forgejo/modules/markup/sanitizer.go

// Copyright 2017 The Gitea Authors. All rights reserved.
// Copyright 2017 The Gogs Authors. All rights reserved.
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file.

package markup

import (
	"bytes"
	"io"
	"regexp"
	"sync"

	"code.gitea.io/gitea/modules/setting"

	"github.com/microcosm-cc/bluemonday"
)

// Sanitizer is a protection wrapper of *bluemonday.Policy which does not allow
// any modification to the underlying policies once it's been created.
type Sanitizer struct {
	policy *bluemonday.Policy
	init   sync.Once
}

var sanitizer = &Sanitizer{}

// NewSanitizer initializes sanitizer with allowed attributes based on settings.
// Multiple calls to this function will only create one instance of Sanitizer during
// entire application lifecycle.
func NewSanitizer() {
	sanitizer.init.Do(func() {
		ReplaceSanitizer()
	})
}

// ReplaceSanitizer replaces the current sanitizer to account for changes in settings
func ReplaceSanitizer() {
	sanitizer.policy = bluemonday.UGCPolicy()
	// For Chroma markdown plugin
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^is-loading$`)).OnElements("pre")
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^(chroma )?language-[\w-]+$`)).OnElements("code")

	// Checkboxes
	sanitizer.policy.AllowAttrs("type").Matching(regexp.MustCompile(`^checkbox$`)).OnElements("input")
	sanitizer.policy.AllowAttrs("checked", "disabled", "data-source-position").OnElements("input")

	// Custom URL-Schemes
	if len(setting.Markdown.CustomURLSchemes) > 0 {
		sanitizer.policy.AllowURLSchemes(setting.Markdown.CustomURLSchemes...)
	}

	// Allow keyword markup
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^` + keywordClass + `$`)).OnElements("span")

	// Allow classes for anchors
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`ref-issue`)).OnElements("a")

	// Allow classes for task lists
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`task-list-item`)).OnElements("li")

	// Allow icons
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^icon(\s+[\p{L}\p{N}_-]+)+$`)).OnElements("i")

	// Allow unlabelled labels
	sanitizer.policy.AllowNoAttrs().OnElements("label")

	// Allow classes for emojis
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`emoji`)).OnElements("img")

	// Allow icons, emojis, and chroma syntax on span
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^((icon(\s+[\p{L}\p{N}_-]+)+)|(emoji))$|^([a-z][a-z0-9]{0,2})$`)).OnElements("span")

	// Allow data tables
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`data-table`)).OnElements("table")
	sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`line-num`)).OnElements("th", "td")

	// Allow generally safe attributes
	generalSafeAttrs := []string{"abbr", "accept", "accept-charset",
		"accesskey", "action", "align", "alt",
		"aria-describedby", "aria-hidden", "aria-label", "aria-labelledby",
		"axis", "border", "cellpadding", "cellspacing", "char",
		"charoff", "charset", "checked",
		"clear", "cols", "colspan", "color",
		"compact", "coords", "datetime", "dir",
		"disabled", "enctype", "for", "frame",
		"headers", "height", "hreflang",
		"hspace", "ismap", "label", "lang",
		"maxlength", "media", "method",
		"multiple", "name", "nohref", "noshade",
		"nowrap", "open", "prompt", "readonly", "rel", "rev",
		"rows", "rowspan", "rules", "scope",
		"selected", "shape", "size", "span",
		"start", "summary", "tabindex", "target",
		"title", "type", "usemap", "valign", "value",
		"vspace", "width", "itemprop",
	}

	generalSafeElements := []string{
		"h1", "h2", "h3", "h4", "h5", "h6", "h7", "h8", "br", "b", "i", "strong", "em", "a", "pre", "code", "img", "tt",
		"div", "ins", "del", "sup", "sub", "p", "ol", "ul", "table", "thead", "tbody", "tfoot", "blockquote",
		"dl", "dt", "dd", "kbd", "q", "samp", "var", "hr", "ruby", "rt", "rp", "li", "tr", "td", "th", "s", "strike", "summary",
		"details", "caption", "figure", "figcaption",
		"abbr", "bdo", "cite", "dfn", "mark", "small", "span", "time", "wbr",
	}

	sanitizer.policy.AllowAttrs(generalSafeAttrs...).OnElements(generalSafeElements...)

	sanitizer.policy.AllowAttrs("itemscope", "itemtype").OnElements("div")

	// FIXME: Need to handle longdesc in img but there is no easy way to do it

	// Custom keyword markup
	for _, rule := range setting.ExternalSanitizerRules {
		if rule.Regexp != nil {
			sanitizer.policy.AllowAttrs(rule.AllowAttr).Matching(rule.Regexp).OnElements(rule.Element)
		} else {
			sanitizer.policy.AllowAttrs(rule.AllowAttr).OnElements(rule.Element)
		}
	}
}

// Sanitize takes a string that contains a HTML fragment or document and applies policy whitelist.
func Sanitize(s string) string {
	NewSanitizer()
	return sanitizer.policy.Sanitize(s)
}

// SanitizeReader sanitizes a Reader
func SanitizeReader(r io.Reader) *bytes.Buffer {
	NewSanitizer()
	return sanitizer.policy.SanitizeReader(r)
}