Csrf not all defenses are created equal
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A description of Cross-site request forgery (CSRF) attacks and defenses, with a focus on the commonly used libraries and functions which are used for CSRF defense. This presentation goes into each of them, and shows it's strengths, weaknesses, and shortcomings.
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Csrf not all defenses are created equal
- 1. CSRF: Not All Defenses Are Created Equal Ari Elias-Bachrach Defensium llc November 2013 CSRF: Not All Defenses Are Created Equal 1
- 2. This Talk is a Review of Current Defensive Options Or the long tail? Is your application one of the 80% CSRF: Not All Defenses Are Created Equal 2
- 3. This Talk Will Cover CSRF Defenses and Their Side Effects What is CSRF General (high level) fixes Code level defenses Server level defenses CSRF: Not All Defenses Are Created Equal 3
- 4. CSRF occurs when an attacker tricks a user's browser into performing an action on a website CSRF: Not All Defenses Are Created Equal 4
- 5. Normally, Browser's Form Submissions are Straightforward and Predictable <form action=submitpage> <input name= amount type=text> <input name=dest type=text> <input type=submit value=Transfer> </form> CSRF: Not All Defenses Are Created Equal 5
- 6. Normally, Browser's Form Submissions are Straightforward and Predictable If action was a POST POST /submitpage Server: server.com amount=100.00&dest=12345 If action was a GET GET /submitpage?amount=100.00&dest=12345 Server: server.com CSRF: Not All Defenses Are Created Equal 6
- 7. If you can predict all the parameters for an action, you can fake it To Fake a GET <img src=”...”> http://server.com/submitpage?amount=100.00&dest=12345 http://webmail.com/sendEmail?dest=boss@work&subj=resignation CSRF: Not All Defenses Are Created Equal 7
- 8. If you can predict all the parameters for an action, you can fake it To Fake a POST <form name=”evil” action=”http://server.com/submitpage” action=POST> <input type=”hidden” name=”amount” value=”100.00”> <input type=”hidden” name=”dest” value=”12345”> </form> <script>document.evil.submit()</script> CSRF: Not All Defenses Are Created Equal 8
- 9. Anatomy of an Attack 1. User navigates to website which attacker has some control over 2. User's browser tries to load content from site 3. Content performs action at a legitimate site CSRF: Not All Defenses Are Created Equal 9
- 10. Anatomy of an Attack <html> Malicious code Legitimate site Session cookie CSRF: Not All Defenses Are Created Equal 10
- 11. In 2008, A CSRF flaw Was Used to Attack Cable Modems Found a CSRF flaw in ADSL modems used by a Brazilian ISP Used it to Change DNS settings Sent users to malicious websites that looked like www.google.br CSRF: Not All Defenses Are Created Equal 11
- 12. High Level Defenses (Design Patterns) CSRF: Not All Defenses Are Created Equal 12
- 13. There are Four Design Patterns Which are Used Synchronizer Token Pattern Double Submit Cookies Challenge Response Check Referrer Header CSRF: Not All Defenses Are Created Equal 13
- 14. Primary Defense is the Synchronizer Token Pattern The most common defense Make at least one parameter unpredictable Upon submission, check to ensure the submitted value matches the generated value <input type="hidden" name="FromEmail" value="president@whitehouse.gov" /> <input type="hidden" name="Subject" value="Do something wild" /> <input type="hidden" name="GUID" value="0f41d8e54aa80b3193c28ed920" /> CSRF: Not All Defenses Are Created Equal 14
- 15. Primary Defense is the Synchronizer Token Pattern The most common defense Things to look out for - How are tokens remembered? - Completeness of coverage CSRF: Not All Defenses Are Created Equal 15
- 16. Second Defensive Option is Double Submit Cookies This option used less often, but useful for things like REST Generate a random value, store it in two places: 1 – a cookie 2 – a hidden form field Upon submission, check to see if they match abc123 <input>=abc123 abc123 <input> abc123 CSRF: Not All Defenses Are Created Equal 16
