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// Setup app
const express = require("express");
const app = express();
const port = 3000;
// Select ejs templating library
app.set('view engine', 'ejs');
// Routes
app.get("/", (req, res) => {
res.render("index");
})
app.get("/vuln", (req, res) => {
// simulate SSPP vulnerability
var a = req.query.a;
var b = req.query.b;
var c = req.query.c;
var obj = {};
obj[a][b] = c; // Hack!!
res.send("OK!");
})
// Start app
app.listen(port, () => {
console.log(`App listening on port ${port}`)
})
위와 같이 prototype pollution이 가능하고 render() 함수가 호출되는 상황에 발생하는 취약점입니다.
어떤 prototype을 name에 입력해야 RCE를 할 수 있을지 찾던 중 멘토님이 아래 코드에 render를 분석해보라고 알려주셔서 render를 분석해봤습니다.
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app.get("/", (req, res) => {
res.render("index");
})
node_modules/ejs/ejs.js 파일에 render 함수 코드입니다.
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exports.render = function (template, d, o) {
var data = d || utils.createNullProtoObjWherePossible();
var opts = o || utils.createNullProtoObjWherePossible();
// No options object -- if there are optiony names
// in the data, copy them to options
if (arguments.length == 2) {
utils.shallowCopyFromList(opts, data, _OPTS_PASSABLE_WITH_DATA);
}
return handleCache(opts, template)(data);
};
각 함수들 코드를 오디팅 하던 중 handleCache() 함수에서 client와 escapeFuntion이 사용되는 Template을 호출하는 compile을 찾았습니다.
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function handleCache(options, template) {
var func;
var filename = options.filename;
var hasTemplate = arguments.length > 1;
if (options.cache) {
if (!filename) {
throw new Error('cache option requires a filename');
}
func = exports.cache.get(filename);
if (func) {
return func;
}
if (!hasTemplate) {
template = fileLoader(filename).toString().replace(_BOM, '');
}
}
else if (!hasTemplate) {
// istanbul ignore if: should not happen at all
if (!filename) {
throw new Error('Internal EJS error: no file name or template ' + 'provided');
}
template = fileLoader(filename).toString().replace(_BOM, '');
}
func = exports.compile(template, options);
if (options.cache) {
exports.cache.set(filename, func);
}
return func;
}
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exports.compile = function compile(template, opts) {
var templ;
// v1 compat
// 'scope' is 'context'
// FIXME: Remove this in a future version
if (opts && opts.scope) {
if (!scopeOptionWarned){
console.warn('`scope` option is deprecated and will be removed in EJS 3');
scopeOptionWarned = true;
}
if (!opts.context) {
opts.context = opts.scope;
}
delete opts.scope;
}
templ = new Template(template, opts);
return templ.compile();
};
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function Template(text, opts) {
opts = opts || utils.createNullProtoObjWherePossible();
var options = utils.createNullProtoObjWherePossible();
this.templateText = text;
/** @type {string | null} */
this.mode = null;
this.truncate = false;
this.currentLine = 1;
this.source = '';
options.client = opts.client || false;
options.escapeFunction = opts.escape || opts.escapeFunction || utils.escapeXML;
options.compileDebug = opts.compileDebug !== false;
options.debug = !!opts.debug;
options.filename = opts.filename;
options.openDelimiter = opts.openDelimiter || exports.openDelimiter || _DEFAULT_OPEN_DELIMITER;
options.closeDelimiter = opts.closeDelimiter || exports.closeDelimiter || _DEFAULT_CLOSE_DELIMITER;
options.delimiter = opts.delimiter || exports.delimiter || _DEFAULT_DELIMITER;
options.strict = opts.strict || false;
options.context = opts.context;
options.cache = opts.cache || false;
options.rmWhitespace = opts.rmWhitespace;
options.root = opts.root;
options.includer = opts.includer;
options.outputFunctionName = opts.outputFunctionName;
options.localsName = opts.localsName || exports.localsName || _DEFAULT_LOCALS_NAME;
options.views = opts.views;
options.async = opts.async;
options.destructuredLocals = opts.destructuredLocals;
options.legacyInclude = typeof opts.legacyInclude != 'undefined' ? !!opts.legacyInclude : true;
if (options.strict) {
options._with = false;
}
else {
options._with = typeof opts._with != 'undefined' ? opts._with : true;
}
this.opts = options;
this.regex = this.createRegex();
}
위 함수를 분석해봤는데 딱히 client와 escapeFuntion prototype을 조작한다고 해서 RCE를 발생시킬 수 있는 부분을 찾지 못했습니다.
