Web Security essentials
PV219, spring 2024
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Agenda
• Cross-origin resource sharing (CORS)
• Cross-site request forgery (CSRF)
• Cross-site scripting (XSS)
• Click-jacking (UI redressing)
• Framing Third-Party Content (+sandboxing)
• SQL injection (SQLi)
• Fingerprinting
• Online exercises
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Cross-origin resource sharing (CORS)
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Cross-origin resource sharing (CORS)
CORS is a browser mechanism
which enables controlled access
to resources located outside of a
given domain. It extends and adds
flexibility to the same-origin policy
(SOP). However, it also provides
potential for cross-domain based
attacks, if a website's CORS policy
is poorly configured and
implemented.
CORS is not a protection against
cross-origin attacks such as CSRF.
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Cross-origin resource sharing (CORS)
Same-Origin Policy (SOP) restricts how a document or script loaded
from one origin can interact with a resource from another origin. It
generally allows a domain to issue requests to other domains, but not
to access the responses.
Let’s consider: http://store.company.com/dir/page.html
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
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Cross-origin resource sharing (CORS)
Why is CORS needed? (video)
• For legitimate and trusted requests to gain access to authorized data
from other domains
• Think cross application data sharing models
• Allows data to be exchanged with trusted sites while using a relaxed
Same-Origin Policy mode
• Application APIs exposed via web services and trusted domains
require CORS to be accessible over the SOP
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Cross-origin resource sharing (CORS)
Example: User visits http://www.example.com and the page attempts a cross-origin
request to fetch the user's data from http://service.example.com. This will happen:
1. The browser sends the GET request with an extra Origin HTTP header to service.example.com
containing the domain that served the parent page:
Origin: http://www.example.com
2. The server at service.example.com may respond with:
a) The requested data along with an Access-Control-Allow-Origin (ACAO) header in its response
indicating the requests from the origin are allowed. For example in this case it should be:
Access-Control-Allow-Origin: http://www.example.com
b) The requested data along with an Access-Control-Allow-Origin (ACAO) header with a wildcard
indicating that the requests from all domains are allowed:
Access-Control-Allow-Origin: *
c) An error page if the server does not allow a cross-origin request
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Cross-origin resource sharing (CORS)
Preflight requests for complex HTTP calls.
If a web app needs a complex HTTP request, the browser adds a preflight request to the
front of the request chain. The CORS specification defines a complex request as
• A request that uses methods other than GET, POST, or HEAD
• A request that includes headers other than Accept, Accept-Language or Content-
Language
• A request that has a Content-Type header other than application/x-www-formurlencoded,
multipart/form-data, or text/plain
Browsers create a preflight request if it is needed. It's an OPTIONS request and is sent
before the actual request message. Have a look on JSONP.
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Cross-origin resource sharing (CORS)
How to prevent CORS-based attacks?
• Proper configuration of cross-domain requests. If a web resource contains sensitive
information, the origin should be properly specified in the Access-Control-Allow-Origin
header.
• Only allow trusted sites. It may seem obvious but origins specified in the Access-ControlAllow-Origin
header should only be sites that are trusted. In particular, dynamically
reflecting origins from cross-domain requests without validation is readily exploitable and
should be avoided.
• Avoid whitelisting null. Avoid using the header Access-Control-Allow-Origin: null. Crossdomain
resource calls from internal documents and sandboxed requests can specify the null
origin. CORS headers should be properly defined in respect of trusted origins for private and
public servers.
• Avoid wildcards in internal networks. Avoid using wildcards in internal networks. Trusting
network configuration alone to protect internal resources is not sufficient when internal
browsers can access untrusted external domains. 10
Cross-site request forgery (CSRF)
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Cross-site request forgery (CSRF)
Cross-site request forgery
allows an attacker to induce
users to perform actions that
they do not intend to perform.
It allows an attacker to partly
circumvent the same origin
policy, which is designed to
prevent different websites from
interfering with each other.
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Cross-site request forgery (CSRF)
For a CSRF attack to be possible, three key conditions must be in place:
1. A relevant action. There is an action within the application that the attacker has a reason to
induce. This might be a privileged action (such as modifying permissions for other users) or any
action on user-specific data (such as changing the user's own password).
2. Cookie-based session handling. Performing the action involves issuing one or more HTTP
requests, and the application relies solely on session cookies to identify the user who has
made the requests. There is no other mechanism in place for tracking sessions or validating
user requests.