- 17. Second Defensive Option is Double Submit Cookies This option used less often, but useful for things like REST Things to look out for: - Do not use the Session ID for this purpose! abc123 <input>=abc123 abc123 <input> abc123 CSRF: Not All Defenses Are Created Equal 17
- 18. A Third Option is Any Form of Challenge Response System Rarely Used Exclusively for CSRF Defense CSRF: Not All Defenses Are Created Equal 18
- 19. A Third Option is Any Form of Challenge Response System Rarely Used Exclusively for CSRF Defense CSRF: Not All Defenses Are Created Equal 19
- 20. A Third Option is Any Form of Challenge Response System Rarely Used Exclusively for CSRF Defense CSRF: Not All Defenses Are Created Equal 20
- 21. A Third Option is Any Form of Challenge Response System Rarely Used Exclusively for CSRF Defense CSRF: Not All Defenses Are Created Equal 21
- 22. A Third Option is Any Form of Challenge Response System Rarely Used Exclusively for CSRF Defense Things to look out for: - User impact CSRF: Not All Defenses Are Created Equal 22
- 23. A Fourth Option is to Check the Referrer Header I Have Never Seen This Implemented GET /services/transfer.jsp HTTP/1.1 Host: mybank.com User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:16.0) Gecko/20100101 Firefox/16.0 Accept: */* Accept-Language: en-US,en;q=0.5 Referer: http://t.co/xblu14l6vL Cookie: JSESSIONID=007f0100547a514c54060044; CSRF: Not All Defenses Are Created Equal 23
- 24. A Fourth Option is to Check the Referrer Header I Have Never Seen This Implemented Things to look out for: - Potential impact on other things which may modify the referer header CSRF: Not All Defenses Are Created Equal 24
- 25. Actually Implementing These Patterns is Where it Gets Fun and Complicated Code Fixes Server Fixes CSRF: Not All Defenses Are Created Equal 25
- 26. We Will Show Five Common Software Libraries That Can Be Used To Do CSRF Defense 1. ViewState User Keys (.net) 2. AntiForgeryToken (.net MVC) 3. AntiCSRF (.net) 4. CSRFGuard (Java, PHP port is in progress) 5. HDIV (Java) CSRF: Not All Defenses Are Created Equal 26
- 27. .net can add CSRF protections to the ViewState Viewstate is meant to maintain a form's state on postbacks Page.aspx CSRF: Not All Defenses Are Created Equal Page.aspx 27
- 28. .net can add CSRF protections to the ViewState Adding the session ID to the view state makes it unpredictable sessionID CSRF: Not All Defenses Are Created Equal 28
- 29. .net can add CSRF protections to the ViewState Add to OnInit for all pages or once to base class protected override OnInit(EventArgs e) { base.OnInit(e); if (User.Identity.IsAuthenticated) ViewStateUserKey = Session.SessionID; } CSRF: Not All Defenses Are Created Equal 29
- 30. .net can add CSRF protections to the ViewState Viewstate User Keys was designed to protect against 1 click attacks, which are a subset of CSRF attacks Only protects postbacks - Won't protect posts to other pages Other.aspx Page.aspx Other.aspx CSRF: Not All Defenses Are Created Equal 30
- 31. .net MVC Applications Can Use AntiForgeryToken What about .net MVC? AntiForgeryToken - Part of the HtmlHelper class CSRF: Not All Defenses Are Created Equal 31
- 32. .net MVC Applications Can Use AntiForgeryToken <% using(Html.Form("UserProfile", "SubmitUpdate")) { %> <%= Html.AntiForgeryToken() %> <!-- rest of form goes here --> <input name="__RequestVerificationToken" type="hidden" value="saTFWpkKN0BYazFtN6c4YbZAmsEwG0srqlUqqloi/fVgeV2ciIFVmelv zwRZ" /> CSRF: Not All Defenses Are Created Equal 32
- 33. .net MVC Applications Can Use AntiForgeryToken Validate the token in the controller [ValidateAntiForgeryToken] public ActionResult FunctionToProtect() { // this is now run only if the token is valid } CSRF: Not All Defenses Are Created Equal 33
