Detecting headless browsers

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This presentation by Sergey Shekyan and Bei Zhang discusses offensive use of headless browsers tools, and how to counteract them in practice.

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Detecting headless browsers

  1. 1. Headless Browser Hide & Seek Sergey Shekyan, Bei Zhang Shape Security
  2. 2. Who We Are • Bei Zhang Senior Software Engineer at Shape Security, focused on analysis and countermeasures of automated web attacks. Previously, he worked at the Chrome team at Google with a focus on the Chrome Apps API. His interests include web security, source code analysis, and algorithms. • Sergey Shekyan Principal Engineer at Shape Security, focused on the development of the new generation web security product. Prior to Shape Security, he spent 4 years at Qualys developing their on demand web application vulnerability scanning service. Sergey presented research at security conferences around the world, covering various information security topics.
  3. 3. Is There A Problem?
  4. 4. What Is a Headless Browser and How it Works Scriptable browser environment that doesn’t require GUI • Existing browser layout engine with bells and whistles (PhantomJS - WebKit, SlimerJS - Gecko, TrifleJS - Trident) • Custom software that models a browser (ZombieJS, HtmlUnit) • Selenium (WebDriver API)
  5. 5. What Is a Headless Browser and How it Works Discussion will focus on PhantomJS: • Backed by WebKit engine • Cross-platform • Popular • True headless
  6. 6. PhantomJS World PhantomJS JavaScript Context QWebFrame QtWebKit Web Page JavaScript Context Control Callback Injection PageEvent Callbacks are serialized var page = require('webpage').create(); page.open(url, function(status) { var title = page.evaluate(function() { return document.title; }); console.log('Page title is ' + title); });
  7. 7. Legitimate uses and how you can benefit • Web Application functional and performance testing • Crawler that can provide certain amount of interaction to reveal web application topology, Automated DOM XSS, CSRF detection • SEO (render dynamic web page into static HTML to feed to search engines) • Reporting, image generation
  8. 8. Malicious Use of Headless Browser • Fuzzing • Botnet • Content scraping • Login brute force attacks • Click fraud • Bidding wars Web admins tend to block PhantomJS in production, so pretending to be a real browser is healthy choice
  9. 9. How It Is Different From a Real Browser • Outdated WebKit engine (close to Safari 5 engine, 4 y.o.) • Uses Qt Framework’s QtWebKit wrapper around WebKit • Qt rendering engine • Qt network stack, SSL implementation • Qt Cookie Jar, that doesn’t support RFC 2965 • No Media Hardware support (no video and audio) • Exposes window.callPhantom and window._phantom • No sandboxing
  10. 10. Good vs. Bad
  11. 11. Headless Browser Seek • Look at user agent string if (/PhantomJS/.test(window.navigator.userAgent)) { console.log(‘PhantomJS environment detected.’); }
  12. 12. Headless Browser Hide • Making user-agent (and navigator.userAgent) a “legitimate” one: var page = require(‘webpage').create(); page.settings.userAgent = ‘Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:30.0) Gecko/20100101 Firefox/30.0';
  13. 13. Score board Phantom Web site User Agent String Win Lose
  14. 14. Headless Browser Seek • Sniff for PluginArray content if (!(navigator.plugins instanceof PluginArray) || navigator.plugins.length == 0) {     console.log("PhantomJS environment detected.");   } else {     console.log("PhantomJS environment not detected.");   }