3. No unpredictable request parameters. The requests that perform the action do not contain
any parameters whose values the attacker cannot determine or guess. For example, when
causing a user to change their password, the function is not vulnerable if an attacker needs to
know the value of the existing password.
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Cross-site request forgery (CSRF)
Application’s HTTP request:
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 30
Cookie: session=yvthwsztyeQkAPzeQ5gHgTvlyxHfsAfE
email=smith@normal-user.com
Attackers web page:
<html>
<body>
<form action="https://vulnerable-website.com/email/change" method="POST">
<input type="hidden" name="email" value="pwned@evil-user.net" />
</form>
<script>
document.forms[0].submit();
</script>
</body>
</html>
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Cross-site request forgery (CSRF)
If a victim user visits the attacker's web page, the following will happen:
• The attacker's page will trigger an HTTP request to the vulnerable web site;
• If the user is logged in to the vulnerable web site, their browser will
automatically include their session cookie in the request;
• The vulnerable web site will process the request in the normal way, treat it as
having been made by the victim user, and change their email address.
Note:
Although CSRF is normally described in relation to cookie-based session handling, it also arises in other
contexts where the application automatically adds some user credentials to requests, such as HTTP
Basic authentication and certificate-based authentication.
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Cross-site request forgery (CSRF)
How to preventing CSRF attacks?
The most robust way to defend against CSRF attacks is to include
a CSRF token within relevant requests. The token should be:
• Unpredictable with high entropy, as for session tokens in general;
• Tied to the user's session;
• Strictly validated in every case before the relevant action is executed.
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Cross-site request forgery (CSRF)
How to preventing CSRF attacks?
POST /email/change HTTP/1.1
Host: vulnerable-website.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 68
Cookie: session=pSJYSScWKpmC60LpFOAHKixuFuM4uXWF;
csrfKey=rZHCnSzEp8dbI6atzagGoSYyqJqTz5dv
csrf=RhV7yQDO0xcq9gLEah2WVbmuFqyOq7tY&email=smith@normal-user.com
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Cross-site scripting (XSS)
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Cross-site scripting (XSS)
XSS allows an attacker to
compromise the interactions
that users have with a
vulnerable application.
It allows an attacker to
circumvent the same-origin
policy, which is designed to
segregate different websites
from each other.
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Cross-site scripting (XSS)
There are three main types of XSS attacks. These are:
1. Reflected XSS, where the malicious script comes from the current HTTP
request.
2. Stored XSS, where the malicious script comes from the website's database (or
another permanent storage).
3. DOM-based XSS, where the vulnerability exists in client-side code rather than
server-side code.
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Cross-site scripting (XSS)
Reflected XSS is the simplest variety of cross-site scripting.
https://insecure-website.com/status?message=All+is+well.
<p>Status: All is well.</p>
The application doesn't perform any other processing of the data, so an
attacker can easily construct an attack like this:
https://insecure-
website.com/status?message=<script>/*+Bad+stuff+here...+*/</script>
<p>Status: <script>/* Bad stuff here... */</script></p>
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Cross-site scripting (XSS)
Stored XSS (also known as persistent or second-order XSS) arises when an
application receives data from an untrusted source and includes that data
within its later HTTP responses in an unsafe way.
A message board application lets users submit messages, which are
displayed to other users:
<p>Hello, this is my message!</p>
The application doesn't perform any other processing of the data, so an
attacker can easily send a message that attacks other users:
<p><script>/* Bad stuff here... */</script></p>
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Cross-site scripting (XSS)
DOM-based XSS arises when an application contains some client-side
JavaScript that processes data from an untrusted source in an unsafe way,
usually by writing the data back to the DOM.
var search = document.getElementById('search').value;
var results = document.getElementById('results');
results.innerHTML = 'You searched for: ' + search;
If the attacker can control the value of the input field, they can easily
construct a malicious value that causes their own script to execute:
You searched for: <img src=1 onerror='/* Bad stuff here... */'>
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Cross-site scripting (XSS)
An attacker who exploits a cross-site scripting vulnerability is typically able
to:
• Impersonate or masquerade as the victim user
• Carry out any action that the user is able to perform
• Read any data that the user is able to access (e.g. cookies)
• Capture the user's login credentials
• Perform virtual defacement of the web site
• Inject trojan functionality into the web site
Visit XSS Vulnerability Payload List to see all variety of possible vectors, or test your skills.
