Double Free in Axum with Dynamodb

A Double Free occurs when a program attempts to free the same memory allocation more than once. In Rust services built with the Axum framework that store or manage state in Amazon DynamoDB, the risk arises from mismatched ownership handling and asynchronous resource management rather than manual memory operations. DynamoDB clients and associated request builders in the AWS SDK for Rust maintain internal buffers and retry state that can be tied to Axum request-scoped data. When developers incorrectly share or clone client wrappers or when Drop implementations interact with request guards, the same underlying allocation can be released multiple times across async tasks or threads.

Consider an Axum handler that clones a DynamoDB client for each request and attaches it to request extensions. If the client or its inner credential provider holds shared references to a region or endpoint configuration, and those references are also held by a background retry or middleware guard, dropping the request may trigger a cleanup path that attempts to release the same region handle. Because Axum’s extractor lifecycle and the SDK’s async retry logic are not always explicitly synchronized, the second drop can occur during shutdown or error handling, leading to undefined behavior that may manifest as crashes or data corruption.

This pattern is especially problematic when integrating DynamoDB with Axum extractors that conditionally create or reuse resources. For example, if a request-scoped cache or session object references a DynamoDB item via a pointer or smart pointer (such as Arc) and that cache is dropped in multiple paths (e.g., both in a custom extractor and in an error handler), the underlying memory can be freed more than once. The vulnerability is exposed when the service processes many concurrent requests or when error paths involving DynamoDB conditional checks intersect with Axum’s guard-based routing, making the attack surface dependent on timing and request ordering.

In the context of API security scanning with middleBrick, such logic flaws are surfaced under checks like BOLA/IDOR and Unsafe Consumption, where the scanner traces resource lifetimes across the API surface. Although middleBrick does not fix the issue, its findings highlight mismanaged resource boundaries and guide developers to validate that each allocation is freed exactly once. The scanner’s inventory management checks can also detect inconsistent client usage patterns across endpoints, helping teams identify where Axum handlers may inadvertently expose double-free conditions through DynamoDB integrations.

To prevent Double Free conditions when using DynamoDB with Axum, ensure that shared resources such as the DynamoDB client are managed with clear ownership semantics and that request-scoped lifetimes do not overlap with background or retry operations. The AWS SDK for Rust provides builders that are designed to be cloned safely; however, how you integrate these builders into Axum extractors determines safety.

Use Arc to wrap the DynamoDB client once at the application level and share it across handlers without cloning the inner state multiple times. Initialize the client in main or during application setup, store it in an Arc, and insert it into Axum’s extension state. This guarantees a single allocation and a single Drop, regardless of how many handlers reference it.

use aws_sdk_dynamodb::Client;
use std::sync::Arc;
use axum::extract::Extension;
use axum::routing::get;
use axum::Router;

async fn handler(Extension(client): Extension>) -> String {
    // Safe: client is Arc-wrapped and will be dropped once.
    let _ = client.list_tables().send().await;
    "ok".to_string()
}

#[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("/tables", get(handler))
        .layer(Extension(client));

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

Avoid passing cloned clients into per-request extractors that may themselves be cloned or moved into multiple async blocks. If you need per-request state, store lightweight references or IDs and retrieve the shared client from extension state within the handler. This eliminates multiple Drop invocations on the same underlying SDK structures.

Additionally, review any custom extractors that interact with DynamoDB conditionals or item collections. Ensure that any temporary buffers or retry guards do not capture references that outlive the request. Prefer composing handlers with tokio::spawn only when the shared client is already Arc-wrapped, and avoid spawning tasks that hold partial references to request data alongside DynamoDB streams or paginated responses.

For teams using middleBrick, the Pro plan’s continuous monitoring and GitHub Action integration can help detect regressions in resource handling by scanning API changes against known patterns. By combining static awareness with runtime findings, you can validate that Axum routes interacting with DynamoDB maintain consistent ownership and do not introduce double-free risks.