Http Request Smuggling in Axum with Dynamodb

HTTP request smuggling arises when an API processes HTTP requests in a way that allows an attacker to smuggle a second request past front‑end protections by manipulating how requests are parsed, framed, or forwarded. In an Axum application that uses Amazon DynamoDB as a backend data store, the risk typically surfaces at the boundary where Axum routes and middleware hand parsed requests to downstream handlers that forward data to DynamoDB via the AWS SDK.

The Axum web framework itself does not introduce parsing deviations, but the way developers compose Axum routes, extract headers, and construct SDK inputs can create conditions where smuggling is possible. For example, if an endpoint accepts a client-supplied header or chunked body and then forwards that data to a DynamoDB operation without strict validation, an attacker may craft requests that are interpreted differently by Axum versus a downstream proxy or load balancer. This mismatch can allow a smuggled request to bypass intended authorization checks or reach an unintended endpoint before the request reaches DynamoDB.

Consider an Axum handler that reads a header such as x-user-id and uses it to build a DynamoDB GetItem input. If the handler does not validate and sanitize the header, and if a fronting proxy normalizes or splits requests differently, an attacker can inject an additional request path or method into the forwarded request stream. When the SDK builds the DynamoDB request, the injected path may map to a different logical operation, exposing data or changing behavior in ways the developer did not intend.

Because DynamoDB is a managed service, the smuggling impact is usually not server-side request forgery into DynamoDB itself, but rather a loss of request boundary control upstream of Axum that results in unintended DynamoDB operations. Common contributing patterns include inconsistent use of axum::extract::HeaderMap without strict allowlists, forwarding raw bytes or bodies to SDK inputs, and failing to enforce strict content-length or transfer-encoding parsing on inputs that ultimately form DynamoDB keys or condition expressions.

To detect this class of issue, middleBrick’s 12 security checks include BOLA/IDOR, Input Validation, and Unsafe Consumption scans, which exercise header and body manipulation against Axum endpoints that produce DynamoDB requests. These tests highlight where request boundaries can be smuggled and where DynamoDB inputs may be driven by attacker-controlled data without sufficient validation.

Remediation focuses on strict request parsing, explicit header allowlists, and defensive construction of DynamoDB inputs so that smuggling attempts cannot alter the intended operation. The following patterns are recommended for Axum services that build DynamoDB requests.

1. Strict header extraction and allowlisting

Do not trust client-supplied headers that influence DynamoDB keys or condition expressions. Extract only the headers you explicitly expect, and reject requests with unexpected or extra values.

use axum::extract::HeaderMap;
use aws_sdk_dynamodb::types::AttributeValue;

async fn get_user(
    headers: HeaderMap,
    user_id: String,
) -> Result {
    let user_header = headers.get("x-user-id");
    let user_header = user_header.ok_or_else(|| (axum::http::StatusCode::BAD_REQUEST, "missing x-user-id".to_string()))?;
    let user_id = user_header.to_str().map_err(|_| (axum::http::StatusCode::BAD_REQUEST, "invalid x-user-id".to_string()))?;

    let item = client
        .get_item()
        .table_name("Users")
        .key("user_id", AttributeValue::S(user_id.to_string()))
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    // handle item
    Ok((axum::http::StatusCode::OK, item))
}

2. Validate and normalize request bodies before building DynamoDB inputs

When a request body influences DynamoDB put or update operations, validate the schema and length, and avoid concatenating or modifying the raw body before deserialization.

use axum::Json;
use serde::Deserialize;
use aws_sdk_dynamodb::types::AttributeValue;

#[derive(Deserialize)]
struct CreateUser {
    user_id: String,
    email: String,
}

async fn create_user(
    Json(payload): Json,
) -> Result {
    if payload.user_id.len() > 256 {
        return Err((axum::http::StatusCode::BAD_REQUEST, "user_id too long".to_string()));
    }
    if !payload.email.contains('@') {
        return Err((axum::http::StatusCode::BAD_REQUEST, "invalid email".to_string()));
    }

    let item = client
        .put_item()
        .table_name("Users")
        .item("user_id", AttributeValue::S(payload.user_id))
        .item("email", AttributeValue::S(payload.email))
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    Ok((axum::http::StatusCode::CREATED, item))
}

3. Avoid forwarding raw bytes or chunked transfer interpretations

When you must accept arbitrary body content, first consume and validate the full body before using it to construct DynamoDB inputs. Do not stream or forward bytes in a way that a proxy may re-chunk, which is a common smuggling vector.

use axum::body::Bytes;
use aws_sdk_dynamodb::types::AttributeValue;

async fn store_raw(
    bytes: Bytes,
) -> Result {
    let body = bytes.to_vec();
    if body.len() > 1_048_576 {
        return Err((axum::http::StatusCode::PAYLOAD_TOO_LARGE, "body too large".to_string()));
    }
    // Validate structure before using as DynamoDB attribute
    let item = client
        .put_item()
        .table_name("Data")
        .item("id", AttributeValue::B(AttributeValue::new().b(body)))
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    Ok((axum::http::StatusCode::ACCEPTED, item))
}

4. Enforce strict routing and method handling

Ensure that Axum’s router does not accidentally expose methods or paths that should be restricted. Explicitly define allowed methods and use nested routers to separate concerns that ultimately map to different DynamoDB tables or operations.

use axum::routing::get;
use axum::Router;

fn app() -> Router {
    Router::new()
        .route("/users/:user_id", get(get_user))
        .route("/users", post(create_user))
}

By combining these practices—explicit header allowlisting, strict body validation, and controlled DynamoDB input construction—you reduce the risk that request boundary manipulation reaches the DynamoDB layer. middleBrick scans can verify that these defenses are present and that no smuggling vectors remain in the unauthenticated attack surface.