_compare(ta, tb, pos + 1, acc && (ha ^^^ hb) == 0)
  end
end

Example 2: Plug-based verification with uniform error handling

defmodule MyAppWeb.HmacAuthPlug do
  import Plug.Conn
  alias Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    case get_req_header(conn, "x-hmac-signature") do
      [received_hmac_b64] ->
        payload = conn.body_params |> Jason.encode!()
        computed_hmac = :crypto.mac(:hmac, :sha256, System.get_env("HMAC_SECRET") |> Base.decode16!(), payload)

        if secure_compare(Base.encode16(computed_hmac), received_hmac_b64) do
          conn # allow request to proceed
        else
          send_error(conn, 401, "invalid_request")
        end

      _ ->
        send_error(conn, 400, "missing_signature")
    end
  end

  defp secure_compare(a, b) when byte_size(a) == byte_size(b) do
    # constant-time compare implementation as above
    do_secure_compare(a, b, 0, true)
  end

  defp secure_compare(_a, _b, _len, _acc), do: false

  defp do_secure_compare(<<>>, <<>>, _pos, acc), do: acc

  defp do_secure_compare(<>, <>, pos, acc) do
    do_secure_compare(ta, tb, pos + 1, acc && (ha ^^^ hb) == 0)
  end

  defp send_error(conn, status, reason) do
    conn
    |> put_status(status)
    |> json(%{error: reason})
    |> halt()
  end
end

Key remediation practices highlighted:

• Use constant-time comparison for HMAC digests to prevent timing-based oracle leakage.
• Return the same HTTP status code and generic error shape for invalid signatures, missing signatures, and malformed requests.
• Avoid branching logic that exposes which step failed (e.g., do not differentiate between bad HMAC and bad JSON).
• Ensure the HMAC secret is loaded securely and is of sufficient entropy; rotate keys as part of operational policy.
• Combine with broader protections—rate limiting, input validation, and authentication checks—as recommended by middleBrick’s findings to reduce overall risk.

By applying these patterns, developers mitigate adaptive forgery and oracle-style attacks against Hmac Signatures in Phoenix, aligning implementation with secure coding practices and reducing the likelihood of successful Bleichenbacher-style exploitation.