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Devlion Memo

どことなくそれっぽい大学生の日記

Golangのbluemondayは注意書きをよく読んで使おう

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
}