Password Spraying in Actix with Hmac Signatures

Password spraying is an authentication abuse pattern where an attacker uses a small set of commonly used passwords against many accounts to avoid account lockouts. When an Actix web service uses Hmac Signatures for request authentication without additional protections, password spraying can be effective if the signature verification logic does not enforce rate limits or otherwise mitigate repeated invalid attempts.

In an Actix-based API, Hmac Signatures are typically computed on a per-request basis using a shared secret (or a derived user secret) combined with request metadata such as timestamp, nonce, and HTTP method. If an attacker can submit crafted requests that include a valid Hmac header but with guessed passwords embedded in the payload or URL parameters, the server may still process the authentication step and reveal whether a user exists or whether the password was incorrect. Because Hmac verification itself may be computationally successful even with a wrong password, the server can return different timing or status cues that an attacker can use to iteratively refine guesses across many accounts.

Moreover, if the Actix endpoint responsible for Hmac verification does not enforce per-user or per-IP rate limiting, an attacker can execute a low-and-slow password spraying campaign across hundreds of accounts within the 5–15 second scan window. The combination of Hmac Signatures and unprotected endpoints means that each request appears cryptographically valid, yet the underlying credential validation may be trivially bypassed by trying common passwords like Password1, Welcome123, or Spring2024. Without correlating Hmac validity with strict authentication rate controls, the attack surface expands to include both endpoint enumeration and credential stuffing.

From an API security scan perspective, middleBrick checks such an endpoint for BFLA/Privilege Escalation, Authentication, and Rate Limiting across 12 parallel checks, including specific analysis of how Hmac Signatures are accepted and validated. The scanner does not attempt to crack passwords but identifies whether the service leaks account existence via timing differences, status codes, or missing rate controls when Hmac-signed requests with sprayed passwords are submitted.

Remediation focuses on ensuring that Hmac verification is tightly coupled with robust authentication and rate-limiting behavior. In Actix, you should validate the Hmac signature early in the request pipeline and enforce per-user and per-IP rate limits before performing any password comparison. This prevents attackers from using valid Hmac headers to probe credentials at scale.

Below are two concrete Actix code examples. The first shows a protected endpoint that verifies an Hmac signature and returns an error if the request is malformed; the second demonstrates how to integrate a per-user rate limiter to mitigate password spraying. Both snippets use realistic dependencies and patterns common in Rust Actix services.

Example 1: Hmac Signature verification in Actix

use actix_web::{web, HttpRequest, HttpResponse, Error};
use hmac::{Hmac, Mac};
use sha2::Sha256;
use std::time::{SystemTime, UNIX_EPOCH};

type HmacSha256 = Hmac;

async fn verify_hmac(req: &HttpRequest, body: &[u8], secret: &[u8]) -> Result {
    let timestamp = req.headers().get("X-Timestamp")
        .and_then(|v| v.to_str().ok())
        .ok_or_else(|| actix_web::error::ErrorBadRequest("Missing timestamp"))?;
    let nonce = req.headers().get("X-Nonce")
        .and_then(|v| v.to_str().ok())
        .ok_or_else(|| actix_web::error::ErrorBadRequest("Missing nonce"))?;
    let signature = req.headers().get("X-Signature")
        .and_then(|v| v.to_str().ok())
        .ok_or_else(|| actix_web::error::ErrorBadRequest("Missing signature"))?;

    let mut mac = HmacSha256::new_from_slice(secret).map_err(|_| actix_web::error::ErrorBadRequest("Hmac init error"))?;
    let data = format!("{}:{}:{}", timestamp, nonce, String::from_utf8_lossy(body));
    mac.update(data.as_bytes());
    let computed = mac.finalize();
    let computed_bytes = computed.into_bytes();

    // Constant-time comparison to avoid timing leaks
    let valid = subtle::ConstantTimeEq::ct_eq(&computed_bytes[..], &hex::decode(signature).unwrap_or_default()[..]);
    Ok(valid == subtle::CtResult::Ok(true))
}

async fn protected_handler(req: HttpRequest, body: web::Bytes) -> HttpResponse {
    let secret = std::env::var("HMAC_SECRET").unwrap_or_else(|_| "defaultsecret".into()).into_bytes();
    match verify_hmac(&req, &body, &secret).unwrap_or(false) {
        true => HttpResponse::Ok().body("Authenticated"),
        false => HttpResponse::Unauthorized().body("Invalid signature"),
    }
}

Example 2: Per-user rate limiting to prevent password spraying

use actix_web::{web, HttpRequest, HttpResponse, Error};
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use std::time::{Duration, Instant};

// Simple in-memory rate limiter: (user_id, Vec)
type RateLimiter = Arc>>>;

fn is_allowed(limiter: &RateLimiter, user_id: &str, max_attempts: usize, window: Duration) -> bool {
    let mut guard = limiter.lock().unwrap();
    let entries = guard.entry(user_id.to_string()).or_default();
    let now = Instant::now();
    entries.retain(|t| now.duration_since(*t) < window);
    if entries.len() >= max_attempts {
        return false;
    }
    entries.push(now);
    true
}

async fn login_with_rate_limit(
    req: HttpRequest,
    body: web::Json,
    limiter: web::Data,
) -> HttpResponse {
    let user_id = match extract_user_id(&body.username) {
        Ok(id) => id,
        Err(_) => return HttpResponse::BadRequest().finish(),
    };
    if !is_allowed(&limiter, &user_id, 5, Duration::from_secs(60)) {
        return HttpResponse::TooManyRequests().body("Rate limit exceeded");
    }
    // Perform password verification here; early exit on failure
    if verify_password(&body.username, &body.password).is_ok() {
        HttpResponse::Ok().json(json!({ "token": "abc123" }))
    } else {
        HttpResponse::Unauthorized().body("Invalid credentials")
    }
}

#[derive(serde::Deserialize)]
struct LoginPayload {
    username: String,
    password: String,
}

These examples illustrate how to bind Hmac verification to authentication logic and add per-user rate limiting to reduce the effectiveness of password spraying. In production, you should also incorporate constant-time comparison, short nonces, replay protection, and centralized secret management.