//! 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 [`CsrfFilterBuilder::allow_xhr()`](struct.CsrfFilterBuilder.html#method.allow_xhr)
//! if you want to allow requests with unsafe methods via
//! [CORS](../cors/struct.Cors.html).
//!
//! # Example
//!
//! ```
//! # extern crate actix_web;
//! use actix_web::{http, Application, HttpRequest, HttpResponse};
//! use actix_web::middleware::csrf;
//!
//! fn handle_post(_: HttpRequest) -> &'static str {
//!     "This action should only be triggered with requests from the same site"
//! }
//!
//! fn main() {
//!     let app = Application::new()
//!         .middleware(
//!             csrf::CsrfFilter::build()
//!                 .allowed_origin("https://www.example.com")
//!                 .finish())
//!         .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::{Result, ResponseError};
use http::{HeaderMap, HttpTryFrom, Uri, header};
use httpmessage::HttpMessage;
use httprequest::HttpRequest;
use httpresponse::HttpResponse;
use middleware::{Middleware, Started};

/// 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.
pub struct CsrfFilter {
    origins: HashSet<String>,
    allow_xhr: bool,
    allow_missing_origin: bool,
    allow_upgrade: bool,
}

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

    fn validate<S>(&self, req: &mut HttpRequest<S>) -> 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: &mut HttpRequest<S>) -> Result<Started> {
        self.validate(req)?;
        Ok(Started::Done)
    }
}

/// Used to build a `CsrfFilter`.
///
/// 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;
///
/// let csrf = csrf::CsrfFilter::build()
///     .allowed_origin("https://www.example.com")
///     .finish();
/// ```
pub struct CsrfFilterBuilder {
    csrf: CsrfFilter,
}

impl CsrfFilterBuilder {
    /// Add an origin that is allowed to make requests. Will be verified
    /// against the `Origin` request header.
    pub fn allowed_origin(mut self, origin: &str) -> CsrfFilterBuilder {
        self.csrf.origins.insert(origin.to_owned());
        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) -> CsrfFilterBuilder {
        self.csrf.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 unsafe requests.
    pub fn allow_missing_origin(mut self) -> CsrfFilterBuilder {
        self.csrf.allow_missing_origin = true;
        self
    }

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

    /// Finishes building the `CsrfFilter` instance.
    pub fn finish(self) -> CsrfFilter {
        self.csrf
    }
}

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

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

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

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

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

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

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

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

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

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

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

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

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

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