This document describes a web cache deception attack where an attacker can exploit how web servers and caching mechanisms handle requests for non-existent files. Specifically, if a request is made for a page plus a non-existent file extension, like http://www.example.com/account.php/stylesheet.css, some systems will return the content of the page rather than a 404. This allows an authenticated user's private page to be cached and then accessed by an attacker. The document provides examples of frameworks like Django and servers like IIS that can be exploited this way. It also discusses how caching services like Cloudflare have addressed this issue. Mitigations are proposed like only caching files if headers allow it and returning 302/404

1. WEB CACHE DECEPTION
ATTACK
Omer Gil

2. No SID

3. About me
• Omer Gil
• 28
• Married + Java
• PT team leader at EY
• Student
@omer_gil omergil.blogspot.com

4. About caching

5. About caching
CDN

6. About caching
CDN

7. About caching
CDN

8. About caching
Load Balancer

9. About caching
Load Balancer

10. About caching
Load Balancer

11. About caching
Load Balancer

12. About caching
Reverse Proxy
DMZ

13. About caching
Reverse Proxy
DMZ

14. About caching
Stylesheet.css
https://www.example.com/stylesheet.css

15. About caching
Stylesheet.css
https://www.example.com/stylesheet.css
Stylesheet.css

16. About caching
https://www.example.com/stylesheet.css
Stylesheet.css

17. About caching
https://www.example.com/stylesheet.css
Stylesheet.cssStylesheet.css

18. /nonexistent.css
Servers’ reactions
http://www.example.com/account.php
account.php
The Spanner: http://www.thespanner.co.uk/2014/03/21/rpo/
XSS Jigsaw: http://blog.innerht.ml/page/2/

19. Servers’ reactions
account.php
/nonexistent.csshttp://www.example.com/account.php

20. Getting down to business
“Hey, access
https://www.bank.com/account.do/stylesheet.css”
WAF!

21. Getting down to business
The user browses to
https://www.bank.com/account.do/stylesheet.css

22. Getting down to business
https://www.bank.com/account.do/stylesheet.css
returns with the content of account.do
and the private page is cached

23. Getting down to business
The attacker browses to
https://www.bank.com/account.do/stylesheet.css
and gets the content of the user’s account.do page

24. Getting down to business
The attacker browses to
https://www.bank.com/account.do/stylesheet.css
and gets the content of the user’s account.do page

25. Demo

27. Conditions
• Web cache functionality is set for the web application to cache static files
based on their extensions, disregarding any caching header.
• When accessing a page like
http://www.example.com/home.php/nonexistent.css, the web server will
return the content of home.php for that URL.
• Victim has to be authenticated while accessing the triggering URL.

28. Why the HELL #1
would a web application react like this?

29. Why the HELL #1
Django:
http://www.sampleapp.com/inbox/

30. Why the HELL #1
Django:
http://www.sampleapp.com/inbox/test.css
urls.py

31. Why the HELL #1
Django:
urls.py
http://www.sampleapp.com/inbox.css

32. Why the HELL #1
Django:
http://www.sampleapp.com/inbox/test.css
urls.py

33. Why the HELL #2
would a caching mechanism react like this?

34. Why the HELL #2
IIS ARR:

35. Why the HELL #2
IIS ARR:
http://www.sampleapp.com/welcome.php/test.css

36. Why the HELL #2
IIS ARR:
http://www.sampleapp.com/welcome.php/test.css

37. Why the HELL #2
Cloudflare:
• Eligibility phase
class, css, jar, js, jpg, jpeg, gif, ico, png, bmp, pict, csv, doc, docx, xls, xlsx, ps, pdf,
pls, ppt, pptx, tif, tiff, ttf, otf, webp, woff, woff2, svg, svgz, eot, eps, ejs, swf, torrent,
midi, mid
https://blog.cloudflare.com/understanding-our-cache-and-the-web-cache-deception-attack/
https://blogs.akamai.com/2017/03/on-web-cache-deception-attacks.html

38. Why the HELL #2
Cloudflare:
• Disqualification phase
‘Edge cache expire TTL’ to the rescue!

39. Why the HELL #2
Cloudflare:
https://blog.cloudflare.com/edge-cache-expire-ttl-easiest-way-to-override/

40. Mitigation
• Only cache files if their HTTP caching headers allow
• Store all static files in a designated directory
• Cache files by their content type
• Don’t accept this! http://www.example.com/home.php/non-existent.css.
Return 302 or 404 instead

41. THANKS
@omer_gil
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omergil.blogspot.com