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lib/nex/csrf.ex
defmodule Nex.CSRF do
@moduledoc """
CSRF (Cross-Site Request Forgery) protection for Nex applications.
Uses **signed tokens** via `Phoenix.Token` — cryptographically verified across
request cycles without server-side session storage. Tokens are signed with a
secret derived from the application's secret key base.
## How it works
1. On page render (`GET`), `generate_token/0` creates a signed token and stores
it in the process dictionary for the current request.
2. The token is injected into `<head>` as a `<meta>` tag and into every form
automatically by `Nex.Handler`.
3. On `POST/PUT/PATCH/DELETE`, `validate/1` verifies the submitted token's
cryptographic signature — no session lookup needed.
## Usage
Everything is automatic. You do not need to call these functions manually.
The framework handles injection and validation transparently.
"""
require Logger
@token_key "_csrf_token"
@header_name "x-csrf-token"
@salt "nex.csrf"
@max_age 86_400
@doc """
Generates a new signed CSRF token for the current request.
The token is a `Phoenix.Token`-signed value containing a random nonce.
It is stored in the process dictionary so helpers (`csrf_input_tag/0` etc.)
can retrieve it without regenerating.
"""
def generate_token do
nonce = :crypto.strong_rand_bytes(16) |> Base.url_encode64(padding: false)
token = Phoenix.Token.sign(endpoint_or_secret(), @salt, nonce)
Process.put(:csrf_token, token)
token
end
@doc """
Gets the current CSRF token, generating one if needed.
"""
def get_token do
case Process.get(:csrf_token) do
nil -> generate_token()
token -> token
end
end
@doc """
Returns an HTML hidden input tag with the CSRF token.
"""
def csrf_input_tag do
token = get_token()
{:safe, ~s(<input type="hidden" name="#{@token_key}" value="#{token}" />)}
end
@doc """
Returns a JSON string for hx-headers attribute with CSRF token.
"""
def hx_headers do
token = get_token()
~s({"#{@header_name}": "#{token}"})
end
@doc """
Returns the meta tag for CSRF token (useful for JavaScript).
"""
def meta_tag do
token = get_token()
{:safe, ~s(<meta name="csrf-token" content="#{token}" />)}
end
@doc """
Validates the CSRF token submitted with a request.
Verifies the cryptographic signature of the token using `Phoenix.Token.verify/4`.
Returns `:ok` if valid, `{:error, reason}` otherwise.
"""
def validate(conn) do
submitted_token = get_submitted_token(conn)
if is_nil(submitted_token) or submitted_token == "" do
{:error, :missing_token}
else
case Phoenix.Token.verify(endpoint_or_secret(), @salt, submitted_token, max_age: @max_age) do
{:ok, _nonce} ->
:ok
{:error, :expired} ->
Logger.debug("[Nex.CSRF] Token expired")
{:error, :invalid_token}
{:error, reason} ->
Logger.debug("[Nex.CSRF] Token invalid: #{inspect(reason)}")
{:error, :invalid_token}
end
end
end
@doc """
Checks if the request method requires CSRF validation.
"""
def protected_method?(method) when is_atom(method) do
method in [:post, :put, :patch, :delete]
end
def protected_method?(method) when is_binary(method) do
method
|> String.downcase()
|> String.to_existing_atom()
|> protected_method?()
rescue
ArgumentError -> false
end
# Private functions
defp get_submitted_token(conn) do
case Plug.Conn.get_req_header(conn, @header_name) do
[token | _] when is_binary(token) and token != "" ->
token
_ ->
case conn.params do
%Plug.Conn.Unfetched{} -> nil
%{} = params -> Map.get(params, @token_key)
_ -> nil
end
end
end
# Returns the secret key base for signing tokens.
# Reads SECRET_KEY_BASE env var, falls back to a deterministic dev secret.
# In production, SECRET_KEY_BASE must be set to a strong random value.
defp endpoint_or_secret do
case System.get_env("SECRET_KEY_BASE") do
nil ->
if dev_env?() do
# In development, generate deterministic dev secret (OK for local dev)
Logger.warning(
"[Nex.CSRF] SECRET_KEY_BASE not set — using insecure dev default. " <>
"Set SECRET_KEY_BASE in production!"
)
end
"nex_dev_secret_key_base_do_not_use_in_production_replace_with_64_char_random_string"
secret ->
secret
end
end
# Runtime-safe environment check
defp dev_env? do
case Application.get_env(:nex_core, :env, :prod) do
:dev -> true
_ -> false
end
end
end