그래서 쭉 코드 오디팅을 하다가 Template.prototype 오브젝트를 찾았습니다.
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Template.prototype = {
createRegex: function () {
var str = _REGEX_STRING;
var delim = utils.escapeRegExpChars(this.opts.delimiter);
var open = utils.escapeRegExpChars(this.opts.openDelimiter);
var close = utils.escapeRegExpChars(this.opts.closeDelimiter);
str = str.replace(/%/g, delim)
.replace(/</g, open)
.replace(/>/g, close);
return new RegExp(str);
},
compile: function () {
/** @type {string} */
var src;
/** @type {ClientFunction} */
var fn;
var opts = this.opts;
var prepended = '';
var appended = '';
/** @type {EscapeCallback} */
var escapeFn = opts.escapeFunction;
/** @type {FunctionConstructor} */
var ctor;
/** @type {string} */
var sanitizedFilename = opts.filename ? JSON.stringify(opts.filename) : 'undefined';
if (!this.source) {
this.generateSource();
prepended +=
' var __output = "";\n' +
' function __append(s) { if (s !== undefined && s !== null) __output += s }\n';
if (opts.outputFunctionName) {
if (!_JS_IDENTIFIER.test(opts.outputFunctionName)) {
throw new Error('outputFunctionName is not a valid JS identifier.');
}
prepended += ' var ' + opts.outputFunctionName + ' = __append;' + '\n';
}
if (opts.localsName && !_JS_IDENTIFIER.test(opts.localsName)) {
throw new Error('localsName is not a valid JS identifier.');
}
if (opts.destructuredLocals && opts.destructuredLocals.length) {
var destructuring = ' var __locals = (' + opts.localsName + ' || {}),\n';
for (var i = 0; i < opts.destructuredLocals.length; i++) {
var name = opts.destructuredLocals[i];
if (!_JS_IDENTIFIER.test(name)) {
throw new Error('destructuredLocals[' + i + '] is not a valid JS identifier.');
}
if (i > 0) {
destructuring += ',\n ';
}
destructuring += name + ' = __locals.' + name;
}
prepended += destructuring + ';\n';
}
if (opts._with !== false) {
prepended += ' with (' + opts.localsName + ' || {}) {' + '\n';
appended += ' }' + '\n';
}
appended += ' return __output;' + '\n';
this.source = prepended + this.source + appended;
}
if (opts.compileDebug) {
src = 'var __line = 1' + '\n'
+ ' , __lines = ' + JSON.stringify(this.templateText) + '\n'
+ ' , __filename = ' + sanitizedFilename + ';' + '\n'
+ 'try {' + '\n'
+ this.source
+ '} catch (e) {' + '\n'
+ ' rethrow(e, __lines, __filename, __line, escapeFn);' + '\n'
+ '}' + '\n';
}
else {
src = this.source;
}
if (opts.client) {
src = 'escapeFn = escapeFn || ' + escapeFn.toString() + ';' + '\n' + src;
if (opts.compileDebug) {
src = 'rethrow = rethrow || ' + rethrow.toString() + ';' + '\n' + src;
}
}
if (opts.strict) {
src = '"use strict";\n' + src;
}
if (opts.debug) {
console.log(src);
}
if (opts.compileDebug && opts.filename) {
src = src + '\n'
+ '//# sourceURL=' + sanitizedFilename + '\n';
}
try {
if (opts.async) {
// Have to use generated function for this, since in envs without support,
// it breaks in parsing
try {
ctor = (new Function('return (async function(){}).constructor;'))();
}
catch(e) {
if (e instanceof SyntaxError) {
throw new Error('This environment does not support async/await');
}
else {
throw e;
}
}
}
else {
ctor = Function;
}
fn = new ctor(opts.localsName + ', escapeFn, include, rethrow', src);
}
catch(e) {
// istanbul ignore else
if (e instanceof SyntaxError) {
if (opts.filename) {
e.message += ' in ' + opts.filename;
}
e.message += ' while compiling ejs\n\n';
e.message += 'If the above error is not helpful, you may want to try EJS-Lint:\n';
e.message += 'https://github.com/RyanZim/EJS-Lint';
if (!opts.async) {
e.message += '\n';
e.message += 'Or, if you meant to create an async function, pass `async: true` as an option.';
}
}
throw e;
}
// Return a callable function which will execute the function
// created by the source-code, with the passed data as locals
// Adds a local `include` function which allows full recursive include
var returnedFn = opts.client ? fn : function anonymous(data) {
var include = function (path, includeData) {
var d = utils.shallowCopy(utils.createNullProtoObjWherePossible(), data);
