microcosm-cc/bluemondayという便利なサニタイザーがある。
文章中に含まれるHTMLタグやクォーテーションなど、脆弱性に繋がる文字列を削除してくれるパッケージだ。
よく他のブログでは、
sanitized := bluemonday.UGCPolicy().Sanitize(query)
のようにUGCPolicyを使っているが、このポリシーは少しだけ注意が必要だ。
https://github.com/microcosm-cc/bluemonday/blob/master/helpers.go#L153にもあるように、
// "id" is permitted. This is pretty much as some HTML elements require this
// to work well ("dfn" is an example of a "id" being value)
// This does create a risk that JavaScript and CSS within your web page
// might identify the wrong elements. Ensure that you select things
// accurately
ID属性はそのまま素通しされる。<div id="hoge"></div>などがそのまま通されてしまう。
よって、javascriptなどでIDを使用して要素を選択する時、間違ってユーザが入力した要素を選択してしまう可能性がある。
そもそもUGCPolicyとは、ブログ記事などに使用することを想定しているため、ID属性は見出しタグに設定する用途などに使えるよう、素通ししていると思われる。
解決方法としては、
1. 新しいPolicyを作る
2. サニタイズした要素以降にID属性を持った要素を配置しない
などがある。
新しいPolicyのサンプルとして、以下を示す。UGCPolicyからAllowStandardAttributesを削除しただけ。
func StandardPolicy() *bluemonday.Policy {
p := bluemonday.NewPolicy()
//////////////////////////////
// Global URL format policy //
//////////////////////////////
p.AllowStandardURLs()
////////////////////////////////
// Declarations and structure //
////////////////////////////////
// "xml" "xslt" "DOCTYPE" "html" "head" are not permitted as we are
// expecting user generated content to be a fragment of HTML and not a full
// document.
//////////////////////////
// Sectioning root tags //
//////////////////////////
// "article" and "aside" are permitted and takes no attributes
p.AllowElements("article", "aside")
// "body" is not permitted as we are expecting user generated content to be a fragment
// of HTML and not a full document.
// "details" is permitted, including the "open" attribute which can either
// be blank or the value "open".
p.AllowAttrs(
"open",
).Matching(regexp.MustCompile(`(?i)^(|open)$`)).OnElements("details")
// "fieldset" is not permitted as we are not allowing forms to be created.
// "figure" is permitted and takes no attributes
p.AllowElements("figure")
// "nav" is not permitted as it is assumed that the site (and not the user)
// has defined navigation elements
// "section" is permitted and takes no attributes
p.AllowElements("section")
// "summary" is permitted and takes no attributes
p.AllowElements("summary")
//////////////////////////
// Headings and footers //
//////////////////////////
// "footer" is not permitted as we expect user content to be a fragment and
// not structural to this extent
// "h1" through "h6" are permitted and take no attributes
p.AllowElements("h1", "h2", "h3", "h4", "h5", "h6")
// "header" is not permitted as we expect user content to be a fragment and
// not structural to this extent
// "hgroup" is permitted and takes no attributes
p.AllowElements("hgroup")
/////////////////////////////////////
// Content grouping and separating //
/////////////////////////////////////
// "blockquote" is permitted, including the "cite" attribute which must be
// a standard URL.
p.AllowAttrs("cite").OnElements("blockquote")
// "br" "div" "hr" "p" "span" "wbr" are permitted and take no attributes
p.AllowElements("br", "div", "hr", "p", "span", "wbr")
///////////
// Links //
///////////
// "a" is permitted
p.AllowAttrs("href").OnElements("a")
// "area" is permitted along with the attributes that map image maps work
p.AllowAttrs("alt").Matching(bluemonday.Paragraph).OnElements("area")
p.AllowAttrs("coords").Matching(
regexp.MustCompile(`^([0-9]+,){2}(,[0-9]+)*$`),
).OnElements("area")
p.AllowAttrs("href").OnElements("area")
p.AllowAttrs("rel").Matching(bluemonday.SpaceSeparatedTokens).OnElements("area")
p.AllowAttrs("shape").Matching(
regexp.MustCompile(`(?i)^(default|circle|rect|poly)$`),
).OnElements("area")
// "link" is not permitted
/////////////////////
// Phrase elements //
/////////////////////
// The following are all inline phrasing elements
p.AllowElements("abbr", "acronym", "cite", "code", "dfn", "em",
"figcaption", "mark", "s", "samp", "strong", "sub", "sup", "var")
// "q" is permitted and "cite" is a URL and handled by URL policies
p.AllowAttrs("cite").OnElements("q")
// "time" is permitted
p.AllowAttrs("datetime").Matching(bluemonday.ISO8601).OnElements("time")
////////////////////
// Style elements //
////////////////////
// block and inline elements that impart no semantic meaning but style the
// document
p.AllowElements("b", "i", "pre", "small", "strike", "tt", "u")
// "style" is not permitted as we are not yet sanitising CSS and it is an
// XSS attack vector
//////////////////////
// HTML5 Formatting //
//////////////////////
// "bdi" "bdo" are permitted
p.AllowAttrs("dir").Matching(bluemonday.Direction).OnElements("bdi", "bdo")
// "rp" "rt" "ruby" are permitted
p.AllowElements("rp", "rt", "ruby")
///////////////////////////
// HTML5 Change tracking //
///////////////////////////
// "del" "ins" are permitted
p.AllowAttrs("cite").Matching(bluemonday.Paragraph).OnElements("del", "ins")
p.AllowAttrs("datetime").Matching(bluemonday.ISO8601).OnElements("del", "ins")
///////////
// Lists //
///////////
p.AllowLists()
////////////
// Tables //
////////////
p.AllowTables()
///////////
// Forms //
///////////
// By and large, forms are not permitted. However there are some form
// elements that can be used to present data, and we do permit those
//
// "button" "fieldset" "input" "keygen" "label" "output" "select" "datalist"
// "textarea" "optgroup" "option" are all not permitted
// "meter" is permitted
p.AllowAttrs(
"value",
"min",
"max",
"low",
"high",
"optimum",
).Matching(bluemonday.Number).OnElements("meter")
// "progress" is permitted
p.AllowAttrs("value", "max").Matching(bluemonday.Number).OnElements("progress")
//////////////////////
// Embedded content //
//////////////////////
// Vast majority not permitted
// "audio" "canvas" "embed" "iframe" "object" "param" "source" "svg" "track"
// "video" are all not permitted
p.AllowImages()
return p
}
