How to fix Broken User Authentication
in Phoenix
Executive Summary
Broken authentication in Phoenix often stems from hand-rolled credential validation or improper session management. To secure an Elixir application, you must mitigate timing attacks, use memory-hard hashing functions like Argon2, and ensure session tokens are cryptographically signed and rotated. Stop building custom login logic and leverage proven primitives.
The Vulnerable Pattern
def login(conn, %{"email" => email, "password" => pass}) {
user = Repo.get_by(User, email: email)
# VULNERABILITY: Direct comparison or weak hash + timing attack vector
if user && user.password_hash == pass do
conn
|> put_session(:user_id, user.id)
|> redirect(to: "/dashboard")
else
render(conn, "login.html", error: "Unauthorized")
end
}The Secure Implementation
The vulnerable code is susceptible to timing attacks because it returns early if a user is not found, allowing an attacker to enumerate valid emails based on response latency. It also lacks a robust hashing mechanism. The secure implementation uses Argon2 for password hashing, which is resistant to GPU cracking. By calling Argon2.no_user_verify() when a user record is missing, we ensure the CPU cycles consumed remain consistent regardless of whether the email exists. Furthermore, using phx.gen.auth patterns ensures session IDs are regenerated upon login to prevent session fixation.
def login(conn, %{"email" => email, "password" => password}) { user = Accounts.get_user_by_email(email) # SECURE: Constant-time verification and Argon2 hashing if UserAuth.verify_password(user, password) do conn |> UserAuth.log_in_user(user) else # Prevent user enumeration via timing attacks Argon2.no_user_verify() render(conn, "login.html", error: "Invalid credentials") end }In UserAuth module:
def verify_password(user, password) do user && Argon2.verify_pass(password, user.password_hash) end
Your Phoenix API
might be exposed to Broken User Authentication
74% of Phoenix apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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