Stack Overflow in Actix with Hmac Signatures

Actix Web is a popular Rust framework for building high-performance HTTP services. When HMAC signatures are used to authenticate requests, a Stack Overflow–style attack can occur if the server performs signature verification on very large payloads or streams without strict size limits. In such setups, an attacker can send a request with an enormous body or many repeated parts, causing the application to consume disproportionate memory and CPU while computing the HMAC, leading to resource exhaustion. This is not a flaw in HMAC itself, but a consequence of unbounded input handling in Actix routes that validate signatures.

Consider an Actix handler that reads the entire request body before verifying an HMAC provided in a custom header. If the endpoint accepts streaming bodies or large file uploads and the HMAC verification logic processes the full payload synchronously, a single malicious request can saturate available worker threads. Because Actix uses an async runtime backed by a thread pool, this can manifest as thread starvation or increased latency, effectively creating a denial-of-service vector similar to a Stack Overflow attack on the service layer.

In the context of middleBrick’s security checks, this pattern would be flagged under the Rate Limiting and Input Validation categories. The scanner detects that the endpoint lacks explicit size constraints and performs HMAC verification on unbounded input. Even though HMAC ensures integrity and authenticity, the runtime behavior of Actix when handling oversized payloads can expose a denial-of-service surface. The scan also notes that without per-request body size limits or streaming signature validation, the service remains vulnerable to resource-based attacks that do not require authentication or exploit the cryptographic properties of HMAC.

An example of a vulnerable Actix route is one that uses web::block to offload HMAC computation to a thread pool while reading the full payload into memory. If no limit is enforced on the payload size, an attacker can send gigabytes of data, forcing the thread pool to process large blocks and increasing memory pressure. MiddleBrick’s checks examine whether the application constrains request size before invoking CPU-intensive operations like HMAC verification, and whether backpressure is applied through Actix’s payload configuration.

To mitigate Stack Overflow–style risks when using HMAC signatures in Actix, apply strict request size limits before performing any cryptographic work. Use Actix’s built-in payload configuration and early validation to reject oversized bodies immediately. This reduces memory and CPU consumption and prevents unbounded processing tied to signature verification.

Below is a concrete, working example of an Actix handler that verifies an HMAC signature safely. The code reads the body only up to a defined limit, computes the signature in a blocking thread, and returns an error if the payload is too large or the signature is invalid.

use actix_web::{web, App, HttpResponse, HttpServer, Error};
use actix_web::http::header::HeaderValue;
use hmac::{Hmac, Mac};
use sha2::Sha256;
use std::str;

type HmacSha256 = Hmac;

async fn verify_hmac(
    payload: web::Bytes,
    signature_header: String,
) -> Result {
    // Enforce a maximum body size before processing
    const MAX_BODY_SIZE: usize = 1024 * 256; // 256 KiB
    if payload.len() > MAX_BODY_SIZE {
        return Ok(HttpResponse::PayloadTooLarge()
            .body("Request body exceeds maximum allowed size"));
    }

    let key = b"my-secret-key";
    let mut mac = HmacSha256::new_from_slice(key)
        .map_err(|_| actix_web::error::ErrorInternalServerError("HMAC init failed"))?;
    mac.update(&payload);

    let signature = signature_header
        .strip_prefix("HMAC ")
        .unwrap_or(&signature_header);
    let decoded_sig = hex::decode(signature)
        .map_err(|_| actix_web::error::ErrorBadRequest("Invalid signature format"))?;

    if mac.verify_slice(&decoded_sig).is_ok() {
        Ok(HttpResponse::Ok().body("Signature valid"))
    } else {
        Ok(HttpResponse::Unauthorized().body("Invalid signature"))
    }
}

async fn upload_handler(
    body: web::Bytes,
    req: actix_web::HttpRequest,
) -> Result {
    match req.headers().get("X-API-Signature") {
        Some(hv) => {
            let sig = hv.to_str().unwrap_or("");
            verify_hmac(body, sig.to_string()).await
        }
        None => Ok(HttpResponse::BadRequest().body("Missing signature")),
    }
}

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

In this example, the request body is capped at 256 KiB before HMAC computation begins. This prevents large payloads from consuming thread-pool resources during signature verification. For streaming scenarios, consider validating signature metadata (such as a content-length header) before fully reading the body, or use middleware that enforces size limits globally.

The middleBrick CLI can be used to scan this Actix service and highlight missing size constraints and improper placement of HMAC verification. By running middlebrick scan <url>, you can detect whether your endpoints are susceptible to resource exhaustion when HMAC checks are involved, and the report will provide prioritized findings with remediation guidance tied to input validation and rate limiting checks.