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Cross-site scripting (XSS)
How to prevent XSS attacks?
• Filter input on arrival. At the point where user input is received, filter as strictly as possible
based on what is expected or valid input.
• Encode data on output. At the point where user-controllable data is output in HTTP
responses, encode the output to prevent it from being interpreted as active content.
Depending on the output context, this might require applying combinations of HTML, URL,
JavaScript, and CSS encoding.
• Use appropriate response headers. To prevent XSS in HTTP responses that aren't intended to
contain any HTML or JavaScript, you can use the Content-Type and X-Content-TypeOptions
headers to ensure that browsers interpret the responses in the way you intend.
• Content Security Policy. As a last line of defense, you can use Content Security Policy (CSP) to
reduce the severity of any XSS vulnerabilities that still occur.
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Click-jacking (UI redressing)
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Click-jacking (UI redressing)
Clickjacking is an interface-based
attack in which a user is tricked into
clicking on actionable content on a
hidden website by clicking on some
other content in a decoy website.
The technique depends upon the
incorporation of an invisible,
actionable web page (or multiple
pages) containing a button or
hidden link, say, within an iframe.
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Click-jacking (UI redressing)
<head>
<style>
#target_website {
position: relative;
width: 128px;
height: 128px;
opacity: 0.00001;
z-index: 2;
}
#decoy-website {
position: absolute;
width: 300px;
height: 400px;
z-index: 1;
}
</style>
</head>
How to construct a basic click-jacking attack?
<body>
<div id="decoy-website">
...decoy web content here...
</div>
<iframe
id="target_website"
src="https://vulnerable-website.com">
</iframe>
</body>
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Click-jacking (UI redressing)
Click-jacking attacks are possible whenever websites can be framed. A
common client-side protection uses frame busting or frame breaking
scripts (via browsers addons). This includes behaviors like:
• check and enforce that the current application window is the main or top window,
• make all frames visible,
• prevent clicking on invisible frames,
• intercept and flag potential clickjacking attacks to the user.
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Click-jacking (UI redressing)
How to prevent click-jacking attacks?
X-Frame-Options
1. X-Frame-Options: deny
2. X-Frame-Options: sameorigin
3. X-Frame-Options: allow-from https://normal-website.com
Content Security Policy
1. Content-Security-Policy: frame-ancestors 'self’;
2. Content-Security-Policy: frame-ancestors normal-website.com;
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Framing Third-Party Content
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Framing Third-Party Content
• Modern web apps often include third-party content
• Social widgets, advertisements, videos, games
• One option is to directly inject this content into the page
• But, this gives complete access to the enclosing DOM and data
(This is sometimes done for ads; why?)
• Instead, untrusted content can be contained in an iframe
• iframe provides separation between the top-level origin and third-party
scripts
• However, this isolation isn't perfect
• e.g. framebusting – if (top != self) {top.location.replace(location);}
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HTML5 Sandbox
HTML5 sandboxes represent a least-privilege approach to securing
third-party scripts
• Allows apps to specify only necessary privileges, and deny the rest
By default, all privileges are revoked and must be selectively enabled
• Synthetic origin, different from enclosing origin
• No JS execution
• No window or dialog creation
• No form submission
• No plugins
• No top navigation
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Sandbox Examples
<iframe src="http://example.com/widget" sandbox></iframe>
<iframe src="http://example.com/widget" sandbox="allow-forms"></iframe>
Other allows:
• allow-same-origin
• allow-scripts
• allow-top-navigation
• allow-popups
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Sandbox Caveats
• Sandbox has drawbacks
• Plugins don't respect browser sandboxes and can bypass their restrictions
• Disabling scripts, forms, and navigation implies that plugins must be disabled
• However, many ads, videos, games use plugins
• Many more approaches to sandboxing JavaScript
• Caja, FBJS2, AdJail
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SQL injection (SQLi)
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SQL injection (SQLi)
SQL injection is a vulnerability that
allows an attacker to interfere with
the queries that an application
makes to its database. It generally
allows an attacker to view data that
they are not normally able to
retrieve.
This might include data belonging
to other users, or any other data
that the application itself is able to
access. In many cases, an attacker
can modify or delete this data,
causing persistent changes to the
application's content or behavior.
