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    //! A filter for cross-site request forgery (CSRF).
    //!
    //! This middleware is stateless and [based on request
    //! headers](https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet#Verifying_Same_Origin_with_Standard_Headers).
    //!
    //! By default requests are allowed only if one of these is true:
    //!
    //! * The request method is safe (`GET`, `HEAD`, `OPTIONS`). It is the
    //!   applications responsibility to ensure these methods cannot be used to
    //!   execute unwanted actions. Note that upgrade requests for websockets are
    //!   also considered safe.
    //! * The `Origin` header (added automatically by the browser) matches one
    //!   of the allowed origins.
    //! * There is no `Origin` header but the `Referer` header matches one of
    //!   the allowed origins.
    //!
    //! Use [`CsrfFilter::allow_xhr()`](struct.CsrfFilter.html#method.allow_xhr)
    //! if you want to allow requests with unprotected methods via
    //! [CORS](../cors/struct.Cors.html).
    //!
    //! # Example
    //!
    //! ```
    //! # extern crate actix_web;
    //! use actix_web::middleware::csrf;
    //! use actix_web::{http, App, HttpRequest, HttpResponse};
    //!
    //! fn handle_post(_: &HttpRequest) -> &'static str {
    //!     "This action should only be triggered with requests from the same site"
    //! }
    //!
    //! fn main() {
    //!     let app = App::new()
    //!         .middleware(
    //!             csrf::CsrfFilter::new().allowed_origin("https://www.example.com"),
    //!         )
    //!         .resource("/", |r| {
    //!             r.method(http::Method::GET).f(|_| HttpResponse::Ok());
    //!             r.method(http::Method::POST).f(handle_post);
    //!         })
    //!         .finish();
    //! }
    //! ```
    //!
    //! In this example the entire application is protected from CSRF.

    use std::borrow::Cow;
    use std::collections::HashSet;

    use bytes::Bytes;
    use error::{ResponseError, Result};
    use http::{header, HeaderMap, HttpTryFrom, Uri};
    use httprequest::HttpRequest;
    use httpresponse::HttpResponse;
    use middleware::{Middleware, Started};
    use server::Request;

    /// Potential cross-site request forgery detected.
    #[derive(Debug, Fail)]
    pub enum CsrfError {
        /// The HTTP request header `Origin` was required but not provided.
        #[fail(display = "Origin header required")]
        MissingOrigin,
        /// The HTTP request header `Origin` could not be parsed correctly.
        #[fail(display = "Could not parse Origin header")]
        BadOrigin,
        /// The cross-site request was denied.
        #[fail(display = "Cross-site request denied")]
        CsrDenied,
    }

    impl ResponseError for CsrfError {
        fn error_response(&self) -> HttpResponse {
            HttpResponse::Forbidden().body(self.to_string())
        }
    }

    fn uri_origin(uri: &Uri) -> Option<String> {
        match (uri.scheme_part(), uri.host(), uri.port()) {
            (Some(scheme), Some(host), Some(port)) => {
                Some(format!("{}://{}:{}", scheme, host, port))
            }
            (Some(scheme), Some(host), None) => Some(format!("{}://{}", scheme, host)),
            _ => None,
        }
    }

    fn origin(headers: &HeaderMap) -> Option<Result<Cow<str>, CsrfError>> {
        headers
            .get(header::ORIGIN)
            .map(|origin| {
                origin
                    .to_str()
                    .map_err(|_| CsrfError::BadOrigin)
                    .map(|o| o.into())
            })
            .or_else(|| {
                headers.get(header::REFERER).map(|referer| {
                    Uri::try_from(Bytes::from(referer.as_bytes()))
                        .ok()
                        .as_ref()
                        .and_then(uri_origin)
                        .ok_or(CsrfError::BadOrigin)
                        .map(|o| o.into())
                })
            })
    }

    /// A middleware that filters cross-site requests.
    ///
    /// To construct a CSRF filter:
    ///
    /// 1. Call [`CsrfFilter::build`](struct.CsrfFilter.html#method.build) to
    ///    start building.
    /// 2. [Add](struct.CsrfFilterBuilder.html#method.allowed_origin) allowed
    ///    origins.
    /// 3. Call [finish](struct.CsrfFilterBuilder.html#method.finish) to retrieve
    ///    the constructed filter.
    ///
    /// # Example
    ///
    /// ```
    /// use actix_web::middleware::csrf;
    /// use actix_web::App;
    ///
    /// # fn main() {
    /// let app = App::new()
    ///     .middleware(csrf::CsrfFilter::new().allowed_origin("https://www.example.com"));
    /// # }
    /// ```
    #[derive(Default)]
    pub struct CsrfFilter {
        origins: HashSet<String>,
        allow_xhr: bool,
        allow_missing_origin: bool,
        allow_upgrade: bool,
    }

    impl CsrfFilter {
        /// Start building a `CsrfFilter`.
        pub fn new() -> CsrfFilter {
            CsrfFilter {
                origins: HashSet::new(),
                allow_xhr: false,
                allow_missing_origin: false,
                allow_upgrade: false,
            }
        }

