25 Million Flows Later – Large-scale Detection of DOM-based XSS
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The document discusses large-scale detection of DOM-based cross-site scripting (DOM XSS) vulnerabilities, focusing on an automated, taint-aware browsing engine designed to analyze and validate these vulnerabilities. The authors conducted an empirical study that analyzed over 4 million web documents, leading to the identification of numerous suspicious data flows and a significant count of actual vulnerabilities within top web domains. The findings indicate that at least 9.6% of the top 5000 web pages contain one or more DOM XSS vulnerabilities, emphasizing the need for improved security measures.
![Validating vulnerabilities
● Current Situation:
● Taint-tracking engine delivers suspicious flows
● Suspicious flow != Vulnerability
● Why may suspicious flows not be exploitable?
● e.g. custom filter, validation or encoding function
● Validation needed: working exploit
<script>!
if (/^[a-z][0-9]+$/.test(location.hash.slice(1)) {!
document.write(location.hash.slice(1));!
}!
</script>
19](https://image.slidesharecdn.com/ccs169-140605003604-phpapp01/85/25-Million-Flows-Later-Large-scale-Detection-of-DOM-based-XSS-19-320.jpg)
25 Million Flows Later – Large-scale Detection of DOM-based XSS
- 1. 25 Million Flows Later – Large-scale Detection of DOM-based XSS CCS 2013, Berlin Sebastian Lekies, Ben Stock, Martin Johns
- 2. Agenda ● XSS & Attacker Scenario ● WebSec guys: wake up once you see a cat ● Motivation ● Our contributions ● Summary 2
- 3. Cross-Site Scripting ● Execution of attacker-controlled code on the client in the context of the vulnerable app ● Three kinds: ● Persistent XSS: guestbook, ... ● Reflected XSS: search forms, ... ● DOM-based XSS: also called local XSS ● content dynamically added by JS (e.g. like button), .. 3 Server side Client side
- 4. Cross-Site Scripting: attacker model ● Attacker wants to inject own code into vuln. app ● steal cookie ● take abritrary action in the name of the user ● pretend to be the server towards the user ● ... 4 Source: http://blogs.sfweekly.com/thesnitch/ cookie_monster.jpg
- 5. Cross-Site Scripting: problem statement ● Main problem: attacker‘s content ends in document and is not properly filtered/encoded ● common for server- and client-side flaws ● Flow of data: from attacker-controllable source to security- sensitive sink ● Our Focus: client side JavaScript code ● Sources: e.g. the URL ● Sinks: e.g. document.write 5
- 6. Example of a DOMXSS vulnerability ● Source: location.hash, Sink: document.write ● Intended usage: ● http://example.org/#mypage ● <img src='//adve.rt/ise?hash=mypage'/>! ● Exploiting the vuln: ● http://example.org/#'/><script>alert(1)</script> ● <img src='//adve.rt/ise?hash='/> <script>alert(1)</script> '/> 6 document.write("<img src='//adve.rt/ise?hash=" + location.hash.slice(1)+ "'/>");!
- 7. How does the attacker exploit this? a. Send a crafted link to the victim b. Embed vulnerable page with payload into his own page 7 h"p://ki"enpics.org Source: http://www.hd-gbpics.de/gbbilder/katzen/katzen2.jpg
- 8. Our motivation and contribution ● Perform Large-scale analysis of DOMXSS vulnerabilities ● Automated, dynamic detection of suspicious flows ● Automated validation of vulnerabilities ● Our key components ● Taint-aware browsing engine ● Crawling infrastructure ● Context-specific exploit generator ● Exploit verification using the crawler 8
- 9. Building a taint-aware browsing engine to find suspicious flows
- 10. Our approach: use dynamic taint tracking ● Taint tracking: Track the flow of marked data from source to sink ● Implementation: into Chromium (Blink+V8) ● Requirements for taint tracking ● Taint all relevent values / propagate taints ● Report all sinks accesses ● be as precise as possible ● taint details on EVERY character 10
- 11. Representing sources ● In terms of DOMXSS, we have 14 sources ● additionally, three relevant, built-in encoding functions ● escape, encodeURI and encodeURIComponent ● .. may prevent XSS vulnerabilites if used properly ● Goal: store source + bitmask of encoding functions for each character 11
