SSRF Protection in Elixir 🛡️

SafeURL is a library that aids developers in protecting against a class of vulnerabilities known as Server Side Request Forgery. It does this by validating a URL against a configurable allow or block list before making an HTTP request.

See the Documentation on HexDocs.

To get started, add safeurl to your project dependencies in mix.exs. Optionally, you may also add HTTPoison to your dependencies for making requests directly through SafeURL:

def deps do
  [
    {:safeurl, "~> 0.3.0"},
    {:httpoison, "~> 1.8"},  # Optional
  ]
end

To use SafeURL with your favorite HTTP Client, see the HTTP Clients section.

SafeURL blocks private/reserved IP addresses are by default, and users can add additional CIDR ranges to the blocklist, or alternatively allow specific CIDR ranges to which the application is allowed to make requests.

You can use allowed?/2 or validate/2 to check if a URL is safe to call. If you have the HTTPoison application available, you can also call get/4 which will validate the host automatically before making a web request, and return an error otherwise.

iex> SafeURL.allowed?("https://includesecurity.com")
true

iex> SafeURL.validate("http://google.com/", schemes: ~w[https])
{:error, :restricted}

iex> SafeURL.validate("http://230.10.10.10/")
{:error, :restricted}

iex> SafeURL.validate("http://230.10.10.10/", block_reserved: false)
:ok

# When HTTPoison is available:

iex> SafeURL.get("https://10.0.0.1/ssrf.txt")
{:error, :restricted}

iex> SafeURL.get("https://google.com/")
{:ok, %HTTPoison.Response{...}}

SafeURL can be configured to customize and override validation behaviour by passing the following options:

• :block_reserved - Block reserved/private IP ranges. Defaults to true.

• :blocklist - List of CIDR ranges to block. This is additive with :block_reserved. Defaults to [].

• :allowlist - List of CIDR ranges to allow. If specified, blocklist will be ignored. Defaults to [].

• :schemes - List of allowed URL schemes. Defaults to ["http, "https"].

• :dns_module - Any module that implements the SafeURL.DNSResolver behaviour. Defaults to DNS from the :dns package.

These options can be passed to the function directly or set globally in your config.exs file:

config :safeurl,
  block_reserved: true,
  blocklist: ~w[100.0.0.0/16],
  schemes: ~w[https],
  dns_module: MyCustomDNSResolver

Find detailed documentation on HexDocs.

While SafeURL already provides a convenient get/4 method to validate hosts before making GET HTTP requests, you can also write your own wrappers, helpers or middleware to work with the HTTP Client of your choice.

For HTTPoison, you can create a wrapper module that validates hosts before making HTTP requests:

defmodule CustomClient do
  def request(method, url, body, headers \\ [], opts \\ []) do
    {safeurl_opts, opts} = Keyword.pop(opts, :safeurl, [])

    with :ok <- SafeURL.validate(url, safeurl_opts) do
      HTTPoison.request(method, url, body, headers, opts)
    end
  end

  def get(url, headers \\ [], opts \\ []),        do: request(:get, url, "", headers, opts)
  def post(url, body, headers \\ [], opts \\ []), do: request(:post, url, body, headers, opts)
  # ...
end

And you can use it as:

iex> CustomClient.get("http://230.10.10.10/data.json", [], safeurl: [block_reserved: false], recv_timeout: 500)
{:ok, %HTTPoison.Response{...}}

For Tesla, you can write a custom middleware to halt requests that are not allowed:

defmodule MyApp.Middleware.SafeURL do
  @behaviour Tesla.Middleware

  @impl true
  def call(env, next, opts) do
    with :ok <- SafeURL.validate(env.url, opts), do: Tesla.run(next)
  end
end

And you can plug it in anywhere you're using Tesla:

defmodule DocumentService do
  use Tesla

  plug Tesla.Middleware.BaseUrl, "https://document-service/"
  plug Tesla.Middleware.JSON
  plug MyApp.Middleware.SafeURL, schemes: ~w[https], allowlist: ["10.0.0.0/24"]

  def fetch(id) do
    get("/documents/#{id}")
  end
end

In some cases you might want to use a custom strategy for DNS resolution. You can do so by passing your own implementation of SafeURL.DNSResolver in the global or local config.

Example use-cases of this are:

• Using a specific DNS server
• Avoiding network access in specific environments
• Mocking DNS resolution in tests

You can do so by implementing DNSResolver:

defmodule TestDNSResolver do
  @behaviour SafeURL.DNSResolver

  @impl true
  def resolve("google.com"), do: {:ok, [{192, 168, 1, 10}]}
  def resolve("github.com"), do: {:ok, [{192, 168, 1, 20}]}
  def resolve(_domain),      do: {:ok, [{192, 168, 1, 99}]}
end

config :safeurl, dns_module: TestDNSResolver

For more examples, see SafeURL.DNSResolver docs.

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SafeURL is officially maintained by the team at Slab. It was originally created by Nick Fox at Include Security.