Fix Logic Flow Bypass in Actix Web

Logic flow bypass in Actix Web occurs when state transitions are improperly managed, allowing an attacker to skip critical checkpoints like authentication, payment, or validation. If your handlers assume a prerequisite was met without server-side verification, you've left the door open. We fix this by enforcing strict state machines via session-backed flags or cryptographically signed state tokens.

The Vulnerable Pattern

use actix_web::{post, web, HttpResponse, Responder};
use serde::Deserialize;
#[derive(Deserialize)]
struct FinalizeRequest {
user_id: i32,
is_payment_verified: bool, // VULNERABLE: Client-controlled logic state
}
#[post(“/finalize”)]
async fn finalize_order(payload: web::Json) -> impl Responder {
if payload.is_payment_verified {
HttpResponse::Ok().body(“Order processed successfully”)
} else {
HttpResponse::BadRequest().body(“Payment required”)
}
}

The Secure Implementation

The vulnerability lies in trusting the client to report its own progress through a multi-step workflow. In the vulnerable snippet, an attacker can simply send 'is_payment_verified: true' in the JSON body to bypass the payment gateway. The secure implementation uses 'actix-session' to track the workflow state server-side. The '/finalize' endpoint checks a session variable that can only be set by the actual payment callback handler. Always use server-side session storage or signed JWTs to maintain state, and ensure state is invalidated (cleared) once the logic flow is completed to prevent reuse.

use actix_web::{post, web, HttpResponse, Error};
use actix_session::Session;
#[post(“/finalize”)]
async fn finalize_order(session: Session) -> Result<HttpResponse, Error> {
// SECURE: State is verified against server-side session, not client input
let payment_status = session.get::(“payment_confirmed”)?.unwrap_or(false);
if !payment_status {
    return Ok(HttpResponse::Forbidden().body("Illegal state transition: Payment not confirmed"));
}

// Clear state after completion to prevent replay
session.remove("payment_confirmed");

Ok(HttpResponse::Ok().body("Order processed successfully"))
}