  15. 15. Headless Browser Hide • Fake navigator object, populate PluginArray with whatever values you need. • Spoofing Plugin objects inside the PluginArray is tedious and hard. • Websites can actually create a plugin to test it. • CONCLUSION: Not a good idea to spoof plugins. page.onInitialized = function () {     page.evaluate(function () {         var oldNavigator = navigator;         var oldPlugins = oldNavigator.plugins;         var plugins = {};         plugins.length = 1;         plugins.__proto__ = oldPlugins.__proto__;         window.navigator = {plugins: plugins};         window.navigator.__proto__ = oldNavigator.__proto__;     }); };
  16. 16. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win
  17. 17. Headless Browser Seek • Alert/prompt/confirm popup suppression timing detection var start = Date.now();   prompt('I`m just kidding');   var elapse = Date.now() - start;   if (elapse < 15) {     console.log("PhantomJS environment detected. #1");   } else {     console.log("PhantomJS environment not detected.");   }
  18. 18. Headless Browser Hide • Can’t use setTimeout, but blocking the callback by all means would work page.onAlert = page.onConfirm = page.onPrompt = function () {     for (var i = 0; i < 1e8; i++) {     }     return "a"; };
  19. 19. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win
  20. 20. Headless Browser Seek • Default order of headers is consistently different in PhantomJS. Camel case in some header values is also a good point to look at. PhantomJS 1.9.7 GET / HTTP/1.1 User-Agent: Accept: Connection: Keep-Alive Accept-Encoding: Accept-Language: Host: Chrome 37 GET / HTTP/1.1 Host: Connection: keep-alive Accept: User-Agent: Accept-Encoding: Accept-Language:
  21. 21. Headless Browser Hide • A custom proxy server in front of PhantomJS instance that makes headers look consistent with user agent string
  22. 22. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose
  23. 23. Headless Browser Seek • PhantomJS exposes APIs: • window.callPhantom • window._phantom // not documented. if (window.callPhantom || window._phantom) {   console.log("PhantomJS environment detected."); } else {  console.log("PhantomJS environment not detected."); }
  24. 24. Headless Browser Hide • store references to original callPhantom, _phantom • delete window.callPhantom, window._phantom page.onInitialized = function () {     page.evaluate(function () {         var p = window.callPhantom;         delete window._phantom;         delete window.callPhantom;         Object.defineProperty(window, "myCallPhantom", {             get: function () { return p;},             set: function () {}, enumerable: false});         setTimeout(function () { window.myCallPhantom();}, 1000);     }); }; page.onCallback = function (obj) { console.log(‘profit!'); }; Unguessable name
  25. 25. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose window.callPhantom Win Lose
  26. 26. Headless Browser Seek • Spoofing DOM API properties of real browsers: • WebAudio • WebRTC • WebSocket • Device APIs • FileAPI • WebGL • CSS3 - not observable. Defeats printing. • Our research on WebSockets: http://goo.gl/degwTr
  27. 27. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose window.callPhantom Win Lose HTML5 features Lose Win
  28. 28. Headless Browser Seek • Significant difference in JavaScript Engine: bind() is not defined in PhantomJS prior to version 2 (function () {     if (!Function.prototype.bind) {       console.log("PhantomJS environment detected.");       return;     }     console.log("PhantomJS environment not detected.");   })(); Function.prototype.bind = function () { var func = this; var self = arguments[0]; var rest = [].slice.call(arguments, 1); return function () { var args = [].slice.call(arguments, 0); return func.apply(self, rest.concat(args)); }; };
  29. 29. Headless Browser Seek • Detecting spoofed Function.prototype.bind for PhantomJS prior to version 2 (function () {     if (!Function.prototype.bind) {       console.log("PhantomJS environment detected. #1");       return;     }     if (Function.prototype.bind.toString().replace(/bind/g, 'Error') != Error.toString()) {       console.log("PhantomJS environment detected. #2");       return;     }     console.log("PhantomJS environment not detected.");   })();