if (includeData) {
d = utils.shallowCopy(d, includeData);
}
return includeFile(path, opts)(d);
};
return fn.apply(opts.context,
[data || utils.createNullProtoObjWherePossible(), escapeFn, include, rethrow]);
};
if (opts.filename && typeof Object.defineProperty === 'function') {
var filename = opts.filename;
var basename = path.basename(filename, path.extname(filename));
try {
Object.defineProperty(returnedFn, 'name', {
value: basename,
writable: false,
enumerable: false,
configurable: true
});
} catch (e) {/* ignore */}
}
return returnedFn;
},
generateSource: function () {
var opts = this.opts;
if (opts.rmWhitespace) {
// Have to use two separate replace here as `^` and `$` operators don't
// work well with `\r` and empty lines don't work well with the `m` flag.
this.templateText =
this.templateText.replace(/[\r\n]+/g, '\n').replace(/^\s+|\s+$/gm, '');
}
// Slurp spaces and tabs before <%_ and after _%>
this.templateText =
this.templateText.replace(/[ \t]*<%_/gm, '<%_').replace(/_%>[ \t]*/gm, '_%>');
var self = this;
var matches = this.parseTemplateText();
var d = this.opts.delimiter;
var o = this.opts.openDelimiter;
var c = this.opts.closeDelimiter;
if (matches && matches.length) {
matches.forEach(function (line, index) {
var closing;
// If this is an opening tag, check for closing tags
// FIXME: May end up with some false positives here
// Better to store modes as k/v with openDelimiter + delimiter as key
// Then this can simply check against the map
if ( line.indexOf(o + d) === 0 // If it is a tag
&& line.indexOf(o + d + d) !== 0) { // and is not escaped
closing = matches[index + 2];
if (!(closing == d + c || closing == '-' + d + c || closing == '_' + d + c)) {
throw new Error('Could not find matching close tag for "' + line + '".');
}
}
self.scanLine(line);
});
}
},
parseTemplateText: function () {
var str = this.templateText;
var pat = this.regex;
var result = pat.exec(str);
var arr = [];
var firstPos;
while (result) {
firstPos = result.index;
if (firstPos !== 0) {
arr.push(str.substring(0, firstPos));
str = str.slice(firstPos);
}
arr.push(result[0]);
str = str.slice(result[0].length);
result = pat.exec(str);
}
if (str) {
arr.push(str);
}
return arr;
},
_addOutput: function (line) {
if (this.truncate) {
// Only replace single leading linebreak in the line after
// -%> tag -- this is the single, trailing linebreak
// after the tag that the truncation mode replaces
// Handle Win / Unix / old Mac linebreaks -- do the \r\n
// combo first in the regex-or
line = line.replace(/^(?:\r\n|\r|\n)/, '');
this.truncate = false;
}
if (!line) {
return line;
}
// Preserve literal slashes
line = line.replace(/\\/g, '\\\\');
// Convert linebreaks
line = line.replace(/\n/g, '\\n');
line = line.replace(/\r/g, '\\r');
// Escape double-quotes
// - this will be the delimiter during execution
line = line.replace(/"/g, '\\"');
this.source += ' ; __append("' + line + '")' + '\n';
},
scanLine: function (line) {
var self = this;
var d = this.opts.delimiter;
var o = this.opts.openDelimiter;
var c = this.opts.closeDelimiter;
var newLineCount = 0;
newLineCount = (line.split('\n').length - 1);
switch (line) {
case o + d:
case o + d + '_':
this.mode = Template.modes.EVAL;
break;
case o + d + '=':
this.mode = Template.modes.ESCAPED;
break;
case o + d + '-':
this.mode = Template.modes.RAW;
break;
case o + d + '#':
this.mode = Template.modes.COMMENT;
break;
case o + d + d:
this.mode = Template.modes.LITERAL;
this.source += ' ; __append("' + line.replace(o + d + d, o + d) + '")' + '\n';
break;
case d + d + c:
this.mode = Template.modes.LITERAL;
this.source += ' ; __append("' + line.replace(d + d + c, d + c) + '")' + '\n';
break;
case d + c:
case '-' + d + c:
case '_' + d + c:
if (this.mode == Template.modes.LITERAL) {
this._addOutput(line);
}
this.mode = null;
this.truncate = line.indexOf('-') === 0 || line.indexOf('_') === 0;
break;
default:
// In script mode, depends on type of tag
if (this.mode) {
// If '//' is found without a line break, add a line break.
