Http Request Smuggling in Actix with Dynamodb

HTTP request smuggling becomes relevant when an Actix-based service sits in front of or behind a proxy or load balancer and forwards requests to a backend that interprets messages as a DynamoDB API call. Although Actix does not implement the protocol that DynamoDB uses directly (it uses HTTP as a transport for JSON), a common pattern is to proxy HTTP requests to a service that translates them into DynamoDB operations. If header parsing and message framing are inconsistent between the front-end and the back-end, an attacker can craft requests where one layer consumes the message differently than the other, causing request splitting, request injection, or response splitting.

In this context, the risk arises when Actix routes or forwards requests to an upstream service that speaks DynamoDB’s JSON-over-HTTPS protocol and header handling diverges. For example, if Actix buffers or normalizes headers differently than the upstream, an attacker can send a request with multiple Content-Length or Transfer-Encoding headers. The front-end may treat the request as a single message, while the back-end may parse it as two separate messages. This can allow an attacker to smuggle a second request into the same connection, potentially reaching an authenticated downstream handler or causing unauthorized DynamoDB operations.

A concrete scenario: an API endpoint in Actix accepts a JSON body intended for DynamoDB operations (such as GetItem or PutItem). If the endpoint forwards this to another service without strictly validating or normalizing headers, and if the upstream service uses chunked encoding while the proxy uses Content-Length, an attacker can inject a second request that references a different DynamoDB table or uses different credentials. The upstream service may process the smuggled request in the context of the same connection, leading to unauthorized data access or privilege escalation. This illustrates why consistent header parsing and strict message framing are essential when combining Actix with any backend that exposes DynamoDB-like semantics over HTTP.

Because middleBrick tests unauthenticated attack surfaces and checks for SSRF and related injection issues among its 12 security checks, this class of misconfiguration can be detected during a scan. A scan will highlight inconsistencies in how headers are handled and surface the risk of request smuggling when front- and back-end interpretations diverge.

To mitigate request smuggling when Actix interacts with a DynamoDB-backed service, enforce strict header normalization and avoid forwarding ambiguous messages. Use a strict content-length policy, reject requests with hop-by-hop headers, and ensure the same parsing rules are applied at every layer. The following examples show how to implement robust parsing and forwarding in Actix.

First, create a function that validates and normalizes headers before forwarding. This function removes ambiguous hop-by-hop headers and ensures a single Content-Length header is present or that Transfer-Encoding is used consistently, but not both.

use actix_web::{http::HeaderValue, HttpRequest, HttpResponse};
use std::collections::HashMap;

/// Normalize headers to reduce request smuggling risk.
/// Removes hop-by-hop headers and ensures consistent framing.
fn normalize_headers(req: &HttpRequest) -> HashMap {
    let hop_by_hops = [
        "connection", "keep-alive", "proxy-authenticate", "proxy-authorization",
        "te", "trailers", "transfer-encoding", "upgrade",
    ];
    let mut headers = HashMap::new();
    for (name, value) in req.headers() {
        let name_l = name.as_str().to_lowercase();
        if hop_by_hops.contains(&name_l.as_str()) {
            continue;
        }
        if let Ok(v) = value.to_str() {
            headers.insert(name_l, v.to_string());
        }
    }
    // Ensure Content-Length is set for clarity when forwarding.
    if !headers.contains_key("content-length") && req.body().size() > 0 {
        headers.insert("content-length".to_string(), req.body().size().to_string());
    }
    headers
}

Second, when forwarding requests to the DynamoDB-compatible backend, use a strict client that does not automatically adopt conflicting framing. Below is an example of building a forwarded request with a single Content-Length header and no Transfer-Encoding, avoiding the conditions that enable smuggling.

use actix_web::web::Bytes;
use reqwest::Client;
use std::time::Duration;

async fn forward_to_dynamodb(
    body: Bytes,
    upstream_url: &str,
    auth_token: &str,
) -> Result {
    let client = Client::builder()
        .timeout(Duration::from_secs(10))
        .build()?;

    let mut request = client.post(upstream_url)
        .header("Content-Length", body.len())
        .header("Authorization", format!("Bearer {}", auth_token))
        .header("Content-Type", "application/x-amz-json-1.0");

    // Explicitly set the body as bytes to avoid chunked encoding ambiguity.
    request = request.body(body.to_vec());

    request.send().await
}

Third, configure Actix routes to reject suspicious requests early. For example, reject requests that include both Content-Length and Transfer-Encoding, or that have mismatched content length values. This prevents malformed or ambiguous messages from being processed or forwarded.

use actix_web::{web, Error, HttpRequest, HttpResponse};

async fn validate_no_smuggling(headers: &actix_web::http::HeaderMap) -> Result<(), HttpResponse> {
    let has_cl = headers.contains_key("content-length");
    let has_te = headers.contains_key("transfer-encoding");

    if has_cl && has_te {
        return Err(HttpResponse::BadRequest().body("Conflicting headers: Content-Length and Transfer-Encoding"));
    }

    if let Some(cl_a) = headers.get("content-length") {
        if let Ok(cl_str) = cl_a.to_str() {
            if cl_str.parse::().is_err() {
                return Err(HttpResponse::BadRequest().body("Invalid Content-Length"));
            }
        }
    }

    Ok(())
}

async fn api_handler(body: web::Bytes, req: HttpRequest) -> Result {
    validate_no_smuggling(req.headers()).await?;
    let headers = normalize_headers(&req);
    // Use headers and body to construct a safe request to DynamoDB backend.
    // Example forwarding omitted for brevity.
    Ok(HttpResponse::Ok().body("Validated and ready to forward"))
}

These steps reduce the risk of request smuggling by ensuring consistent message framing and header interpretation between Actix and the DynamoDB-facing backend. They complement the checks provided by middleBrick, which can identify header inconsistencies and SSRF-related risks in unauthenticated scans.