Privilege Escalation in Actix with Hmac Signatures

In Actix web applications, using Hmac signatures for request authentication can introduce privilege escalation risks when the implementation does not strictly validate the scope and context of the signed payload. A common pattern is to sign user roles or permissions as part of a JWT or custom Hmac payload and use that signature to authorize access to elevated endpoints. If the server only verifies the cryptographic validity of the Hmac but does not re-check the associated authorization claims on each request, an attacker who obtains a valid signature can manipulate path parameters, headers, or query strings to escalate privileges.

Consider an Actix handler that relies on a client-supplied header containing a user role, for example X-User-Role: admin, with an Hmac signature in X-Signature. If the server reuses a static secret to verify the Hmac but does not bind the role claim to a server-side session or enforce role checks via middleware, an attacker can change the header to X-User-Role: superadmin while keeping a valid signature for a different role. Because the signature may have been generated over a subset of headers or a limited set of claims, the server may accept the modified request as authorized, effectively escalating the user’s privileges.

Another scenario involves ID-style parameters in URLs where an Hmac covers an identifier but not the intended action. For example, an endpoint GET /accounts/{id}/export might use an Hmac signed over id and a timestamp, but if the server does not also validate that the authenticated principal owns that id, an attacker can iterate over IDs and use the same valid Hmac pattern to export data belonging to other users. This is a BOLA/IDOR behavior enabled by trusting the signature alone for authorization rather than coupling it with resource ownership checks.

These patterns are relevant to the broader middleBrick scan coverage, which includes Authentication, BOLA/IDOR, and BFLA/Privilege Escalation checks running in parallel. The scanner evaluates whether an API endpoint trusts client-controlled claims after signature verification without additional authorization enforcement. Because Actix applications often handle high-privilege operations, such unchecked delegation can lead to unauthorized access to administrative functions or sensitive data, which would be flagged with high severity by the security risk scoring engine.

To mitigate privilege escalation when using Hmac signatures in Actix, ensure that the signature covers all inputs that affect authorization and that authorization checks are performed independently of signature validation. The following examples show a secure approach: include user identity, intended action, and resource identifiers in the signed payload; verify the signature; then enforce role- and ownership-based checks using Actix middleware or guards.

Example 1: Signing and verifying a payload that includes role and resource scope.

use hmac::{Hmac, Mac};
use sha2::Sha256;
use actix_web::{web, HttpRequest, HttpResponse};
use serde::{Deserialize, Serialize};

type HmacSha256 = Hmac;

#[derive(Serialize, Deserialize)]
struct AuthPayload {
    sub: String,
    role: String,
    resource: String,
    exp: usize,
}

fn verify_hmac_signature(secret: &[u8], payload: &AuthPayload, received_sig: &str) -> bool {
    let mut mac = HmacSha256::new_from_slice(secret).expect("Hmac can take key of any size");
    let data = serde_json::to_string(payload).expect("serialize payload");
    mac.update(data.as_bytes());
    match hex::decode(received_sig) {
        Ok(sig_bytes) => mac.verify_slice(&sig_bytes).is_ok(),
        Err(_) => false,
    }
}

async fn export_handler(
    req: HttpRequest,
    body: web::Json,
) -> HttpResponse {
    let secret = include_bytes!("./hmac_secret.key");
    let received_sig = req.headers().get("X-Signature")
        .and_then(|v| v.to_str().ok())
        .unwrap_or("");

    if !verify_hmac_signature(secret, &body, received_sig) {
        return HttpResponse::Unauthorized().body("invalid signature");
    }

    // Enforce authorization after signature verification
    if body.role != "admin" {
        return HttpResponse::Forbidden().body("insufficient scope");
    }
    if !resource_owner_matches(&body.sub, &body.resource) {
        return HttpResponse::Forbidden().body("access to resource denied");
    }

    HttpResponse::Ok().body(format!("exporting {}", body.resource))
}

fn resource_owner_matches(user_id: &str, resource_id: &str) -> bool {
    // Server-side check linking user_id to resource_id
    true
}

Example 2: Middleware that validates Hmac for specific routes and binds claims to session context.

use actix_web::{dev::ServiceRequest, Error, middleware::Next};
use actix_web::http::header::HeaderValue;
use actix_web::web::Data;

async fn hmac_auth_middleware(
    req: ServiceRequest,
    next: Next<impl actix_web::body::MessageBody>
) -> Result<actix_web::dev::ServiceResponse, Error> {
    let secret = std::env::var("HMAC_SECRET").expect("HMAC_SECRET must be set");
    let sig = req.headers().get("X-Signature")
        .ok_or(actix_web::error::ErrorUnauthorized("missing signature"))?;
    let payload_header = req.headers().get("X-Payload")
        .ok_or(actix_web::error::ErrorUnauthorized("missing payload"))?;
    let payload_str = payload_header.to_str().map_err(|_| actix_web::error::ErrorBadRequest("invalid header"))?;
    let payload: AuthPayload = serde_json::from_str(payload_str).map_err(|_| actix_web::error::ErrorBadRequest("invalid json"))?;

    // Verify Hmac over the full payload as in Example 1
    let valid = verify_hmac_signature(secret.as_bytes(), &payload, sig.to_str().unwrap_or(""));
    if !valid {
        return Err(actix_web::error::ErrorUnauthorized("invalid hmac"));
    }

    // Attach verified claims to request extensions for downstream guards
    req.extensions_mut().insert(payload);
    next.call(req).await
}

Key remediation steps reflected in middleBrick findings include ensuring the Hmac covers all authorization-determining fields, avoiding signature reuse across different resources or actions, and enforcing server-side ownership and role checks. These controls reduce the risk of privilege escalation and align with findings mapped to OWASP API Top 10 and relevant compliance frameworks.