Dns Cache Poisoning in Actix with Dynamodb

DNS cache poisoning is a network-layer attack where false DNS responses cause a resolver to cache an incorrect IP mapping. In an Actix web service that relies on Amazon DynamoDB for data storage, this attack surface intersects with application behavior in several concrete ways.

When an Actix application resolves a backend hostname—such as a DynamoDB endpoint in a region like us-east-1—it typically depends on the operating system or runtime DNS resolver. If an attacker can poison the resolver cache, the Actix service may be directed to a malicious host that impersonates DynamoDB. Because the application trusts the resolved address, it may send database requests over an unencrypted or attacker-controlled channel, effectively bypassing intended network segmentation.

DynamoDB does not directly participate in DNS resolution, but the way an Actix client is configured to reach DynamoDB determines exposure. For example, if the client uses a region-specific hostname such as dynamodb.us-east-1.amazonaws.com, a poisoned cache entry can redirect traffic to a rogue server that mimics the AWS endpoint. In environments where TLS certificate validation is not strictly enforced or where custom HTTP clients skip hostname verification, the poisoned DNS entry can lead to traffic interception or manipulation. This is particularly relevant when the Actix runtime uses a minimal DNS cache with short time-to-live (TTL) settings, causing frequent re-resolution and increasing the window for exploitation.

Additionally, if the Actix application integrates with DynamoDB using an SDK that relies on the system resolver, and the deployment environment shares DNS infrastructure with other services, a single poisoned cache can affect multiple components. An attacker might craft a response that maps the DynamoDB hostname to an IP under their control, then proxy or modify requests to gain access to sensitive data or inject malicious instructions. Because the vulnerability resides in the network path and caching behavior rather than in DynamoDB itself, the risk is realized at the point where the Actix client establishes a connection.

To detect this class of issue using middleBrick, you can scan the public-facing API surface of your Actix service without authentication. The scan will flag weaknesses in network-level protections and highlight missing transport hardening, such as inadequate DNS validation or lack of explicit TLS hostname verification when communicating with DynamoDB.

Remediation focuses on ensuring that the Actix runtime validates DNS responses and enforces strict, application-controlled resolution when interacting with DynamoDB. Below are concrete, idiomatic examples using the official AWS SDK for Rust and Actix web.

1. Enforce TLS and explicit endpoint configuration

Configure the DynamoDB client to use a known, hardcoded endpoint and require TLS. This reduces reliance on runtime DNS for critical resolution decisions.

use aws_config::meta::region::RegionProviderChain;
use aws_sdk_dynamodb::Client;
use std::net::SocketAddr;

async fn create_dynamodb_client() -> Client {
    let region_provider = RegionProviderChain::first_try(Some("us-east-1".parse().unwrap()));
    let config = aws_config::from_env().region(region_provider).load().await;
    // Explicit endpoint ensures the client does not follow a potentially poisoned DNS redirect
    let endpoint = "https://dynamodb.us-east-1.amazonaws.com";
    Client::new(&config).with_endpoint_url(endpoint)
}

2. Custom DNS resolver with validation in Actix

Use a trusted DNS-over-HTTPS (DoH) resolver to obtain IPs for DynamoDB and validate responses before use. This example integrates a synchronous resolver into Actix configuration.

use actix_web::{web, App, HttpServer, Responder};
use trust_dns_resolver::Resolver;
use trust_dns_resolver::config::{ResolverConfig, ResolverOpts, NameServerConfig};
use std::net::IpAddr;

async fn resolve_dynamodb_address() -> std::io::Result {
    let mut opts = ResolverOpts::default();
    opts.attempts = 2;
    opts.timeout = std::time::Duration::from_secs(2);
    // Use a reliable DoH server to avoid poisoned cache
    let ns = NameServerConfig::new("8.8.8.8:443".parse().unwrap(), trust_dns_resolver::xfer::DnsHandle::new);
    let config = ResolverConfig::from_parts(None, vec![ns], vec![]);
    let resolver = Resolver::new(config, opts)?;
    let response = resolver.lookup_ip("dynamodb.us-east-1.amazonaws.com")?;
    response.iter().next().ok_or_else(|| std::io::Error::new(std::io::ErrorKind::NotFound, "no IP"))
}

async fn index() -> impl Responder {
    match resolve_dynamodb_address().await {
        Ok(addr) => format!("Resolved DynamoDB address: {}", addr),
        Err(e) => format!("Resolution failed: {}", e),
    }
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    HttpServer::new(|| App::new().route("/", web::get().to(index)))
        .bind("127.0.0.1:8080")?
        .run()
        .await
}

3. Pin certificate and hostname during HTTPS requests

When the Actix client communicates with DynamoDB via HTTPS, enforce certificate pinning and strict hostname checks to mitigate the impact of a poisoned DNS record that might present a valid but fraudulent certificate.

use reqwest::Client;
use std::time::Duration;

async fn create_pinned_client() -> Client {
    let mut builder = reqwest::Client::builder()
        .timeout(Duration::from_secs(5))
        .danger_accept_invalid_certs(false);
    // Example of pinning a certificate hash (SHA-256) for dynamodb.us-east-1.amazonaws.com
    // builder = builder.add_root_certificate(pinned_cert);
    builder.build().unwrap()
}

4. Monitor and rotate resolution paths

Implement runtime checks that validate the resolved IP against an allowlist or a second, independent resolution source. Combine this with regular rotation of DNS servers used by the resolver to reduce the effectiveness of cache poisoning attacks targeting a specific DNS provider.

middleBrick can complement these efforts by scanning your Actix service endpoints continuously. Using the CLI (middlebrick scan <url>) or the GitHub Action, you can ensure that exposed surfaces do not rely on insecure DNS resolution when reaching DynamoDB.