Buffer Overflow in Phoenix with Hmac Signatures

A buffer overflow in a Phoenix application that uses Hmac Signatures can occur when unchecked input is used to construct cryptographic material or when the runtime buffers data associated with signature verification. In Phoenix, this typically surfaces in controller actions that read request payloads into binaries or binaries via Plug.Parsers and then pass data to HMAC functions without length validation. Because Hmac Signatures rely on exact byte-for-byte input to reproduce the expected digest, attackers may exploit oversized or malformed payloads to force the runtime to allocate or copy data beyond intended memory boundaries, potentially leading to information disclosure or code execution. The risk is heightened when developers concatenate user-supplied data (e.g., headers, JSON fields) directly into the binary used for signing, especially when using binaries built incrementally via <> in a loop over large inputs.

Consider a scenario where a Phoenix controller computes an Hmac over raw request body to validate integrity. If the body is read into an IO list or binary without size limits and then passed to :crypto.mac(:hmac, :sha256, key, data), a large payload can trigger excessive memory growth or binary copying. Because the binary is constructed from external input, an oversized payload can expose implementation details or crash the runtime, which may be interpreted as a denial-of-service vector. Even when the cryptographic library itself does not overflow, the surrounding code that assembles the signed data can do so unsafely, particularly when using functions that concatenate or slice binaries without boundary checks.

Moreover, when Hmac Signatures are used for routing or parameter authentication (e.g., signing query parameters or headers), attackers may send specially crafted parameter sets that increase the size of the canonical string used to compute the signature. If the code that builds this canonical string uses unsafe binary operations or unbounded list concatenation, the intermediate representation can grow unchecked. Although Phoenix and the BEAM provide memory isolation, a buffer overflow in the underlying NIFs or linked native code called during Hmac computation can still compromise the process. Therefore, validating input length, avoiding unchecked binary concatenation, and using constant-time comparison for signatures are essential to mitigate this specific combination of a buffer overflow in Phoenix with Hmac Signatures.

To remediate buffer overflow risks in Phoenix when using Hmac Signatures, ensure input size validation, safe binary handling, and constant-time verification. Always set explicit size limits on request bodies before processing them for signing, and avoid building large binaries via repeated concatenation. Use binary pattern matching with size constraints and prefer functions from the crypto module that operate on binaries without intermediate copying.

Example 1: Safe Hmac computation with size-limited body

defmodule MyAppWeb.AuthController do
  use MyAppWeb, :controller

  @max_body_size 1_048_576 # 1 MB limit

  def verify(conn, _opts) do
    case read_body_with_limit(conn, @max_body_size) do
      {:ok, body} ->
        key = System.get_env("HMAC_KEY") |> Base.decode16!()
        signature = conn.req_headers["x-signature"]
        computed = :crypto.mac(:hmac, :sha256, key, body)
        if constant_time_equal(computed, Base.decode16!(signature)) do
          json(conn, %{status: "valid"})
        else
          send_resp(conn, 401, "Invalid signature")
        end

      {:error, :too_large} ->
        send_resp(conn, 413, "Payload too large")
    end
  end

  defp read_body_with_limit(conn, limit) do
    conn
    |> Plug.Conn.read_body(limit: limit)
    |> case do
      {:ok, body, _conn} -> {:ok, body}
      {:more, _body, _conn} -> {:error, :too_large}
      {:error, _reason} -> {:error, :too_large}
    end
  end

  defp constant_time_equal(a, b) when byte_size(a) == byte_size(b) do
    :crypto.verify(a, :hmac, :sha256, b) # placeholder for constant-time compare
  end
  defp constant_time_equal(_, _), do: false
end

Example 2: Canonical string construction with binary pattern matching

defmodule MyAppWeb.Signature do
  @moduledoc """
  Build canonical string safely and compute Hmac without unsafe concatenation.
  """

  def build_canonical(params) when is_list(params) do
    # Ensure fixed-size encoding and avoid incremental list growth
    parts =
      Enum.map(params, fn {k, v} ->
        <<(byte_size(k))::16>> <> k <> <<(byte_size(v))::16>> <> v
      end)
    :erlang.list_to_bitstring(parts)
  end

  def compute_hmac(key, canonical) when is_binary(key) and is_binary(canonical) do
    :crypto.mac(:hmac, :sha256, key, canonical)
  end
end

# Usage in a controller
canonical = MyAppWeb.Signature.build_canonical([{"param1", "value1"}, {"param2", "value2"}])
signature = Base.encode16(MyAppWeb.Signature.compute_hmac(key, canonical))

These examples emphasize bounded reads, explicit size limits, and avoiding runtime list-to-binary expansion that can lead to buffer overflow conditions. By combining input validation with safe binary construction and constant-time comparison, you reduce the attack surface for buffer overflow in Phoenix applications that rely on Hmac Signatures.