Race Condition Exploit in Axum

Detecting race conditions in Axum requires both code review and behavioral testing. In code, look for Arc-wrapped types that are not Sync, such as Cell, RefCell, or even Rc. These are red flags because Axum's async handlers can execute concurrently on different threads. Also check for missing synchronization around shared state mutations, even with Mutex if the lock scope doesn't cover the entire critical section.

middleBrick's black-box scanner tests for race conditions by simulating concurrent requests to state-modifying endpoints. For the vulnerable /withdraw example, it sends multiple simultaneous requests and analyzes whether the final balance reflects an inconsistent state (e.g., negative balance after two withdrawals from a 100 balance). This falls under its BOLA/IDOR and Rate Limiting checks, as race conditions often bypass authorization or quota enforcement.

To manually test, you can use a simple script:

#!/usr/bin/env bash
URL="http://localhost:3000/withdraw"
# Fire 10 concurrent requests
for i in {1..10}; do    curl -X POST $URL &donedone

Then inspect the application's state. If the balance drops below zero or more than the initial amount, a race condition exists. middleBrick automates this with controlled concurrency and statistical analysis to distinguish random errors from deterministic race windows.

Fix race conditions in Axum by ensuring all shared mutable state is accessed atomically. For simple numeric counters, use atomic types from std::sync::atomic. For complex state, use a database with transactions or an async-aware mutex like tokio::sync::Mutex with a lock that spans the entire check-then-act sequence.

Atomic Fix: Replace Cell<i32> with AtomicI32. The entire operation becomes lock-free and thread-safe:

use std::sync::atomic::{AtomicI32, Ordering};
#[derive(Clone)]struct AppState {    balance: Arc<AtomicI32>,}

async fn withdraw(State(state): State<AppState>) -> Json<String> {    let mut current = state.balance.load(Ordering::Relaxed);    loop {        if current < 100 {            return Json("Insufficient funds".into());        }        // Attempt to decrement atomically        match state.balance.compare_exchange(            current,            current - 100,            Ordering::Relaxed,            Ordering::Relaxed        ) {            Ok(_) => return Json("Withdrawn 100".into()),            Err(new_current) => current = new_current, // Retry with updated value        }    }}

Database Transaction Fix: For financial operations, move the logic to the database:

// Using SQLx with a transaction
async fn withdraw(State(pool): State<sqlx::PgPool>) -> Json<String> {    let mut tx = pool.begin().await.unwrap();    let balance: i32 = sqlx::query_scalar!("SELECT balance FROM accounts WHERE id = $1 FOR UPDATE", 1)        .fetch_one(&mut tx)        .await.unwrap();    if balance >= 100 {        sqlx::query!("UPDATE accounts SET balance = balance - 100 WHERE id = $1", 1)            .execute(&mut tx)            .await.unwrap();        tx.commit().await.unwrap();        Json("Withdrawn 100".into())    } else {        tx.rollback().await.unwrap();        Json("Insufficient funds".into())    }}

Always ensure the critical section (check + update) is indivisible. tokio::sync::Mutex works but can introduce priority inversion; atomics or database transactions are preferred for high-contention endpoints.