switch (this.mode) {
case Template.modes.EVAL:
case Template.modes.ESCAPED:
case Template.modes.RAW:
if (line.lastIndexOf('//') > line.lastIndexOf('\n')) {
line += '\n';
}
}
switch (this.mode) {
// Just executing code
case Template.modes.EVAL:
this.source += ' ; ' + line + '\n';
break;
// Exec, esc, and output
case Template.modes.ESCAPED:
this.source += ' ; __append(escapeFn(' + stripSemi(line) + '))' + '\n';
break;
// Exec and output
case Template.modes.RAW:
this.source += ' ; __append(' + stripSemi(line) + ')' + '\n';
break;
case Template.modes.COMMENT:
// Do nothing
break;
// Literal <%% mode, append as raw output
case Template.modes.LITERAL:
this._addOutput(line);
break;
}
}
// In string mode, just add the output
else {
this._addOutput(line);
}
}
if (self.opts.compileDebug && newLineCount) {
this.currentLine += newLineCount;
this.source += ' ; __line = ' + this.currentLine + '\n';
}
}
};
위 코드에서 중요한 부분은 compile: function 부분입니다.
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compile: function () {
/** @type {string} */
var src;
/** @type {ClientFunction} */
var fn;
var opts = this.opts;
var prepended = '';
var appended = '';
/** @type {EscapeCallback} */
var escapeFn = opts.escapeFunction;
/** @type {FunctionConstructor} */
var ctor;
/** @type {string} */
var sanitizedFilename = opts.filename ? JSON.stringify(opts.filename) : 'undefined';
if (!this.source) {
this.generateSource();
prepended +=
' var __output = "";\n' +
' function __append(s) { if (s !== undefined && s !== null) __output += s }\n';
if (opts.outputFunctionName) {
if (!_JS_IDENTIFIER.test(opts.outputFunctionName)) {
throw new Error('outputFunctionName is not a valid JS identifier.');
}
prepended += ' var ' + opts.outputFunctionName + ' = __append;' + '\n';
}
if (opts.localsName && !_JS_IDENTIFIER.test(opts.localsName)) {
throw new Error('localsName is not a valid JS identifier.');
}
if (opts.destructuredLocals && opts.destructuredLocals.length) {
var destructuring = ' var __locals = (' + opts.localsName + ' || {}),\n';
for (var i = 0; i < opts.destructuredLocals.length; i++) {
var name = opts.destructuredLocals[i];
if (!_JS_IDENTIFIER.test(name)) {
throw new Error('destructuredLocals[' + i + '] is not a valid JS identifier.');
}
if (i > 0) {
destructuring += ',\n ';
}
destructuring += name + ' = __locals.' + name;
}
prepended += destructuring + ';\n';
}
if (opts._with !== false) {
prepended += ' with (' + opts.localsName + ' || {}) {' + '\n';
appended += ' }' + '\n';
}
appended += ' return __output;' + '\n';
this.source = prepended + this.source + appended;
}
if (opts.compileDebug) {
src = 'var __line = 1' + '\n'
+ ' , __lines = ' + JSON.stringify(this.templateText) + '\n'
+ ' , __filename = ' + sanitizedFilename + ';' + '\n'
+ 'try {' + '\n'
+ this.source
+ '} catch (e) {' + '\n'
+ ' rethrow(e, __lines, __filename, __line, escapeFn);' + '\n'
+ '}' + '\n';
}
else {
src = this.source;
}
if (opts.client) {
src = 'escapeFn = escapeFn || ' + escapeFn.toString() + ';' + '\n' + src;
if (opts.compileDebug) {
src = 'rethrow = rethrow || ' + rethrow.toString() + ';' + '\n' + src;
}
}
if (opts.strict) {
src = '"use strict";\n' + src;
}
if (opts.debug) {
console.log(src);
}
if (opts.compileDebug && opts.filename) {
