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Fix Broken User Authentication in Axum

Broken Authentication remains a top-tier vulnerability in Axum applications when developers bypass robust middleware for 'roll-your-own' logic. Common fails include plaintext password comparisons, weak session ID generation, and lack of constant-time verification. If you're not using Argon2id and secure cookie attributes, you're leaving the door wide open for credential stuffing and session hijacking.

The Vulnerable Pattern

use axum::{routing::post, Json, response::IntoResponse, http::StatusCode, Router};
use serde::Deserialize;

#[derive(Deserialize)]
struct LoginRequest {
username: String,
password: String,
}


// VULNERABLE: Plaintext comparison and no secure session management
async fn login_vulnerable(Json(payload): Json) -> impl IntoResponse {
let stored_password = “p@ssword123”; // Mocked DB lookup


if payload.password == stored_password {
    (StatusCode::OK, "Login successful")
} else {
    (StatusCode::UNAUTHORIZED, "Invalid credentials")
}


}

The Secure Implementation

The vulnerable code fails by performing a direct string comparison on plaintext passwords, which is susceptible to timing attacks and database leaks. The secure implementation utilizes the Argon2id hashing algorithm, which is CPU/Memory hard, mitigating brute-force and GPU-accelerated attacks. Furthermore, it leverages 'tower-sessions' to handle stateful authentication via secure, HttpOnly, and SameSite cookies, rather than returning a raw success string. Always ensure your session store is backed by a secure DB (like Redis or Postgres) and that session IDs are sufficiently random (128-bit entropy).

use axum::{routing::post, Json, response::IntoResponse, http::StatusCode};
use argon2::{password_hash::{PasswordHash, PasswordHasher, PasswordVerifier, SaltString}, Argon2};
use tower_sessions::{Session, SessionManagerLayer};

// SECURE: Argon2id hashing and proper session handling
async fn login_secure(session: Session, Json(payload): Json) -> impl IntoResponse {
let stored_hash = “$argon2id$v=19$m=19456,t=2,p=1$VE9… ”; // Fetch from DB


let parsed_hash = PasswordHash::new(stored_hash).map_err(|_| StatusCode::INTERNAL_SERVER_ERROR)?;

if Argon2::default().verify_password(payload.password.as_bytes(), &parsed_hash).is_ok() {
    session.insert("user_id", 1337).await.unwrap();
    Ok((StatusCode::OK, "Authenticated"))
} else {
    Err(StatusCode::UNAUTHORIZED)
}


}

System Alert • ID: 6157
Target: Axum API
Potential Vulnerability

Your Axum API might be exposed to Broken User Authentication

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

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