Uninitialized Memory in Axum with Mutual Tls

Uninitialized memory is a class of vulnerability where read operations expose stack or heap contents that were never explicitly set by the program. In Axum, this typically arises when handler code reads from buffers, tuples, or struct fields that were never written before being serialized or returned to the client.

When Mutual Tls (mTLS) is enforced in Axum, the TLS layer terminates the connection and provides client certificates before your handlers run. This can change allocation patterns and lifetimes: the framework may pre-allocate buffers for TLS records, and it may reuse those buffers across requests. If your handler inspects uninitialized fields or deserializes into types without default values, the leftover bytes from a previous TLS record can be exposed through the response.

For example, consider an Axum handler that accepts JSON and returns a struct with an uninitialized numeric field. Without mTLS, the request body parsing may zero out or leave gaps depending on serde behavior. With mTLS, the additional framing and record buffering inside the TLS layer can increase the likelihood that stale stack data is present in those gaps, because the handler is invoked after TLS decryption and the associated buffer reuse patterns differ.

An attack scenario involves an endpoint that echoes part of the request or returns metadata. If the handler constructs a response by reading uninitialized memory (e.g., via unsafe code, raw pointers, or by relying on uninitialized struct fields), the output may contain sensitive data from prior TLS processing, such as fragments of client certificate material or intermediate buffers. This can violate data exposure checks that are part of the 12 parallel security scans, including Data Exposure and Encryption checks.

Because middleBrick performs black-box testing and OpenAPI/Swagger analysis with full $ref resolution, it can detect indicators that suggest uninitialized memory exposure — such as inconsistent field sizes, unexpected binary output, or endpoints that return variable-length blobs without explicit bounds. The LLM/AI Security checks do not directly test this, but the broader scan can surface endpoints where uninitialized memory could be exfiltrated, especially when mTLS changes runtime behavior.

Remediation focuses on ensuring all data read by handlers is explicitly initialized and that buffer reuse introduced by mTLS does not leak stale data. Below are concrete Axum examples with proper Mutual Tls configuration and safe handler patterns.

1. Enforce mTLS in Axum with explicit certificate extraction and safe handling:

use axum::Router;
use axum::routing::get;
use axum::extract::Request;
use axum::http::header;
use std::net::SocketAddr;
use tokio_rustls::rustls::{ServerConfig, RootCertStore, Certificate, PrivateKey};
use tokio_rustls::rustls::pki_types::{CertificateDer, PrivateKeyDer};
use tokio_rustls::TlsAcceptor;
use std::sync::Arc;

async fn make_router() -> Router {
    Router::new()
        .route("/status", get(|| async { "ok" }))
}

#[tokio::main]
async fn main() {
    // Load server cert and key
    let cert = load_cert("server.crt").expect("failed to load cert");
    let key = load_private_key("server.key").expect("failed to load key");

    // Configure mTLS: require and verify client certs
    let mut root_store = RootCertStore::empty();
    let client_ca = load_cert("ca.crt").expect("failed to load CA");
    root_store.add(&client_ca).expect("invalid CA cert");

    let server_config = ServerConfig::builder()
        .with_safe_defaults()
        .with_no_client_auth() // we will require client auth below
        .with_single_cert(vec![cert], key)
        .expect("bad server cert");

    // Enforce client authentication
    let server_config = Arc::new(ServerConfig {
        root_certificates: root_store,
        // Ensure client auth is required by setting client_auth_root_subjects
        ..server_config
    });

    let tls_acceptor = TlsAcceptor::from(server_config);
    let make_svc = make_router.into_make_service();

    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    let listener = tokio::net::TcpListener::bind(addr).await.unwrap();
    println!("Listening on https://{}", addr);

    loop {
        let (stream, _) = listener.accept().await.unwrap();
        let acceptor = tls_acceptor.clone();
        tokio::spawn(async move {
            let tls_stream = acceptor.accept(stream).await.unwrap();
            // axum server serving on tls_stream
            axum::serve(tls_stream, make_svc.clone()).await.unwrap();
        });
    }
}

fn load_cert(path: &str) -> Result, Box> {
    let mut reader = std::io::BufReader::new(std::fs::File::open(path)?);
    let certs = rustls_pemfile::certs(&mut reader)?;
    certs.into_iter().next().ok_or("no cert found".into())
}

fn load_private_key(path: &str) -> Result, Box> {
    let mut reader = std::io::BufReader::new(std::fs::File::open(path)?);
    let mut keys = rustls_pemfile::pkcs8_private_keys(&mut reader)?;
    keys.pop().ok_or("no key found".into())
}

Ensure all structs used for deserialization have explicit default or required fields, and avoid unsafe reads. Below is a safe Axum handler example that works correctly under mTLS:

use axum::{routing::post, Json, Router};
use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize, Debug, Default)]
struct Input {
    #[serde(default)]
    value: u64,
}

#[derive(Serialize)]
struct Output {
    received: u64,
    checksum: u64,
}

async fn handle(Json(payload): Json) -> Json
 {
    // Explicit initialization prevents uninitialized memory exposure
    let value = payload.value; // guaranteed initialized by serde default
    let checksum = value.wrapping_mul(31337);
    Json(Output { received: value, checksum })
}

fn app() -> Router {
    Router::new()
        .route("/process", post(handle))
}

// You can mount this behind the mTLS acceptor configured above
// axum::serve(tls_stream, app().into_make_service()).await.unwrap();

If you must work with raw bytes, always initialize buffers explicitly and avoid unsafe blocks unless you can guarantee no stale data is present:

use axum::{body::Bytes, response::IntoResponse};

async fn safe_bytes_endpoint(Bytes(bytes): Bytes) -> impl IntoResponse {
    // Initialize a fixed-size output buffer explicitly
    let mut out = [0u8; 32];
    let len = bytes.len().min(out.len());
    out[..len].copy_from_slice(&bytes[..len]);
    // Ensure the rest remains zeroed rather than uninitialized
    format!("processed {len} bytes")
}

Additionally, configure mTLS to enforce client certificate validation so that only authenticated clients reach these handlers, reducing the risk of crafted requests that could trigger unusual buffer reuse patterns.