- 34. .net MVC Applications Can Use AntiForgeryToken By Default, will only work for POST Not a problem if GET is idempotent Can be hacked to work, google for details CSRF: Not All Defenses Are Created Equal 34
- 35. .net MVC Applications Can Use AntiForgeryToken Obvious problem: the forgetful programmer - must add to every controller and function that needs to be protected CSRF: Not All Defenses Are Created Equal 35
- 36. Anticsrf for .net implements the double submit cookies pattern Anticsrf - For .net - Has no other requirements (like viewstate enabled, MVC, etc.) - Open source - Developed in C# Available from http://anticsrf.codeplex.com/ CSRF: Not All Defenses Are Created Equal 36
- 37. Anticsrf for .net implements the double submit cookies pattern Generates string using Guid.NewGuid() Cookie: __CSRFCOOKIE=a22b81af-74f0-45ee-b2fd-1ead5f31f1c2; in POST __CSRFTOKEN=a22b81af-74f0-45ee-b2fd-1ead5f31f1c2 abc123 <input>=abc123 abc123 <input> abc123 CSRF: Not All Defenses Are Created Equal 37
- 38. Anticsrf for .net implements the double submit cookies pattern Can be used in a .net web app New Token for each session Only protects POST (not a problem if GET is idempotent) - Won't work for Rest (unless you hack it) abc123 <input>=abc123 abc123 <input> abc123 CSRF: Not All Defenses Are Created Equal 38
- 39. CSRFGuard Implements the Synchronizer Token Pattern and Makes a New Token For Each Session Made By OWASP (open source, BSD license) Java currently, PHP and .net port in progress Keeps one token per session, stored in the session - exposure of token compromises entire session CSRF: Not All Defenses Are Created Equal 39
- 40. CSRFGuard Implements the Synchronizer Token Pattern and Makes a New Token For Each Session Modifies existing GET and POST requests Keeps one token per session, stored in the session - exposure of token compromises entire session CSRF: Not All Defenses Are Created Equal link=nonce1 action=nonce1 40
- 41. CSRFGuard Can Also be Configured to Generate a New Token For Each Page Each link or action would get a unique token value Stored in session Feature is still experimental link=page?nonce1 action=page2?nonce2 CSRF: Not All Defenses Are Created Equal 41
- 42. CSRFGuard Can Also be Configured to Generate a New Token For Each Page Also supports AJAX Sets the token value in an HTTP header CSRF: Not All Defenses Are Created Equal 42
- 43. HDIV Uses Tokens With a Queue Based Expiry HDIV is a Java library that provides several security functions, including CSRF defense using the Synchronizer Token Pattern. The queue includes all generated tokens (could be dozens per page). link=page?nonce1 action=page2?nonce2 CSRF: Not All Defenses Are Created Equal 43
- 44. These Five Libraries All Have Different Approaches To CSRF Defense ViewState User Keys (.net) Synchronizer Token Pattern - only postbacks AntiForgeryToken (.net MVC) Synchronizer Token Pattern - needs lots of code changes AntiCSRF (.net) Double Submit Cookies CSRF: Not All Defenses Are Created Equal 44
- 45. These Five Libraries All Have Different Approaches To CSRF Defense CSRFGuard (Java) Synchronizer Token Pattern - can be done per session or page HDIV Synchronizer Token Pattern - per link/action - queue based expiry CSRF: Not All Defenses Are Created Equal 45
- 46. We Can Also Implement CSRF Protection on the Server Changing code on existing applications is hard What if we asked the server to do CSRF protection CSRF: Not All Defenses Are Created Equal 46
- 47. Tomcat 7 Includes a CSRF Prevention Filter Generates a new UUID for each page loaded - default generator is java.security.SecureRandom) Protects GET and POST - modifies links and form actions Stores the last n UUIDs in the session - default for n is 5 link=nonce1 http://server/page?org.apache.catalina.filters.CSRF_NONCE=31ACB2CA0A9... CSRF: Not All Defenses Are Created Equal 47
