Use After Free in Axum with Dynamodb

Use After Free (UAF) occurs when memory is deallocated but references to it remain in use, leading to undefined behavior or data corruption. In the context of an Axum application interacting with Amazon DynamoDB, UAF can arise through improper handling of asynchronous resources, SDK client lifetimes, or deserialized request data that outlives its intended scope.

Axum is a Rust web framework that relies on async runtimes and shared state. When integrating with DynamoDB via the AWS SDK for Rust, developers often hold SDK clients or DynamoDB item structs in application state or request handlers. If these references are cloned or moved across async tasks without ensuring proper ownership and lifetimes, it can create situations where underlying buffers are freed while still being accessed. For example, extracting a DynamoDB attribute into a raw pointer or a long-lived async block without pinning or proper synchronization can expose UAF when the original allocation is dropped prematurely.

Consider an Axum handler that deserializes a DynamoDB GetItem response into a struct and then spawns a tokio task to process it. If the handler captures a reference to the deserialized data without ensuring it lives long enough, and the original data is dropped at the end of the handler, the spawned task may attempt to read freed memory. This is especially risky when using Arc incorrectly or when mixing synchronous SDK calls with async processing pipelines. The DynamoDB SDK for Rust manages its own memory, but developers must ensure that any extracted data is either owned or properly synchronized across async boundaries.

Another exposure path involves the SDK’s internal buffering for paginated queries. If an Axum service iterates over DynamoDB Scan results and stores references to items in a global cache without deep cloning or serialization, those references can become dangling once the pagination stream completes and internal buffers are released. This pattern can lead to information disclosure or crashes when cached data is later accessed. The risk is compounded when the application uses lightweight request-scoped objects that do not enforce strict lifetimes.

To detect such issues, middleBrick scans Axum endpoints that interact with DynamoDB, checking for unsafe patterns such as raw pointer usage, improper async task captures, and missing lifetime annotations in handler signatures. The scanner correlates these patterns with DynamoDB request and response structures to identify potential UAF conditions during black-box testing of the unauthenticated attack surface.

Remediation focuses on ensuring data ownership and lifetime safety when integrating Axum with DynamoDB. The key is to avoid holding references that may dangle and instead use owned types or explicit synchronization primitives.

1. Use owned data across async boundaries

When extracting data from a DynamoDB response, clone or deserialize into owned structures before moving them into async tasks. This prevents use-after-free when the original response buffer is dropped.

use aws_sdk_dynamodb::Client;
use axum::{routing::get, Router};
use serde::Deserialize;
use std::sync::Arc;

#[derive(Clone, Debug, Deserialize)]
struct UserProfile {
    user_id: String,
    email: String,
}

async fn get_user(client: Arc<Client>, user_id: String) -> UserProfile {
    let resp = client.get_item()
        .table_name("Users")
        .key("user_id", aws_sdk_dynamodb::types::AttributeValue::S(user_id))
        .send()
        .await
        .expect("failed to get item");

    let item = resp.item.expect("item not found");
    // Deserialize into an owned struct
    let user: UserProfile = serde_dynamodb::from_hashmap(item).expect("failed to deserialize");
    user // owned, safe to move
}

#[tokio::main]
async fn main() {
    let config = aws_config::load_from_env().await;
    let client = Arc::new(Client::new(&config));

    let app = Router::new()
        .route("/users/:id", get(|user_id| {
            let client = client.clone();
            async move {
                let user = get_user(client, user_id).await;
                // user is safely owned here
                format!("{}", user.email)
            }
        }));
}

2. Avoid storing references in shared state

Do not store references to DynamoDB items in static or global state. If caching is required, use serialized formats or deep clones with proper locking.

use std::collections::HashMap;
use std::sync::Mutex;

// Safe: owned data in Arc>
type SafeCache = Arc<Mutex<HashMap<String, UserProfile>>>;

async fn update_cache(cache: SafeCache, user: UserProfile) {
    let mut map = cache.lock().unwrap();
    map.insert(user.user_id.clone(), user); // owned copy stored
}

// Unsafe pattern to avoid:
// let cache: Arc<Mutex<HashMap<String, &HashMap<String, AttributeValue>>>> = ...;
// This holds references that may dangle.

3. Validate lifetimes in handler signatures

Ensure Axum extractors do not inadvertently borrow data with insufficient lifetimes. Prefer extracting owned query parameters or using State with owned configurations.

use axum::{extract::State, http::StatusCode};

struct AppState {
    client: Client,
    default_table: String, // owned, not a reference
}

async fn handler(
    State(state): State<Arc<AppState>>,
    // Use query extractor to get owned values
    query: axum::extract::Query<HashMap<String, String>>,
) -> Result<(), (StatusCode, String)> {
    // Safe: all data owned
    Ok(())
}

4. Use serde_dynamodb or safe deserialization

When converting DynamoDB attribute maps to Rust structs, use libraries that enforce ownership and avoid unsafe transmutes. This prevents internal buffers from being freed while still in use.

let user_map: HashMap<String, AttributeValue> = resp.item.context("missing item")?;
let user: UserProfile = serde_dynamodb::from_hashmap(user_map)?;
// serde_dynamodb consumes the map, ensuring no lingering references