  30. 30. Headless Browser Hide • Spoofing Function.prototype.toString function functionToString() {     if (this === bind) {         return nativeFunctionString;     }     return oldCall.call(oldToString, this); }
  31. 31. Headless Browser Seek • Detecting spoofed Function.prototype.bind for PhantomJS prior to version 2 (function () {     if (!Function.prototype.bind) {       console.log("PhantomJS environment detected. #1");       return;     }     if (Function.prototype.bind.toString().replace(/bind/g, 'Error') != Error.toString()) {       console.log("PhantomJS environment detected. #2");       return;     }     if (Function.prototype.toString.toString().replace(/toString/g, 'Error') != Error.toString()) {       console.log("PhantomJS environment detected. #3");       return;     }     console.log("PhantomJS environment not detected.");   })();
  32. 32. Headless Browser Hide • Spoofing Function.prototype.toString.toString function functionToString() {     if (this === bind) {         return nativeFunctionString;     }     if (this === functionToString) {         return nativeToStringFunctionString;     }     if (this === call) {         return nativeCallFunctionString;     }     if (this === apply) {         return nativeApplyFunctionString;     }     var idx = indexOfArray(bound, this);     if (idx >= 0) {         return nativeBoundFunctionString;     }     return oldCall.call(oldToString, this); }
  33. 33. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose window.callPhantom Win Lose HTML5 features Lose Win Function.prototype.bind Win Lose
  34. 34. Headless Browser Seek • PhantomJS2 is very EcmaScript5 spec compliant, so checking for outstanding JavaScript features is not going to work
  35. 35. Headless Browser Seek • Stack trace generated by PhantomJs • Stack trace generated by SlimerJS • Hmm, there is something common… at querySelectorAll (phantomjs://webpage.evaluate():9:10) at phantomjs://webpage.evaluate():19:30 at phantomjs://webpage.evaluate():20:7 at global code (phantomjs://webpage.evaluate():20:13) at evaluateJavaScript ([native code]) at global code (/Users/sshekyan/Projects/phantomjs/spoof.js:8:14) Element.prototype.querySelectorAll@phantomjs://webpage.evaluate():9 @phantomjs://webpage.evaluate():19 @phantomjs://webpage.evaluate():20
  36. 36. Headless Browser Seek var err; try { null[0](); } catch (e) { err = e; } if (indexOfString(err.stack, 'phantomjs') > -1) { console.log("PhantomJS environment detected."); } else {  console.log("PhantomJS environment is not detected."); It is not possible to override the thrown TypeError generic indexOf(), can be spoofed, define your own
  37. 37. Headless Browser Hide • Modifying webpage.cpp: QVariant WebPage::evaluateJavaScript(const QString &code) { QVariant evalResult; QString function = "(" + code + ")()"; evalResult = m_currentFrame->evaluateJavaScript(function, QString("phantomjs://webpage.evaluate()")); return evalResult; } at at at at at evaluateJavaScript ([native code]) at at global code (/Users/sshekyan/Projects/phantomjs/spoof.js:8:14) • Produces
  38. 38. Headless Browser Hide • Spoofed PhantomJs vs Chrome: at querySelectorAll (Object.InjectedScript:9:10) at Object.InjectedScript:19:30 at Object.InjectedScript:20:7 at global code (Object.InjectedScript:20:13) at evaluateJavaScript ([native code]) at at global code (/Users/sshekyan/Projects/phantomjs/spoof.js:8:14) TypeError: Cannot read property '0' of null at HTMLDocument.Document.querySelectorAll.Element.querySelectorAll [as querySelectorAll] (<anonymous>:21:5) at <anonymous>:2:10 at Object.InjectedScript._evaluateOn (<anonymous>:730:39) at Object.InjectedScript._evaluateAndWrap (<anonymous>:669:52) at Object.InjectedScript.evaluate (<anonymous>:581:21)