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SQL injection (SQLi)
There are a wide variety of SQL injection vulnerabilities, attacks, and techniques,
which arise in different situations. Some common SQL injection examples include:
• Retrieving hidden data, where you can modify an SQL query to return additional
results.
• Subverting application logic, where you can change a query to interfere with the
application's logic.
• UNION attacks, where you can retrieve data from different database tables.
• Examining the database, where you can extract information about the version
and structure of the database.
• Blind SQL injection, where the results of a query you control are not returned in
the application's responses.
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SQL injection (SQLi)
Retrieving hidden data, where you can modify an SQL query to return
additional results.
https://insecure-website.com/products?category=Gifts
SELECT * FROM products WHERE category = 'Gifts' AND released = 1
https://insecure-website.com/products?category=Gifts'+OR+1=1--
SELECT * FROM products WHERE category = 'Gifts' OR 1=1--' AND released = 1
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SQL injection (SQLi)
Subverting application logic, where you can change a query to interfere with
the application's logic.
SELECT * FROM users WHERE username = 'smith' AND password = 'locksmith'
To remove the password check from the WHERE clause of the query. For
example, submitting the username administrator'-- and a blank password
results in the following query:
SELECT * FROM users WHERE username = 'administrator'--' AND password = ''
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SQL injection (SQLi)
UNION attacks, where you can retrieve data from different database tables.
For example, if an application executes the following query containing the
user input "Gifts":
SELECT name, description FROM products WHERE category = 'Gifts'
then an attacker can submit the input:
' UNION SELECT username, password FROM users--
This will cause the application to return all usernames and passwords along
with the names and descriptions of products.
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SQL injection (SQLi)
Examining the database, where you can extract information about the
version and structure of the database. For example, on Oracle you can
execute:
SELECT * FROM v$version WHERE banner LIKE 'Oracle%'
You can also determine what database tables exist, and which columns they
contain. For example, on most databases you can execute the following
query to list the tables:
SELECT * FROM information_schema.tables
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SQL injection (SQLi)
How to detect SQL injection vulnerabilities?
SQL injection can be detected automatically by tools (scanners) or manually by
using a systematic set of tests against every entry point in the application. This
typically involves:
• Submitting the single quote character ' and looking for errors or other anomalies.
• Submitting some SQL-specific syntax that evaluates to the base (original) value of
the entry point, and to a different value, and looking for systematic differences in
the resulting application responses.
• Submitting Boolean conditions such as OR 1=1 and OR 1=2, and looking for
differences in the application's responses.
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SQL injection (SQLi)
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Fingerprinting
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Fingerprinting
A device fingerprint, machine
fingerprint or browser fingerprint is
information collected about a remote
computing device for the purpose of
identification.
Fingerprints can be used to fully or
partially identify individual users or
devices even when cookies are
turned off. Websites identify unique
users and track their online behavior.
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Fingerprinting
The collection of large amount of diverse and stable information from web
browsers is possible thanks for most part to client-side scripting languages. The
data to be extracted:
• Browser version (name, compatibility info, …)
• Browser extensions (list of plugins and extensions)
• Hardware properties (phone model, screen size, screen orientation, …)
• Browsing history (which site s were already visited, CSS :visited)
• Font metrics (the letter bounding boxes differs based on hinting & anti-aliasing)
• Canvas and WebGL (GPU/CPU type, drivers, canvas.toDataURL)
• Hardware benchmarking (battery API, OscillatorNode, crypto algorithms)
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Fingerprinting
Example: How to get list of installed plugins.
var a = new Array();
try {
for (var i = 0; i < navigator.plugins.length; i++) {
a.push(navigator.plugins[i].name + ': '
+ navigator.plugins[i].description
+ ' (' + navigator.plugins[i].filename +')');
}
alert (a.toString());
} catch(e) {}
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Fingerprinting
The international advertising industry and marketing machines love your
data. Fingerprinting could by also used for many useful cases, like:
• Account fraud, confirm that every visitor is real and not a bot.
• Payment processing, to recognize e.g. repeated card testing activity
• Cryptocurrency, to secure trading, exchange and transfer operations
• Gaming, to catch users trying break the system via multiple accounts
• Paywall, to ensure users pay a fair price for the content
Fun fact: Only 1 in 286 777 other browsers will share the same fingerprint as
another user.
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Online Exercises
Web Security Crossroads (MDN)
Awesome AppSec (curated list of resources)
OWASP
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