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XSS flaws occur whenever an application takes untrusted data and sends it to a web browser without proper validation or escaping. XSS allows attackers to execute scripts in the victims' browser, which can access any cookies, session tokens, or other sensitive information retained by the browser, or redirect user to malicious sites.
New Terms:
- XSS: Cross-Site Scripting, or XSS, occur when a web applications takes untrusted data and sends it to a web browser without proper validation or escaping.
- Stored XSS: XSS that can persist in an application and be run on a user’s page after being loaded from a database, server-side endpoint, or local browser storage.
- Reflected XSS: XSS that can be injected by an attacker and is returned in an immediate HTTP response to the targeted user.
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On the modern web, web applications
on the client usually run JavaScript,
0:00
and they're built out of HTML and CSS.
0:04
However, even the simplest of
websites that accept input
0:07
can be compromised when developers
forget to sanitize user inputs.
0:11
According to OWASP, the third most common
vulnerability in web applications is
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cross-site scripting, or XSS.
0:19
XSS is exploited easily by
attackers with little work,
0:22
leading to a wide-spread
presence across the web.
0:26
However, XSS has a moderate impact.
0:29
Though, particularly severe cases have led
to vulnerabilities on sites like Facebook
0:32
where any user could post on another
user's wall without their permission.
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When the researcher posted
as Mark Zuckerberg,
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after Facebook was slow to
respond to his bug report.
0:45
XSS has flaws occur when the web
application takes untrusted data and
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sends it to the web browser,
without proper validation or escaping.
0:54
XSS allows attackers to execute
JavaScript in the victim's browser,
0:58
which can access any available cookies,
1:02
session tokens where any other
sensitive information retained.
1:04
This also includes local store
mechanisms used by frameworks like
1:08
AngularJS and React.
1:12
Even worse, malicious scripts can
redirect users to malicious sites, and
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completely compromise their computer by
gaining access to the operating system
1:18
outside the browser.
1:21
In general,
the two most common types of XSS flaws
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around today are reflected XSS and
stored XSS.
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Reflected XSS is where the malicious
data is echoed back by the server
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in an immediate response to
an HTTP request from the victim.
1:36
Stored XSS is where the malicious
data is stored on the server or
1:40
in the browser and later gets embedded
in HTML pages provided to the victim.
1:44
Each reflected and stored XSS can
occur on the server or on the client.
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When it's on the client,
it is known as dom based XSS,
1:54
depending on when the malicious
data gets injected in HTML markup.
1:57
Both reflected and
stored XSS are critical vulnerabilities.
2:02
The stored XSS is far worse in most cases.
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For example, imagine that an attacker
discovers that they can maliciously
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inject a script into tweets on
Twitter when they're posted.
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Also imagine this attacker has compromised
the Twitter account of a user with
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tens of thousands of followers, which as
we discussed in our Treehouse course on
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data security, it's not all that hard when
the user has poorly chosen passwords.
2:25
With this kind of access, the attacker
can post a tweet with a malicious script
2:29
hidden inside of it, which will then
be stored inside of Twitter's database.
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Then, when the tens of thousands of
followers of this compromised user see
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this tweet in their feed, the script
will execute in each of their browsers,
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which could do everything from
download malware onto their machines,
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to takeover their accounts.
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Now you may be thinking, this seems
far out of the realm of possibility.
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However, studys conducted by prominent
security firms have found that
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even in2015, XSS likely exists
in 47% of existing websites.
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Hopefully, now you understand the
consequences of a successful XSS attack.
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So let's dig deeper into how
these attacks actually work.
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The only way for an attacker to run
malicious JavaScript in a victim's browser
3:12
is to inject it into one of
the pages that the victim uses.
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Again, this can happen if
the website directly includes
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user input into its pages.
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Since the attacker can insert a string,
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they'll be treated as code
by the victim's browser.
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Here, we can see a simple server-side
script is used to display the latest
3:30
comment on a website.
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However, this script assumes that
a comment consist of only text.
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The sense user input is included directly,
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an attacker could submit
a comment with an embedded script.
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At this point any user visiting the page
would now see that embedded script
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into the page.
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Now, when a user's browser loads
the page the user will be executing
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whatever the attacker included
inside those JavaScript script tags.
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So that's how exercise
works in practice and
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it isn't only possible in a simple
case we just demonstrated.
4:00
XSS occurs in the context
when the developer forgets
4:04
to sanitize user input and
most of this code can be inserted.
4:07
This means it can occur in HTML tags and
attributes, CCS style sheets, XML tags and
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last but not least any place with a client
style application using JavaScript.
4:16
Now that you should have an understanding
of how XSS works, let's look back at our
4:21
application and see a more complex
storage XSS vulnerability in action.
4:25
And then we'll look at
how to prevent them.
4:29
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