  39. 39. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose window.callPhantom Win Lose HTML5 features Lose Win Function.prototype.bind Win Lose Stack trace Lose Win
  40. 40. Score board Phantom Web site User Agent String Win Lose Inspect PluginArray Lose Win Timed alert() Lose Win HTTP Header order Win Lose window.callPhantom Win Lose HTML5 features Lose Win Function.prototype.bind Win Lose Stack trace Lose Win SCORE: 4 4
  41. 41. Attacking PhantomJS
  42. 42. How to turn a headless browser against the attacker The usual picture: • PhantomJS running as root • No sandboxing in PhantomJS • Blindly executing untrusted JavaScript • outdated third party libs (libpng, libxml, etc.) Can possibly lead to: • abuse of PhantomJS • abuse of OS running PhantomJS
  43. 43. Headless Browser Seek var html = document.querySelectorAll('html'); var oldQSA = document.querySelectorAll; Document.prototype.querySelectorAll = Element.prototype.querySelectorAll = function () { var err; try { null[0](); } catch (e) { err = e; } if (indexOfString(err.stack, 'phantomjs') > -1) { return html; } else { return oldQSA.apply(this, arguments); } }; It is not possible to override the thrown TypeError generic indexOf(), can be spoofed, define your
  44. 44. Headless Browser Seek • In a lot of cases --web-security=false is used in PhantomJS var xhr = new XMLHttpRequest(); xhr.open('GET', 'file:/etc/hosts', false); xhr.onload = function () { console.log(xhr.responseText); }; xhr.onerror = function (e) { console.log('Error: ' + JSON.stringify(e)); }; xhr.send();
  45. 45. Headless Browser Seek • Obfuscate, randomize the output, randomize the modified API call var _0x34c7=["x68x74x6D x6C","x71x75x65x72x79x53x65x6Cx65x63x74x6F x72x41x6Cx6C","x70x72x6Fx74x6F x74x79x70x65","x73x74x61x63x6B","x70x68x61x6E x74x6Fx6Dx6Ax73","x61x70x70x6Cx79"];var html=document[_0x34c7[1]](_0x34c7[0]);var apsdk=document[_0x34c7[1]];Document[_0x34c7[2]] [_0x34c7[1]]=Element[_0x34c7[2]][_0x34c7[1]]=function () {var _0xad6dx3;try{null[0]();} catch(e) {_0xad6dx3=e;} ;if(indexOfString(_0xad6dx3[_0x34c7[3]],_0 x34c7[4])>-1){return html;} else {return apsdk[_0x34c7[5]] (this,arguments);};};
  46. 46. Tips for Using Headless Browsers Safely • If you don’t need a full blown browser engine, don’t use it • Do not run with ‘- -web-security=false’ in production, try not to do that in tests as well. • Avoid opening arbitrary page from the Internet • No root or use chroot • Use child processes to run webpages • If security is a requirement, use alternatives or on a controlled environment, use Selenium
  47. 47. Tips For Web Admins • It is not easy to unveil a user agent. Make sure you want to do it. • Do sniff for headless browsers (some will bail out) • Combine several detection techniques • Reject known unwanted user agents (5G Blacklist 2013 is a good start) • Alter DOM API if headless browser is detected • DoS • Pwn
  48. 48. Start Detecting Already Ready to use examples: github.com/ikarienator/phantomjs_hide_and_seek
  49. 49. References • http://www.sba-research.org/wp-content/uploads/publications/jsfingerprinting.pdf • https://kangax.github.io/compat-table/es5/ • https://media.blackhat.com/us-13/us-13-Grossman-Million-Browser-Botnet.pdf • http://ariya.ofilabs.com/2011/10/detecting-browser-sniffing-2.html • http://www.darkreading.com/attacks-breaches/ddos-attack-used-headless-browsers- in-150-hour-siege/d/d-id/1140696? • http://vamsoft.com/downloads/articles/vamsoft-headless-browsers-in-forum-spam.pdf • http://blog.spiderlabs.com/2013/02/server-site-xss-attack-detection-with-modsecurity- and-phantomjs.html • http://googleprojectzero.blogspot.com/2014/07/pwn4fun-spring-2014-safari-part- i_24.html
  50. 50. Thank you! @ikarienator @sshekyan

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