Use After Free in Axum with Api Keys

A use-after-free condition occurs when memory is accessed after it has been deallocated. In an Axum application that relies on API keys for authorization, this typically arises when key material or associated metadata is freed while a request or background task still holds a pointer to it. Because Axum handlers are often asynchronous and may capture references to data for use across async boundaries, improper lifetime management can keep references alive after the owning structure has been dropped.

Consider an Axum extractor that retrieves an API key from headers, validates it, and then schedules further work, such as logging or rate-limiting checks. If the key is stored in a reference-counted wrapper (e.g., Arc) but some inner component holds a raw pointer or a shorter-lived borrow into that wrapper, the inner component may access the key after the Arc is dropped. This can expose key bytes or lead to undefined behavior, including information disclosure or crashes.

Another scenario involves middleware that clones an Arc containing the key into multiple handlers or background jobs. If one job completes and drops its Arc while another job is still running, the latter may read or write to freed memory. This is especially risky when API keys are cached or partially redacted, as partial state can remain accessible after deallocation.

From a scanning perspective, middleBrick evaluates whether API endpoints that use keys exhibit unsafe patterns such as unchecked async handoffs or missing synchronization. While middleBrick does not fix these issues, its findings highlight risky code paths and provide remediation guidance aligned with secure memory handling practices.

To prevent use-after-free in Axum when handling API keys, ensure that all key material and related state have clear ownership and lifetime guarantees. Prefer shared, thread-safe containers such as Arc and avoid mixing raw pointers with managed lifetimes. Below are concrete, idiomatic examples that demonstrate safe patterns.

1. Safe key extraction and validation with Arc

Keep API keys wrapped in Arc for the duration of the request and any spawned tasks. Clone the Arc before moving it into async blocks to ensure the underlying data stays alive as long as any reference exists.

use std::sync::Arc;
use axum::{routing::get, Router, extract::State};

struct ApiKey(String);

#[tokio::main]
async fn main() {
    let key = Arc::new(ApiKey("secret-key-123".to_string()));
    let app = Router::new()
        .route("/validate", get(validate_key))
        .with_state(key);

    axum::Server::bind(&"0.0.0.0:3000".parse().unwrap())
        .serve(app.into_make_service())
        .await
        .unwrap();
}

async fn validate_key(State(key): State>) -> String {
    format!("Validated key: {}", key.0)
}

2. Avoiding use-after-free in spawned tasks

When spawning async tasks that need API key data, clone the Arc to extend the lifetime explicitly. Do not rely on references that may outlive the original scope.

use std::sync::Arc;
use axum::extract::State;

async fn spawn_handler(key: Arc) {
    let key_clone = Arc::clone(&key);
    tokio::spawn(async move {
        // key_clone is safely moved into the task and will live until the task finishes
        tracing::info!("Processing with key: {}", key_clone.0);
    });
}

3. Using channels with owned data

If you need to pass API keys between components, prefer owned messages over borrowed ones. This avoids situations where a receiver accesses memory after the sender has dropped its references.

use tokio::sync::mpsc;

async fn channel_example() {
    let (tx, mut rx) = mpsc::channel(32);
    let key = Arc::new(ApiKey("channel-key".to_string()));

    // Send an owned clone
    let tx_key = Arc::clone(&key);
    tx.send(tx_key).await.unwrap();

    while let Some(received_key) = rx.recv().await {
        // received_key is a new Arc, independent of the original scope
        tracing::info!("Received key: {}", received_key.0);
    }
}

4. Middleware with proper lifetimes

When implementing middleware that inspects or logs API keys, ensure that you do not store references beyond the request lifecycle. Capture data by value where necessary and avoid returning closures that borrow key material.

use axum::{extract::Request, middleware::Next};

async fn key_aware_middleware(
    request: Request,
    next: Next,
) -> Result