// Copyright 2025 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package http

import (
	
	
	
	
	
)

// CrossOriginProtection implements protections against [Cross-Site Request
// Forgery (CSRF)] by rejecting non-safe cross-origin browser requests.
//
// Cross-origin requests are currently detected with the [Sec-Fetch-Site]
// header, available in all browsers since 2023, or by comparing the hostname of
// the [Origin] header with the Host header.
//
// The GET, HEAD, and OPTIONS methods are [safe methods] and are always allowed.
// It's important that applications do not perform any state changing actions
// due to requests with safe methods.
//
// Requests without Sec-Fetch-Site or Origin headers are currently assumed to be
// either same-origin or non-browser requests, and are allowed.
//
// The zero value of CrossOriginProtection is valid and has no trusted origins
// or bypass patterns.
//
// [Sec-Fetch-Site]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Site
// [Origin]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin
// [Cross-Site Request Forgery (CSRF)]: https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/CSRF
// [safe methods]: https://developer.mozilla.org/en-US/docs/Glossary/Safe/HTTP
type CrossOriginProtection struct {
	bypass    atomic.Pointer[ServeMux]
	trustedMu sync.RWMutex
	trusted   map[string]bool
	deny      atomic.Pointer[Handler]
}

// NewCrossOriginProtection returns a new [CrossOriginProtection] value.
func () *CrossOriginProtection {
	return &CrossOriginProtection{}
}

// AddTrustedOrigin allows all requests with an [Origin] header
// which exactly matches the given value.
//
// Origin header values are of the form "scheme://host[:port]".
//
// AddTrustedOrigin can be called concurrently with other methods
// or request handling, and applies to future requests.
//
// [Origin]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin
func ( *CrossOriginProtection) ( string) error {
	,  := url.Parse()
	if  != nil {
		return fmt.Errorf("invalid origin %q: %w", , )
	}
	if .Scheme == "" {
		return fmt.Errorf("invalid origin %q: scheme is required", )
	}
	if .Host == "" {
		return fmt.Errorf("invalid origin %q: host is required", )
	}
	if .Path != "" || .RawQuery != "" || .Fragment != "" {
		return fmt.Errorf("invalid origin %q: path, query, and fragment are not allowed", )
	}
	.trustedMu.Lock()
	defer .trustedMu.Unlock()
	if .trusted == nil {
		.trusted = make(map[string]bool)
	}
	.trusted[] = true
	return nil
}

var noopHandler = HandlerFunc(func( ResponseWriter,  *Request) {})

// AddInsecureBypassPattern permits all requests that match the given pattern.
// The pattern syntax and precedence rules are the same as [ServeMux].
//
// AddInsecureBypassPattern can be called concurrently with other methods
// or request handling, and applies to future requests.
func ( *CrossOriginProtection) ( string) {
	var  *ServeMux

	// Lazily initialize c.bypass
	for {
		 = .bypass.Load()
		if  != nil {
			break
		}
		 = NewServeMux()
		if .bypass.CompareAndSwap(nil, ) {
			break
		}
	}

	.Handle(, noopHandler)
}

// SetDenyHandler sets a handler to invoke when a request is rejected.
// The default error handler responds with a 403 Forbidden status.
//
// SetDenyHandler can be called concurrently with other methods
// or request handling, and applies to future requests.
//
// Check does not call the error handler.
func ( *CrossOriginProtection) ( Handler) {
	if  == nil {
		.deny.Store(nil)
		return
	}
	.deny.Store(&)
}

// Check applies cross-origin checks to a request.
// It returns an error if the request should be rejected.
func ( *CrossOriginProtection) ( *Request) error {
	switch .Method {
	case "GET", "HEAD", "OPTIONS":
		// Safe methods are always allowed.
		return nil
	}

	switch .Header.Get("Sec-Fetch-Site") {
	case "":
		// No Sec-Fetch-Site header is present.
		// Fallthrough to check the Origin header.
	case "same-origin", "none":
		return nil
	default:
		if .isRequestExempt() {
			return nil
		}
		return errCrossOriginRequest
	}

	 := .Header.Get("Origin")
	if  == "" {
		// Neither Sec-Fetch-Site nor Origin headers are present.
		// Either the request is same-origin or not a browser request.
		return nil
	}

	if ,  := url.Parse();  == nil && .Host == .Host {
		// The Origin header matches the Host header. Note that the Host header
		// doesn't include the scheme, so we don't know if this might be an
		// HTTP→HTTPS cross-origin request. We fail open, since all modern
		// browsers support Sec-Fetch-Site since 2023, and running an older
		// browser makes a clear security trade-off already. Sites can mitigate
		// this with HTTP Strict Transport Security (HSTS).
		return nil
	}

	if .isRequestExempt() {
		return nil
	}
	return errCrossOriginRequestFromOldBrowser
}

var (
	errCrossOriginRequest               = errors.New("cross-origin request detected from Sec-Fetch-Site header")
	errCrossOriginRequestFromOldBrowser = errors.New("cross-origin request detected, and/or browser is out of date: " +
		"Sec-Fetch-Site is missing, and Origin does not match Host")
)

// isRequestExempt checks the bypasses which require taking a lock, and should
// be deferred until the last moment.
func ( *CrossOriginProtection) ( *Request) bool {
	if  := .bypass.Load();  != nil {
		if ,  := .Handler();  != "" {
			// The request matches a bypass pattern.
			return true
		}
	}

	.trustedMu.RLock()
	defer .trustedMu.RUnlock()
	 := .Header.Get("Origin")
	// The request matches a trusted origin.
	return  != "" && .trusted[]
}

// Handler returns a handler that applies cross-origin checks
// before invoking the handler h.
//
// If a request fails cross-origin checks, the request is rejected
// with a 403 Forbidden status or handled with the handler passed
// to [CrossOriginProtection.SetDenyHandler].
func ( *CrossOriginProtection) ( Handler) Handler {
	return HandlerFunc(func( ResponseWriter,  *Request) {
		if  := .Check();  != nil {
			if  := .deny.Load();  != nil {
				(*).ServeHTTP(, )
				return
			}
			Error(, .Error(), StatusForbidden)
			return
		}
		.ServeHTTP(, )
	})
}