src = src + '\n'
+ '//# sourceURL=' + sanitizedFilename + '\n';
}
try {
if (opts.async) {
// Have to use generated function for this, since in envs without support,
// it breaks in parsing
try {
ctor = (new Function('return (async function(){}).constructor;'))();
}
catch(e) {
if (e instanceof SyntaxError) {
throw new Error('This environment does not support async/await');
}
else {
throw e;
}
}
}
else {
ctor = Function;
}
fn = new ctor(opts.localsName + ', escapeFn, include, rethrow', src);
}
catch(e) {
// istanbul ignore else
if (e instanceof SyntaxError) {
if (opts.filename) {
e.message += ' in ' + opts.filename;
}
e.message += ' while compiling ejs\n\n';
e.message += 'If the above error is not helpful, you may want to try EJS-Lint:\n';
e.message += 'https://github.com/RyanZim/EJS-Lint';
if (!opts.async) {
e.message += '\n';
e.message += 'Or, if you meant to create an async function, pass `async: true` as an option.';
}
}
throw e;
}
// Return a callable function which will execute the function
// created by the source-code, with the passed data as locals
// Adds a local `include` function which allows full recursive include
var returnedFn = opts.client ? fn : function anonymous(data) {
var include = function (path, includeData) {
var d = utils.shallowCopy(utils.createNullProtoObjWherePossible(), data);
if (includeData) {
d = utils.shallowCopy(d, includeData);
}
return includeFile(path, opts)(d);
};
return fn.apply(opts.context,
[data || utils.createNullProtoObjWherePossible(), escapeFn, include, rethrow]);
};
if (opts.filename && typeof Object.defineProperty === 'function') {
var filename = opts.filename;
var basename = path.basename(filename, path.extname(filename));
try {
Object.defineProperty(returnedFn, 'name', {
value: basename,
writable: false,
enumerable: false,
configurable: true
});
} catch (e) {/* ignore */}
}
return returnedFn;
},
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compile: function () {
/** @type {string} */
var src;
/** @type {ClientFunction} */
var fn;
var opts = this.opts;
var prepended = '';
var appended = '';
/** @type {EscapeCallback} */
••• var escapeFn = opts.escapeFunction; •••
/** @type {FunctionConstructor} */
var ctor;
/** @type {string} */
var sanitizedFilename = opts.filename ? JSON.stringify(opts.filename) : 'undefined';
if (!this.source) {
this.generateSource();
prepended +=
' var __output = "";\n' +
' function __append(s) { if (s !== undefined && s !== null) __output += s }\n';
if (opts.outputFunctionName) {
if (!_JS_IDENTIFIER.test(opts.outputFunctionName)) {
throw new Error('outputFunctionName is not a valid JS identifier.');
}
prepended += ' var ' + opts.outputFunctionName + ' = __append;' + '\n';
}
if (opts.localsName && !_JS_IDENTIFIER.test(opts.localsName)) {
throw new Error('localsName is not a valid JS identifier.');
}
if (opts.destructuredLocals && opts.destructuredLocals.length) {
var destructuring = ' var __locals = (' + opts.localsName + ' || {}),\n';
for (var i = 0; i < opts.destructuredLocals.length; i++) {
var name = opts.destructuredLocals[i];
if (!_JS_IDENTIFIER.test(name)) {
throw new Error('destructuredLocals[' + i + '] is not a valid JS identifier.');
}
if (i > 0) {
destructuring += ',\n ';
}
