probo/pkg/saferedirect/saferedirect.go

// Copyright (c) 2025-2026 Probo Inc <hello@probo.com>.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

package saferedirect

import (
	"context"
	"net/http"
	"net/url"
	"path"
	"strings"
)

type (
	AllowedHostFunc func(ctx context.Context, host string) bool

	SafeRedirect struct {
		allowedHost AllowedHostFunc
	}
)

func New(allowedHost AllowedHostFunc) *SafeRedirect {
	return &SafeRedirect{allowedHost: allowedHost}
}

// StaticHosts returns an AllowedHost function that matches against a fixed
// list of hosts.
func StaticHosts(hosts ...string) AllowedHostFunc {
	allowed := make(map[string]bool, len(hosts))
	for _, h := range hosts {
		if h != "" {
			allowed[h] = true
		}
	}

	return func(_ context.Context, host string) bool {
		return allowed[host]
	}
}

func (sr *SafeRedirect) Validate(ctx context.Context, redirectURL string) (string, bool) {
	if redirectURL == "" {
		return "", false
	}

	if strings.HasPrefix(redirectURL, "/") {
		safePath, ok := normalizeRelativePath(redirectURL)
		if !ok {
			return "", false
		}

		return safePath, true
	}

	parsedURL, err := url.Parse(redirectURL)
	if err != nil {
		return "", false
	}

	if parsedURL.IsAbs() {
		if parsedURL.Scheme != "http" && parsedURL.Scheme != "https" {
			return "", false
		}

		if sr.allowedHost != nil && !sr.allowedHost(ctx, parsedURL.Host) {
			return "", false
		}

		return redirectURL, true
	}

	return "", false
}

func (sr *SafeRedirect) GetSafeRedirectURL(ctx context.Context, redirectURL, fallbackURL string) string {
	if safeURL, isValid := sr.Validate(ctx, redirectURL); isValid {
		return safeURL
	}

	return fallbackURL
}

func (sr *SafeRedirect) Redirect(w http.ResponseWriter, r *http.Request, redirectURL, fallbackURL string, statusCode int) {
	safeURL := sr.GetSafeRedirectURL(r.Context(), redirectURL, fallbackURL)
	http.Redirect(w, r, safeURL, statusCode)
}

func normalizeRelativePath(redirectURL string) (string, bool) {
	if strings.HasPrefix(redirectURL, "//") {
		return "", false
	}

	if strings.Contains(redirectURL, `\`) || strings.Contains(strings.ToLower(redirectURL), "%5c") {
		return "", false
	}

	cleaned := path.Clean(redirectURL)
	if !strings.HasPrefix(cleaned, "/") {
		return "", false
	}

	if len(cleaned) > 1 && (cleaned[1] == '/' || cleaned[1] == '\\') {
		return "", false
	}

	if strings.Contains(cleaned, `\`) {
		return "", false
	}

	return cleaned, true
}