Use After Free in Axum with Basic Auth

Use After Free (UAF) occurs when memory is deallocated but subsequent code continues to reference it. In Axum, combining Basic Auth parsing with asynchronous Rust can expose UAF when authorization state outlives the data it references. Consider a handler that reads credentials from an Authorization header, deserializes them into a short-lived structure, and then passes a reference into an async block or stores it in session-like state without ensuring the referenced data remains valid for the entire lifetime of the future.

With Basic Auth, the typical pattern is to extract a header value, split on :, and keep a reference to the decoded username or password. If the handler spawns a task, uses tokio::spawn, or stores the reference in a cache keyed by request ID, the referenced string data may be freed when the original request’s stack frame is dropped. The spawned future may then read or compare against that dangling reference, leading to UAF behavior. This is more likely when middleware or extractors return references tied to the request rather than owned values, and when Basic Auth credentials are kept as slices rather than cloned strings.

Although Axum itself does not manage memory unsafely, Rust’s lifetime parameters can inadvertently allow references to outlive their source if extractors are written to return borrowed data and are used across await points. An example risk pattern is an extractor that returns &str pointing into the request body or header, which is later used inside an async move block after the request context is gone. A runtime scan with middleBrick can surface findings related to unsafe patterns by correlating input validation checks with the use of borrowed data across asynchronous boundaries.

To detect such issues, middleBrick runs 12 security checks in parallel, including Input Validation and Unsafe Consumption, and can identify risky combinations of authentication mechanisms and async handling. Its OpenAPI/Swagger spec analysis resolves $ref definitions and cross-references them with runtime findings, helping to highlight endpoints where Basic Auth data flows into complex lifetimes or shared state. This is valuable because the scanner operates without agents or credentials, providing a black-box view of how an endpoint behaves when presented with crafted Authorization headers.

Real-world attack patterns that resemble UAF in this context include scenarios where an application reuses buffers or caches keyed by usernames without ensuring the underlying data remains stable. For instance, if an endpoint decodes Basic Auth, stores a reference in a rate-limiting map keyed by username, and the request ends before the map entry is cleaned up, subsequent operations might read freed memory. OWASP API Top 10 categories such as Security Misconfiguration and Improper Asset Management intersect with this risk when authentication logic interacts with storage or logging subsystems.

middleBrick’s findings include severity ratings and remediation guidance mapped to compliance frameworks like OWASP API Top 10 and SOC2. For endpoints using Basic Auth in Axum, this means highlighting where borrowed data crosses async boundaries and advising to prefer owned types. The tool does not fix or block, but it provides concrete guidance so developers can adjust extractor designs and data lifetimes to avoid UAF-like conditions.

Remediation focuses on ensuring that credentials extracted from the Authorization header are owned and do not contain references that can dangle across await points. Instead of returning borrowed slices from extractors, clone the necessary strings or use wrapper types that own their data. Below are concrete, working Axum examples that demonstrate safe patterns.

Insecure pattern to avoid:

use axum::{routing::get, Router};
use std::net::SocketAddr;

async fn handler(auth_user: &str) -> String {
    format!("Hello {}", auth_user)
}

#[tokio::main]
async fn main() { 
    let app = Router::new().route("/", get(handler));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr).serve(axum::routing::get_service(app)).await.unwrap();
}

In this snippet, handler accepts a borrowed &str that would typically come from an extractor. If this reference were tied to the request and used across an async boundary, it could become invalid.

Secure pattern using owned String:

use axum::{routing::get, Router, extract::Extension, Json};
use std::net::SocketAddr;

struct AppState { }

async fn handler(Json(payload): Json<AuthPayload>, Extension(state): Extension<AppState>) -> String {
    let username = payload.username.clone(); // owned copy
    format!("Hello {}", username)
}

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

#[tokio::main]
async fn main() { 
    let app = Router::new()
        .route("/", get(handler))
        .layer(Extension(AppState {}));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr).serve(axum::routing::get_service(app)).await.unwrap();
}

Here, AuthPayload owns its fields, so the handler works with String rather than references. This avoids lifetime issues when the handler is async and data may be moved into the future.

Secure pattern using Basic Auth header parsing with owned values:

use axum::{routing::get, Router, extract::Extension, async_trait};
use axum::headers::{authorization::Basic, Authorization};
use std::net::SocketAddr;

async fn handler(Authorization(basic): Authorization<Basic>) -> String {
    let username = basic.username().to_string(); // clone to own
    format!("User: {}", username)
}

#[tokio::main]
async fn main() { 
    let app = Router::new().route("/", get(handler));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr).serve(axum::routing::get_service(app)).await.unwrap();
}

This approach uses Axum’s built-in Authorization<Basic> extractor, which decodes the header and provides owned methods like username() that return String-like data via to_string(). The handler then works with an owned copy, eliminating lifetime risks.

For middleware or shared caches, store owned values (e.g., String or Arc<str>) rather than references. When integrating with other systems, middleBrick can validate that authentication-related data flows remain within safe ownership boundaries by analyzing endpoint behavior against crafted Basic Auth inputs.