Regex Dos in Actix with Hmac Signatures

Regex-based denial-of-service (Regex Dos) can occur in Actix web applications when route path patterns or extractor logic rely on regular expressions that exhibit catastrophic backtracking. When such routes are combined with Hmac Signatures used for request authentication, an attacker can craft inputs that cause exponential time complexity in signature verification or parameter extraction, leading to severe latency or process exhaustion. This specific combination becomes dangerous because Hmac validation often runs on the full request path, query string, or headers, and if those inputs are processed by vulnerable regexes, the computational cost multiplies.

In Actix, routes can define path parameters using regex patterns (e.g., web::resource("/api/{id:^[0-9]+(?!0$)}{len=1,64}$")). If the regex is poorly constructed—such as using nested quantifiers like (a+)+ on user-controlled segments—an attacker can send a long string of characters that causes the regex engine to backtrack extensively. When Hmac Signatures are validated in middleware or an extractor, the request may already have been routed and partially parsed, meaning the expensive regex work has occurred before the signature is even checked. This means the server does expensive work for unauthenticated or malicious requests, enabling resource exhaustion without consuming authenticated capacity.

Moreover, Hmac Signatures often include query parameters or path segments in the signed string. If those values are later matched against complex regex rules in handlers or guards, an attacker can probe for patterns that maximize backtracking while still producing a plausible signed request. Because Actix processes routes and extractors in a pipeline, the interaction between routing regexes and Hmac verification creates a window where CPU time is spent before authentication decisions are finalized. This can amplify the impact of a single malicious request, especially when combined with connection-level concurrency limits.

The risk is particularly pronounced when signature schemes embed variable-length nonces or timestamps into paths or headers that are then validated with overly permissive regexes. For example, a signature that includes a timestamp may be parsed with a regex that attempts to validate multiple optional formats, introducing alternation and repetition that can be abused. In scanning terms, middleBrick checks for unauthenticated endpoints and input validation issues; when Regex Dos intersects with Hmac flows, findings may highlight path-based DoSS risks and insecure consumption patterns that expose expensive parsing logic.

Real-world analogues include CVE scenarios where crafted paths trigger exponential runtime in route matchers. Remediation requires avoiding regex features known for catastrophic backtracking and ensuring Hmac validation occurs on normalized, bounded inputs. middleBrick’s checks for Input Validation and Unsafe Consumption can surface these risks when scanning Actix services, providing prioritized findings with severity and remediation guidance rather than attempting to fix the runtime behavior directly.

To mitigate Regex Dos risks in Actix when using Hmac Signatures, focus on simplifying route definitions, bounding input sizes, and moving expensive validation after lightweight checks. Use simple path extractors where possible, validate length and character sets before regex matching, and ensure Hmac verification operates on canonical, trimmed, and bounded inputs. Below are concrete patterns and code examples for secure Actix implementations.

1. Prefer built-in extractors over custom regex paths

Avoid defining routes with complex regex path patterns. Use typed extractors and constrain numeric or alphanumeric segments with simple guards.

use actix_web::{web, App, HttpResponse, HttpServer, Responder};
use hmac::{Hmac, Mac};
use sha2::Sha256;

type HmacSha256 = Hmac;

// Safe: use typed path parameters and length checks instead of open regex
async fn get_item(
    id: web::Path,
    query: web::Query>,
) -> impl Responder {
    let id = id.into_inner();
    // Bounded length and character class to prevent backtracking
    if id.len() > 32 || !id.chars().all(|c| c.is_ascii_alphanumeric()) {
        return HttpResponse::BadRequest().body("invalid id");
    }
    // Hmac verification on canonical string
    let mut mac = HmacSha256::new_from_slice(b"secret-key").expect("valid key");
    mac.update(id.as_bytes());
    // signature from headers or query, compare in constant time
    HttpResponse::Ok().body("ok")
}

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

2. Validate and normalize before Hmac verification

Normalize inputs (trim, limit length, reject unexpected characters) before building the signed string. This prevents regex-heavy normalization logic from being invoked on attacker-controlled data during route matching.

fn normalize_and_sign(input: &str) -> (String, String) {
    // Trim and enforce bounds to avoid excessive work
    let trimmed = input.trim();
    let bounded = if trimmed.len() > 128 { &trimmed[..128] } else { trimmed };
    // Build canonical form without complex regex
    let canonical = bounded.replace(" ", "");
    let mut mac = HmacSha256::new_from_slice(b"secret-key").expect("valid key");
    mac.update(canonical.as_bytes());
    let result = mac.finalize();
    let signature = hex::encode(result.into_bytes());
    (canonical, signature)
}

async fn verify_request(
    path: web::Path,
    headers: web::Header,
) -> HttpResponse {
    let (canonical, sig) = normalize_and_sign(&path);
    // Perform constant-time comparison elsewhere; placeholder
    HttpResponse::Ok().body(canonical)
}

3. Use middleware to reject malformed patterns early

Implement lightweight middleware that checks path and header lengths and known dangerous patterns before routing proceeds to Hmac validation. This reduces work for clearly malicious payloads.

use actix_web::dev::{Service, ServiceResponse, Transform};
use actix_web::Error;
use futures::future::{ok, Ready};

pub struct GuardMiddleware;

impl Transform for GuardMiddleware
where
    S: Service, Error = Error>,
    S::Future: 'static,
    B: 'static,
{
    type Response = ServiceResponse;
    type Error = Error;
    type InitError = ();
    type Transform = GuardMiddlewareImpl;
    type Future = Ready>;

    fn new_transform(&self, service: S) -> Self::Future {
        ok(GuardMiddlewareImpl { service })
    }
}

pub struct GuardMiddlewareImpl {
    service: S,
}

impl Service for GuardMiddlewareImpl
where
    S: Service, Error = Error>,
    S::Future: 'static,
    B: 'static,
{
    type Response = ServiceResponse;
    type Error = Error;
    type Future = futures::future::LocalBoxFuture<'static, Result>;

    fn poll_ready(&self, cx: &mut core::task::Context<'_>) -> std::task::Poll> {
        self.service.poll_ready(cx)
    }

    fn call(&self, req: ServiceRequest) -> Self::Future {
        // Lightweight guard: reject paths longer than a threshold or containing suspicious repeats
        let path = req.path();
        if path.len() > 256 || path.contains("../") {
            let response = HttpResponse::BadRequest().body("rejected");
            return Box::pin(async { Ok(req.into_response(response)) });
        }
        let fut = self.service.call(req);
        Box::pin(async move {
            let res = fut.await?;
            Ok(res)
        })
    }
}