Dns Rebinding in Axum with Dynamodb

DNS rebinding occurs when an attacker manipulates DNS responses to make a victim browser believe a remote host is reachable at an attacker-controlled IP after the initial request. In an Axum service that uses Amazon DynamoDB, this can expose internal or restricted endpoints when the application resolves service endpoints dynamically or uses instance metadata endpoints for configuration.

Consider an Axum API that retrieves database configuration from a metadata service at startup or per-request. If the application performs a DNS lookup for a hostname such as metadata.service.internal and caches the result, an attacker on the same network can respond with a rebounded IP (e.g., 127.0.0.1 or a reachable internal service). Because Axum routes requests through your application logic, a crafted request can cause the app to issue a DynamoDB operation to an unintended endpoint. This can lead to confused deputy behavior where the application inadvertently accesses or modifies DynamoDB resources it should not, especially when combined with broad IAM permissions assigned to the host.

In a black-box scan using middleBrick, the DNS rebinding check tests whether an endpoint allows resolution changes between request and response. For Axum services integrated with DynamoDB, the risk is elevated when environment-based configuration (such as AWS_REGION or DYNAMODB_ENDPOINT) is mutable at runtime or when service discovery is used. An attacker can probe for DynamoDB-related endpoints by triggering HTTP requests that cause the server to perform AWS SDK calls, and the scan’s SSRF and input validation checks can surface whether rebinding leads to unauthorized DynamoDB operations.

An example scenario: an Axum handler obtains a table name from a query parameter and uses a region derived from a DNS-resolved service. If DNS is rebinded to a local AWS metadata service endpoint, the SDK may attempt operations against a local DynamoDB instance or a different region, leading to data exposure or privilege escalation. middleBrick’s LLM/AI security checks are not relevant here, but its input validation and property authorization tests help detect whether user-controlled inputs can influence AWS resource targeting in unsafe ways.

To mitigate DNS rebinding risks in an Axum application using DynamoDB, ensure that service endpoints and configuration are static and validated before use. Avoid runtime DNS-based discovery for AWS resources, and enforce strict input validation for any user-controlled data that influences AWS SDK calls.

Use hardcoded or securely injected configuration for the DynamoDB endpoint and region. In Axum, you can initialize the AWS SDK client once during application startup and reuse it across handlers. This prevents repeated DNS lookups and reduces the attack surface.

The following example shows a safe Axum setup with a statically configured DynamoDB client. It avoids runtime hostname resolution for AWS endpoints and validates table names against a whitelist pattern before use.

use aws_sdk_dynamodb::Client;
use aws_config::meta::region::RegionProviderChain;
use axum::{routing::get, Router};
use std::net::SocketAddr;
use aws_sdk_dynamodb::types::TableName;

async fn build_client() -> Client {
    let region_provider = RegionProviderChain::first_try(Some("us-east-1".parse().unwrap()))
        .or_default_provider()
        .and_then_with(|| {
            // Avoid dynamic DNS-based region resolution
            Some("us-east-1".parse().unwrap())
        });
    let config = aws_config::from_env().region(region_provider).load().await;
    Client::new(&config)
}

async fn get_item_handler(
    client: &Client,
    table_name: TableName,
    key: String,
) -> Result {
    // Validate table name against allowed pattern
    if !table_name.chars().all(|c| c.is_alphanumeric() || c == '-' || c == '_') {
        return Err("Invalid table name".into());
    }
    let output = client.get_item()
        .table_name(table_name)
        .key("id", aws_sdk_dynamodb::types::AttributeValue::S(key))
        .send()
        .await
        .map_err(|e| e.to_string())?;
    // Process output safely
    Ok(format!("{:?}", output.item))
}

#[tokio::main]
async fn main() {
    let client = build_client().await;
    let app = Router::new()
        .route("/item/:id", get(|path: axum::extract::Path| async move {
            let table_name = TableName::from("myapp_items"); // static table name
            get_item_handler(&client, table_name, path.into_inner()).await
        }));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

Additionally, apply middleware in Axum to reject requests with suspicious Host headers or unexpected origin values that could facilitate rebinding attacks. Combine this with IAM policies that limit DynamoDB permissions to the least privilege required, ensuring that even if rebinding occurs, the impact is contained.

middleBrick’s dashboard and CLI can be used to verify that your endpoints do not reflect user-controlled inputs into AWS SDK configurations. By scanning your Axum service, you can identify whether input validation and property authorization checks are sufficient to prevent unintended DynamoDB access.