Credential Stuffing in Actix with Hmac Signatures

Credential stuffing takes advantage of reused passwords across services. When an Actix API uses Hmac Signatures for request authentication but does not adequately protect the initial credential validation step, attackers can bypass signature checks by leveraging valid username/password pairs obtained from other breaches.

Hmac Signatures typically involve a shared secret combined with request metadata (method, path, timestamp, nonce) to produce a signature sent in a header. In Actix, if the server validates the Hmac correctly but still allows unlimited login or token exchange attempts without rate limiting or multi-factor enforcement, attackers can automate submissions of stolen credentials and iterate through them rapidly. The signature mechanism may appear strong, but the overall authentication flow remains weak where credential validation is guessable or unthrottled.

Another risk pattern arises when the Hmac is computed over a subset of request data that does not include the timestamp or a server-side nonce. Without these anti-replay protections, intercepted requests can be replayed to reuse valid credentials. Even if the signature itself is valid, replayed requests may be accepted if the server does not enforce strict one-time-use timestamps or nonce tracking.

During a black-box scan, middleBrick tests for these conditions by submitting requests with reused credentials and varying nonces or timestamps, checking whether the server enforces rate limits, rejects replayed signatures, and ties authentication state to session or token binding. Findings often highlight missing account lockout, absent CAPTCHA, or weak origin checks on the credential verification endpoint, even when Hmac Signatures are in place.

Additionally, if the application exposes an unauthenticated endpoint that reports whether a username exists (user enumeration), attackers can iteratively probe valid usernames before launching credential stuffing. The presence of Hmac Signatures on other endpoints does not prevent this enumeration if the login or verification response differs based on account existence.

middleBrick’s 12 security checks run in parallel to surface these issues, including Authentication, BOLA/IDOR, Rate Limiting, and Input Validation. The scan identifies whether Hmac implementations are coupled with proper throttling, replay prevention, and secure credential handling, providing prioritized findings and remediation guidance mapped to frameworks like OWASP API Top 10 and PCI-DSS.

Remediation centers on tightening the authentication flow around Hmac Signatures, ensuring each request is tied to a short-lived context and that credential validation is protected against automation.

Include timestamp and nonce in Hmac calculation

Ensure the signature covers a server-supplied nonce or a strict timestamp to prevent replay. Example in Actix using chrono and hmac:

use actix_web::{web, HttpResponse, HttpRequest};
use hmac::{Hmac, Mac};
use sha2::Sha256;
type HmacSha256 = Hmac<Sha256>;
use chrono::{Utc, Duration};

fn verify_hmac(
    req: &HttpRequest,
    body: &str,
    provided_signature: &str,
    secret: &[u8],
) -> bool {
    let timestamp = req.headers().get("X-Timestamp")
        .and_then(|v| v.to_str().ok())
        .and_then(|s| s.parse::

use actix_web::middleware::Logger;
use actix_web::web::Data;
use std::collections::HashMap;
use std::sync::Mutex;

struct RateLimiter {
    attempts: Mutex<HashMap<String, Vec<chrono::DateTime<chrono::Utc>>>>
}

fn check_rate_limit(limiter: Data<RateLimiter>, key: String) -> bool {
    let mut attempts = limiter.attempts.lock().unwrap();
    let now = chrono::Utc::now();
    let window = now - chrono::Duration::seconds(60);
    let entry = attempts.entry(key.clone()).or_default();
    entry.retain(|&t| t > window);
    if entry.len() >= 10 {
        return false;
    }
    entry.push(now);
    true
}