- 12. Representing sources (cntd) ● 14 sources è 4 bits sufficient ● 3 relevant built-in functions è 3 bits sufficient 7 bits < 1 byte ● è 1 Byte sufficient to store source + encoding functions ● encoding functions and counterparts set/unset bits ● hard-coded characters have source 0 12 enconding functions Source
- 13. Representing sources (cntd) ● Each source API (e.g. URL or cookie) attaches taint bytes ● identifing the source of a char ● var x = location.hash.slice(1);! ● x = escape(x);! 13 t e s ' 1 1 1 1 t e s % 65 65 65 65 2 7 65 65 0 1 0 0 0 0 0 1
- 14. Detecting sink access ● Taint propagated through all relevant functions ● Security-sensitive sinks report flow and details ● such as text, taint information, source code location ● Chrome extension to handle reporting ● keep core changes as small as possible ● repack information in JavaScript ● stub function directly inside V8 14 Extension V8 JS eval report WebKit document.write
- 15. Empirical study on suspicious flows
- 16. Crawling the Web (at University scale) ● Crawler infrastructure constisting of ● modified, taint-aware browsing engine ● browser extension to direct the engine ● Dispatching and reporting backend ● In total, we ran 6 machines Control'backend' Background'script' Tab'1' content'' script' Web' page''' &'' user' script' Tab'n' content'' script' Web' page''' &'' user' script'…' Background'script' Tab'1' content'' script' Web' page''' &'' user' script' Tab'n' content'' script' Web' page''' &'' user' script'…'…' Browser'1' Browser'm' 16
- 17. Empirical study ● Shallow crawl of Alexa Top 5000 Web Sites ● Main page + first level of links ● 504,275 URLs scanned in roughly 5 days ● on average containing ~8,64 frames ● total of 4,358,031 analyzed documents ● Step 1: Flow detection ● 24,474,306 data flows from possibly attacker-controllable input to security-sensitive sinks 17
- 18. Context-Sensitive Generation of Cross-Site Scripting Payloads
- 19. Validating vulnerabilities ● Current Situation: ● Taint-tracking engine delivers suspicious flows ● Suspicious flow != Vulnerability ● Why may suspicious flows not be exploitable? ● e.g. custom filter, validation or encoding function ● Validation needed: working exploit <script>! if (/^[a-z][0-9]+$/.test(location.hash.slice(1)) {! document.write(location.hash.slice(1));! }! </script> 19
- 20. Anatomy of an XSS Exploit ● Cross-Site Scripting exploits are context-specific: ● HTML Context ● Vulnerability: ● Exploit: ● JavaScript Context ● Vulnerability: ● Exploit: eval("var x = '" + location.hash + "';"); '; alert(1); // document.write("<img src='pic.jpg?hash=" ! !+ location.hash.slice(1) + "'>"); '><script>alert(1)</script><textarea> 20
- 21. Anatomy of an XSS Exploit ● Context-Sensitivity ● Breakout-Sequence: Highly context sensitive (generation is difficult) ● Payload: Not context sensitive (arbitrary JavaScript code) ● Comment Sequence: Very easy to generate (choose from a handful of options) Payload Break-in / Comment SequenceBreak-out Sequence '; alert(1); // '><script> alert(1); </script><textarea> 21
- 22. Breaking out of JavaScript contexts ● JavaScript Context ● Visiting http://example.org/ in our engine <script>! var code = 'function test(){' ! ! + 'var x = "' + location.href + '";'! ! //inside function test! ! + 'doSomething(x);'! ! + '}'; ! //top level ! eval(code);! </script> eval(' function test() {! var x = "http://example.org";! doSomething(x);! } '); 22
- 23. Syntax tree to working exploit ● Two options here: ● break out of string ● break out of function definition ● Latter is more reliable ● function test not necessarily called automatically on „normal“ execution 23 Tainted value aka injecAon point function test() {! var x = "http://example.org";! doSomething(x);! }
- 24. Generating a valid exploit ● Traverse the AST upwards and “end” the branches ● Breakout Sequence: “;} ● Comment: // ● Exploit: ";}alert(1);// ● Visit: http://example.org/#";}alert(1);// } ; “ 24 function test() {! var x = "http://example.org";! }! alert(1);//“; doSomething(x); }
