Fix Improper Error Handling in Warp
Warp's rejection system is a double-edged sword. While it allows for modular error handling, failing to implement a global 'recover' filter often leads to Information Disclosure (CWE-209). By default, passing raw error messages into custom rejections can leak database connection strings, file paths, or internal logic traces directly to the HTTP response body. A hardened Warp application must intercept all rejections and map them to sanitized, generic responses.
The Vulnerable Pattern
use warp::Filter;#[tokio::main] async fn main() { // VULNERABLE: Directly passing sensitive error details to the client let route = warp::path(“user”) .and_then(|| async move { let db_err = “Connection failed: postgres://admin:p@ssword123@internal-db:5432/prod”; // This custom rejection leaks the internal connection string to the requester Err::<String, _>(warp::reject::custom(db_err)) });
warp::serve(route).run(([127, 0, 0, 1], 3030)).await;
}
The Secure Implementation
The secure implementation utilizes Warp's `.recover()` filter to catch all `Rejection` types before they reach the client. Instead of returning raw error strings, we define a custom `InternalError` struct that implements `warp::reject::Reject`. The `handle_rejection` function acts as a security boundary: it inspects the rejection type and returns a sanitized JSON payload with a generic message and appropriate HTTP status code. This ensures that even if a database or third-party API fails, the attacker only sees a controlled 'Internal Server Error' message rather than sensitive system internals.
use warp::{Filter, Rejection, Reply, http::StatusCode}; use serde::Serialize;#[derive(Debug)] struct InternalError; impl warp::reject::Reject for InternalError {}
#[derive(Serialize)] struct ErrorResponse { message: String }
// SECURE: Global rejection handler that sanitizes output async fn handle_rejection(err: Rejection) -> Result<impl Reply, std::convert::Infallible> { let (code, message) = if err.is_not_found() { (StatusCode::NOT_FOUND, “Resource not found”) } else if let Some(_) = err.find::
() { // Log the actual error internally here, but return generic message to user (StatusCode::INTERNAL_SERVER_ERROR, “An internal server error occurred”) } else { (StatusCode::INTERNAL_SERVER_ERROR, “Unhandled rejection”) }; let json = warp::reply::json(&ErrorResponse { message: message.into() }); Ok(warp::reply::with_status(json, code))}
#[tokio::main] async fn main() { let route = warp::path(“user”) .and_then(|| async move { // Map sensitive errors to a generic InternalError type Err::<String, _>(warp::reject::custom(InternalError)) }) .recover(handle_rejection);
warp::serve(route).run(([127, 0, 0, 1], 3030)).await;
}
Your Warp API
might be exposed to Improper Error Handling
74% of Warp apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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