destructuring += name + ' = __locals.' + name;
}
prepended += destructuring + ';\n';
}
if (opts._with !== false) {
prepended += ' with (' + opts.localsName + ' || {}) {' + '\n';
appended += ' }' + '\n';
}
appended += ' return __output;' + '\n';
this.source = prepended + this.source + appended;
}
if (opts.compileDebug) {
src = 'var __line = 1' + '\n'
+ ' , __lines = ' + JSON.stringify(this.templateText) + '\n'
+ ' , __filename = ' + sanitizedFilename + ';' + '\n'
+ 'try {' + '\n'
+ this.source
+ '} catch (e) {' + '\n'
+ ' rethrow(e, __lines, __filename, __line, escapeFn);' + '\n'
+ '}' + '\n';
}
else {
src = this.source;
}
•••
if (opts.client) {
src = 'escapeFn = escapeFn || ' + escapeFn.toString() + ';' + '\n' + src;
if (opts.compileDebug) {
src = 'rethrow = rethrow || ' + rethrow.toString() + ';' + '\n' + src;
}
}
//- `escapeFn`이 존재하면 그대로 사용하고, 없으면 `escapeFn.toString()`의 결과를 문자열로 변환하여 사용합니다. 그리고 이를 `src` 문자열 앞에 추가합니다.
•••
if (opts.strict) {
src = '"use strict";\n' + src;
}
if (opts.debug) {
console.log(src);
}
if (opts.compileDebug && opts.filename) {
src = src + '\n'
+ '//# sourceURL=' + sanitizedFilename + '\n';
}
try {
if (opts.async) {
// Have to use generated function for this, since in envs without support,
// it breaks in parsing
try {
ctor = (new Function('return (async function(){}).constructor;'))();
}
catch(e) {
if (e instanceof SyntaxError) {
throw new Error('This environment does not support async/await');
}
else {
throw e;
}
}
}
else {
ctor = Function;
}
•••
fn = new ctor(opts.localsName + ', escapeFn, include, rethrow', src);
//`opts.localsName + ', escapeFn, include, rethrow'` 부분이 생성되는 함수의 인수 이름을 나타내며, `src`는 함수 본문을 나타냅니다. 이렇게 생성된 함수 객체는 `fn` 변수에 할당됩니다.
•••
}
catch(e) {
// istanbul ignore else
if (e instanceof SyntaxError) {
if (opts.filename) {
e.message += ' in ' + opts.filename;
}
e.message += ' while compiling ejs\n\n';
e.message += 'If the above error is not helpful, you may want to try EJS-Lint:\n';
e.message += 'https://github.com/RyanZim/EJS-Lint';
if (!opts.async) {
e.message += '\n';
e.message += 'Or, if you meant to create an async function, pass `async: true` as an option.';
}
}
throw e;
}
// Return a callable function which will execute the function
// created by the source-code, with the passed data as locals
// Adds a local `include` function which allows full recursive include
••• // fn 함수를 returnedFn에 할당합니다.
var returnedFn = opts.client ? fn : function anonymous(data) {
var include = function (path, includeData) {
var d = utils.shallowCopy(utils.createNullProtoObjWherePossible(), data);
if (includeData) {
d = utils.shallowCopy(d, includeData);
}
return includeFile(path, opts)(d);
};
return fn.apply(opts.context,
[data || utils.createNullProtoObjWherePossible(), escapeFn, include, rethrow]);
};
if (opts.filename && typeof Object.defineProperty === 'function') {
var filename = opts.filename;
var basename = path.basename(filename, path.extname(filename));
try {
Object.defineProperty(returnedFn, 'name', {
value: basename,
writable: false,
enumerable: false,
configurable: true
});
} catch (e) {/* ignore */}
}
return returnedFn; // 할당된 함수를 return
•••
},
마지막 return 부분에서 입력할 value 값을 return 하는 것을 확인하고 즉시 실행하는 함수 표현식을 이용해서 RCE가 가능하다는 것을 알았습니다.