- 25. Validating vulnerabilities ● Our focus: directly controllable exploits ● Sinks: direct execution sinks ● HTML sinks (document.write, innerHTML ,...) ● JavaScript sinks (eval, ...) ● Sources: location and referrer ● Only unencoded strings ● Not in the focus (yet): second-order vulnerabilities ● to cookie and from cookie to eval ● ... 25
- 26. Empirical study ● Step 2: Flow reduction ● Only JavaScript and HTML sinks: 24,474,306 è 4,948,264 ● Only directly controllable sources: 4,948,264 è 1,825,598 ● Only unencoded flows: 1,825,598 è 313,794 ● Step 3: Precise exploit generation ● Generated a total of 181,238 unique test cases ● rest were duplicates (same URL and payload) ● basically same vuln twice in same page 26
- 27. Empirical study ● Step 4: Exploit validation ● 69,987 out of 181,238 unique test cases triggered a vulnerability ● Step 5: Further analysis ● 8,163 unique vulnerabilities affecting 701 domains ● …of all loaded frames (i.e. also from outside Top 5000) ● 6,167 unique vulnerabilities affecting 480 Alexa top 5000 domains ● At least, 9.6 % of the top 5000 Web pages contain one or more XSS problems ● This number only represents the lower bound (!) 27
- 28. Limitations ● No assured code coverage ● e.g. debug GET-param needed? ● also, not all pages visited (esp. stateful applications) ● Fuzzing might get better results ● does not scale as well ● Not yet looking at the „harder“ flows ● found one URL è Cookie è eval „by accident“ 28
- 29. Summary ● We built a tool capable of detecting flows ● taint-aware Chromium ● Chrome extension for crawling and reporting ● We built an automated exploit generator ● taking into account the exact taint information ● ... and specific contexts ● We found that at least 480 of the top 5000 domains carry a DOM-XSS vuln 29
- 30. Thank you very much for your attention! Ben Stock @kcotsneb ben.stock@fau.de
- 31. 31 URL Cookie Referrer window.name postMessage WebStorage Total HTML 1,356,796 1,535,299 240,341 35,446 35,103 16,387 3,219,392 JavaScript 22,962 359,962 511 617,743 448,311 279,383 1,728,872 URL 3,798,228 2,556,709 313,617 83,218 18,919 28,052 6,798,743 Cookie 220,300 10,227,050 25,062 1,328,634 2,554 5,618 11,809,218 post Message 451,170 77,202 696 45,220 11,053 117,575 702,916 Web Storage 41,739 65,772 1,586 434 194 105,440 215,165 Total 5,891,195 14,821,994 581,813 2,110,715 516,134 552,455 24,474,306 Encoded 64,78% 52,81% 83,99% 57,69% 1,57% 30,31% Sinks Sources Outlook on future work
- 32. 32 URL Cookie Referrer window.name postMessage WebStorage Total HTML 1,356,796 1,535,299 240,341 35,446 35,103 16,387 3,219,392 JavaScript 22,962 359,962 511 617,743 448,311 279,383 1,728,872 URL 3,798,228 2,556,709 313,617 83,218 18,919 28,052 6,798,743 Cookie 220,300 10,227,050 25,062 1,328,634 2,554 5,618 11,809,218 post Message 451,170 77,202 696 45,220 11,053 117,575 702,916 Web Storage 41,739 65,772 1,586 434 194 105,440 215,165 Total 5,891,195 14,821,994 581,813 2,110,715 516,134 552,455 24,474,306 Encoded 64,78% 52,81% 83,99% 57,69% 1,57% 30,31% Sinks Sources Outlook on future work
- 33. 33 URL Cookie Referrer window.name postMessage WebStorage Total HTML 1,356,796 1,535,299 240,341 35,446 35,103 16,387 3,219,392 JavaScript 22,962 359,962 511 617,743 448,311 279,383 1,728,872 URL 3,798,228 2,556,709 313,617 83,218 18,919 28,052 6,798,743 Cookie 220,300 10,227,050 25,062 1,328,634 2,554 5,618 11,809,218 post Message 451,170 77,202 696 45,220 11,053 117,575 702,916 Web Storage 41,739 65,772 1,586 434 194 105,440 215,165 Total 5,891,195 14,821,994 581,813 2,110,715 516,134 552,455 24,474,306 Encoded 64,78% 52,81% 83,99% 57,69% 1,57% 30,31% Sinks Sources Outlook on future work
- 34. 34 URL Cookie Referrer window.name postMessage WebStorage Total HTML 1,356,796 1,535,299 240,341 35,446 35,103 16,387 3,219,392 JavaScript 22,962 359,962 511 617,743 448,311 279,383 1,728,872 URL 3,798,228 2,556,709 313,617 83,218 18,919 28,052 6,798,743 Cookie 220,300 10,227,050 25,062 1,328,634 2,554 5,618 11,809,218 post Message 451,170 77,202 696 45,220 11,053 117,575 702,916 Web Storage 41,739 65,772 1,586 434 194 105,440 215,165 Total 5,891,195 14,821,994 581,813 2,110,715 516,134 552,455 24,474,306 Encoded 64,78% 52,81% 83,99% 57,69% 1,57% 30,31% Sinks Sources Outlook on future work