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How to fix BFLA (Broken Function Level Authorization)
in Plug

Executive Summary

Broken Function Level Authorization (BFLA) occurs when an application exposes sensitive administrative or management functions to unauthorized users. In Elixir/Plug, this usually manifests as a route that checks for a valid session but fails to verify if the user's role permits the specific action. Attackers exploit this by guessing administrative endpoints or manipulating HTTP methods to escalate privileges.

The Vulnerable Pattern

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

VULNERABLE: This route only assumes the user is authenticated via earlier plugs,


but it never verifies if the user has the ‘admin’ role before executing a delete.


delete “/api/v1/users/:id” do
user_id = conn.params[“id”]
MyApp.DB.delete_user(user_id)
send_resp(conn, 200, “User deleted”)
end
end

The Secure Implementation

To mitigate BFLA, you must implement a 'Deny-by-Default' strategy. In Plug, create a dedicated authorization module that inspects the 'conn.assigns' for user metadata. Use 'Plug.Conn.halt/1' to stop the request pipeline immediately if the user's role does not match the required privilege level for the function. Never rely on the UI to hide administrative features; every API endpoint must be its own gatekeeper.

SECURE CODE
defmodule MyApp.Auth do
  import Plug.Conn

def restrict_to_role(conn, role) do
user = conn.assigns[:current_user]
if user && user.role == role do
conn
else
conn
|> send_resp(403, “Forbidden: Insufficient Permissions”)
|> halt()
end
end
end


defmodule MyApp.Router do
use Plug.Router
import MyApp.Auth


plug :match
plug :dispatch


SECURE: Explicitly verify the ‘admin’ role and halt the pipeline if unauthorized.


delete “/api/v1/users/:id” do
conn
|> restrict_to_role(“admin”)
|> handle_delete()
end


defp handle_delete(%{halted: true} = conn), do: conn
defp handle_delete(conn) do
user_id = conn.params[“id”]
MyApp.DB.delete_user(user_id)
send_resp(conn, 200, “User deleted”)
end
end

System Alert • ID: 1642
Target: Plug API
Potential Vulnerability

Your Plug API might be exposed to BFLA (Broken Function Level Authorization)

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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