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How to fix SSRF (Server Side Request Forgery)
in Plug

Executive Summary

SSRF in the Elixir/Plug ecosystem typically occurs when user-controlled input is passed directly to HTTP clients like HTTPoison, Req, or Mint. Attackers exploit this to pivot into your internal network, hitting sensitive endpoints like AWS/GCP metadata services (169.254.169.254), local databases, or internal management APIs. To fix this, you must implement a strict allowlist or a blacklist that resolves hostnames to IPs before the request is dispatched.

The Vulnerable Pattern

VULNERABLE CODE
defmodule MyApp.Router do
  use Plug.Router
  plug :match
  plug :dispatch

get “/fetch_metadata” do
# DANGER: Direct injection of user-provided URL into HTTP client
url = conn.params[“target_url”]
case HTTPoison.get(url) do
{:ok, %{body: body}} -> send_resp(conn, 200, body)
{:error, _} -> send_resp(conn, 500, “Error”)
end
end
end

The Secure Implementation

The secure implementation mitigates SSRF by decoupling the user input from the request execution. 1. Scheme Validation: Only 'https' is allowed to prevent protocol smuggling (e.g., file:// or gopher://). 2. DNS Resolution: We resolve the hostname to an IP address using :inet.getaddr. 3. IP Blacklisting: We check the resolved IP against RFC 1918 private ranges and the cloud metadata IP. This prevents attackers from using DNS rebinding to bypass initial hostname checks, as the validation is performed on the actual IP address that would be contacted.

SECURE CODE
defmodule MyApp.SSRFGuard do
  @forbidden_ips [{10, 0, 0, 0}, {172, 16, 0, 0}, {192, 168, 0, 0}, {127, 0, 0, 1}, {169, 254, 169, 254}]

def safe_get(url) do
uri = URI.parse(url)
with :ok <- validate_scheme(uri.scheme),
{:ok, ip} <- resolve_ip(uri.host),
:ok <- check_ip_blacklist(ip) do
HTTPoison.get(url)
else
{:error, reason} -> {:error, reason}
end
end


defp validate_scheme(“https”), do: :ok
defp validate_scheme(_), do: {:error, “Invalid scheme”}


defp resolve_ip(host) do
case :inet.getaddr(to_charlist(host), :inet) do
{:ok, ip} -> {:ok, ip}
_ -> {:error, “Resolution failed”}
end
end


defp check_ip_blacklist(ip) do
if ip in @forbidden_ips, do: {:error, “Prohibited IP”}, else: :ok
end
end

System Alert • ID: 2836
Target: Plug API
Potential Vulnerability

Your Plug API might be exposed to SSRF (Server Side Request Forgery)

74% of Plug apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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