- 48. Tomcat's CSRF Prevention Filter Can Cause Usability Issues for User's With Multiple Browser Tabs Open User opens a second tab (same session, same cookies, etc.) Makes n mouse clicks (default n is 5) Original tab is now broken nonce1 CSRF: Not All Defenses Are Created Equal nonce2 nonce3 nonce4 nonce5 nonce6 48
- 49. F5's ASM Can Insert a Token in All Links and Forms to Implement the Synchronizer Token Pattern <form action=”foo”> <a href=”bar”> <form action=”foo”> <input type=”hidden” value=”12345”> <a href=”bar?csrt=12345”> CSRF: Not All Defenses Are Created Equal 49
- 50. F5's ASM Can Insert a Token in All Links and Forms to Implement the Synchronizer Token Pattern Will protect all GET and POST requests Token are generated per session, and have an expiry time (configurable from 1-99999 seconds). Default is 600 seconds Obvious problem of timeouts CSRF: Not All Defenses Are Created Equal 50
- 51. Imperva SecureSphere Can Detect CSRF Attacks by Checking the Referrer Header SecureSphere (Imperva's WAF) can alert and block when the referrer header of a request is from an external site GET /services/transfer.jsp HTTP/1.1 Host: mybank.com User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:16.0) Gecko/20100101 Firefox/16.0 Accept-Language: en-US,en;q=0.5 Referer: http://t.co/xblu14l6vL Cookie: JSESSIONID=007f0100547a514c54060044; CSRF: Not All Defenses Are Created Equal 51
- 52. Imperva SecureSphere Can Detect CSRF Attacks by Checking the Referrer Header The referrer header is not respected in all situations Bookmarks, links from external sites, and plugins that stop or tamper with the referrer header can all cause false positives CSRF: Not All Defenses Are Created Equal 52
- 53. All Three Of The Servers We Looked At Do CSRF Defense Differently Synchronizer Token Pattern - Queue based expiry Synchronizer Token Pattern - Time based expiry Check Referrer Header - Is intended for detection, not prevention CSRF: Not All Defenses Are Created Equal 53
- 54. CSRF Token Names Can Reveal What Library You Are Using CSRF: Not All Defenses Are Created Equal 54
- 55. CSRF Token Names Can Reveal What Library You Are Using CSRF: Not All Defenses Are Created Equal 55
- 56. CSRF Token Names Can Reveal What Library You Are Using Tomcat CSRFGuard 513 results 126,000 results CSRF: Not All Defenses Are Created Equal 56
- 57. CSRF Token Names Can Reveal What Library You Are Using Almost all of the solutions we've mentioned that use tokens allow you to customize the name of the token Some require you to edit source code to do it... CSRF: Not All Defenses Are Created Equal 57
- 58. A single XSS flaw makes all of these CSRF defenses useless There are numerous ways for a script to access the CSRF token value document.cookie document.getElementByID('csrftoken') document.forms[0].elements[0] CSRF: Not All Defenses Are Created Equal 58
- 59. Protecting GET Requests Comes At A Cost CSRF tokens can be leaked through the referer header, and can be reused if they're still valid GET /page HTTP/1.1 Host: othersite.com Referer: http://mysite.com/page?CSRF_TOKEN=1ba5690d4ea45fbab3 CSRF: Not All Defenses Are Created Equal 59
- 60. We Have Seen Seven Widely Used Implementations of CSRF Defense Know your defenses – which solution you select will depend on your application How many of these solutions were perfect? Security is rarely 'plug n play' CSRF: Not All Defenses Are Created Equal 60
- 61. We Have Seen Seven Widely Used Implementations of CSRF Defense Know your defenses – which solution you select will depend on your application Environment and language used Whether this is a new app or a retrofit of an old one Idempotence Potential user impact of some solutions CSRF: Not All Defenses Are Created Equal 61
- 62. CSRF: Not All Defenses Are Created Equal Ari Elias-Bachrach ari@defensium.com @angelofsecurity Defensium llc http://www.defensium.com CSRF: Not All Defenses Are Created Equal 62
Editor's Notes
- Request.Params versus Request.Form – param does GET r POST, form does only POST