        /// Add an origin that is allowed to make requests. Will be verified
        /// against the `Origin` request header.
        pub fn allowed_origin<T: Into<String>>(mut self, origin: T) -> CsrfFilter {
            self.origins.insert(origin.into());
            self
        }

        /// Allow all requests with an `X-Requested-With` header.
        ///
        /// A cross-site attacker should not be able to send requests with custom
        /// headers unless a CORS policy whitelists them. Therefore it should be
        /// safe to allow requests with an `X-Requested-With` header (added
        /// automatically by many JavaScript libraries).
        ///
        /// This is disabled by default, because in Safari it is possible to
        /// circumvent this using redirects and Flash.
        ///
        /// Use this method to enable more lax filtering.
        pub fn allow_xhr(mut self) -> CsrfFilter {
            self.allow_xhr = true;
            self
        }

        /// Allow requests if the expected `Origin` header is missing (and
        /// there is no `Referer` to fall back on).
        ///
        /// The filter is conservative by default, but it should be safe to allow
        /// missing `Origin` headers because a cross-site attacker cannot prevent
        /// the browser from sending `Origin` on unprotected requests.
        pub fn allow_missing_origin(mut self) -> CsrfFilter {
            self.allow_missing_origin = true;
            self
        }

        /// Allow cross-site upgrade requests (for example to open a WebSocket).
        pub fn allow_upgrade(mut self) -> CsrfFilter {
            self.allow_upgrade = true;
            self
        }

        fn validate(&self, req: &Request) -> Result<(), CsrfError> {
            let is_upgrade = req.headers().contains_key(header::UPGRADE);
            let is_safe = req.method().is_safe() && (self.allow_upgrade || !is_upgrade);

            if is_safe || (self.allow_xhr && req.headers().contains_key("x-requested-with"))
            {
                Ok(())
            } else if let Some(header) = origin(req.headers()) {
                match header {
                    Ok(ref origin) if self.origins.contains(origin.as_ref()) => Ok(()),
                    Ok(_) => Err(CsrfError::CsrDenied),
                    Err(err) => Err(err),
                }
            } else if self.allow_missing_origin {
                Ok(())
            } else {
                Err(CsrfError::MissingOrigin)
            }
        }
    }

    impl<S> Middleware<S> for CsrfFilter {
        fn start(&self, req: &HttpRequest<S>) -> Result<Started> {
            self.validate(req)?;
            Ok(Started::Done)
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use http::Method;
        use test::TestRequest;

        #[test]
        fn test_safe() {
            let csrf = CsrfFilter::new().allowed_origin("https://www.example.com");

            let req = TestRequest::with_header("Origin", "https://www.w3.org")
                .method(Method::HEAD)
                .finish();

            assert!(csrf.start(&req).is_ok());
        }

        #[test]
        fn test_csrf() {
            let csrf = CsrfFilter::new().allowed_origin("https://www.example.com");

            let req = TestRequest::with_header("Origin", "https://www.w3.org")
                .method(Method::POST)
                .finish();

            assert!(csrf.start(&req).is_err());
        }

        #[test]
        fn test_referer() {
            let csrf = CsrfFilter::new().allowed_origin("https://www.example.com");

            let req = TestRequest::with_header(
                "Referer",
                "https://www.example.com/some/path?query=param",
            ).method(Method::POST)
                .finish();

            assert!(csrf.start(&req).is_ok());
        }

        #[test]
        fn test_upgrade() {
            let strict_csrf = CsrfFilter::new().allowed_origin("https://www.example.com");

            let lax_csrf = CsrfFilter::new()
                .allowed_origin("https://www.example.com")
                .allow_upgrade();

            let req = TestRequest::with_header("Origin", "https://cswsh.com")
                .header("Connection", "Upgrade")
                .header("Upgrade", "websocket")
                .method(Method::GET)
                .finish();

            assert!(strict_csrf.start(&req).is_err());
            assert!(lax_csrf.start(&req).is_ok());
        }
    }

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