Cross-site scripting (XSS) in Cybersecurity - Step-by-Step Execution

Concept Flow - Cross-site scripting (XSS)

User inputs data

↓

Data sent to web server

↓

Server stores or reflects data

↓

Web page displays data

↓

Browser executes page content

↓

Malicious script runs if input not sanitized

↓

Attacker steals info or manipulates page

This flow shows how user input can travel from input to execution in a browser, leading to XSS if not properly handled.

Execution Sample

Cybersecurity

<script>alert('XSS')</script>
User input -> Web page output
Browser runs script if unsafe

This example shows a malicious script entered as user input that runs in the browser if the site does not block it.

Analysis Table

Step	Action	Data Content	Result	Security Check
1	User inputs data	<script>alert('XSS')</script>	Data sent to server	No check yet
2	Server receives data	<script>alert('XSS')</script>	Stores or reflects data	No sanitization
3	Web page renders data	<script>alert('XSS')</script>	Script included in page	No escaping
4	Browser loads page	Page with script tag	Executes alert popup	Malicious script runs
5	Attacker impact	Alert popup shown	User info can be stolen or page altered	Security breached

💡 Execution stops after malicious script runs in browser causing security breach

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
User Input	empty	<script>alert('XSS')</script>	<script>alert('XSS')</script>	<script>alert('XSS')</script>	<script>alert('XSS')</script>	Executed script
Server Data	empty	empty	<script>alert('XSS')</script>	<script>alert('XSS')</script>	<script>alert('XSS')</script>	Stored unsafe data
Web Page Content	empty	empty	empty	<script>alert('XSS')</script>	<script>alert('XSS')</script>	Page includes script
Browser Execution	none	none	none	alert popup	alert popup	Script runs

Key Insights - 3 Insights

Why does the malicious script run in the browser?

Because the server did not sanitize or escape the user input before including it in the web page, the browser treats it as code and executes it (see execution_table step 4).

What is the difference between storing data and executing script?

Storing data means saving user input as-is (step 2), but executing script happens only when the browser renders the page and runs the embedded code (step 4).

How can this attack be prevented?

By sanitizing or escaping user input on the server before displaying it, so the browser treats it as text, not code (missing in execution_table steps).

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What happens to the user input?

A It is safely removed from the page

B It is encrypted before display

C It is included in the web page without changes

D It is blocked by the server

💡 Hint

Check the 'Data Content' and 'Result' columns at step 3 in execution_table

At which step does the browser execute the malicious script?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Browser Execution' and 'Result' columns in execution_table

If the server sanitized input before storing, how would the 'Server Data' variable change after step 2?

A It would contain the raw script tags

B It would contain escaped text, not executable code

C It would be empty

D It would cause the server to crash

💡 Hint

Refer to variable_tracker 'Server Data' row and think about sanitization effects

Concept Snapshot

Cross-site scripting (XSS) happens when malicious scripts enter web pages via user input.
If input is not sanitized, browsers run this code, risking user data.
Prevent by validating and escaping input before display.
Always treat user input as unsafe until cleaned.
XSS can steal info or change page behavior.

Full Transcript

Cross-site scripting, or XSS, is a security problem where attackers put harmful code into websites through user input. The flow starts when a user enters data, which the server then stores or reflects without cleaning. When the web page shows this data, the browser runs the harmful script. This can steal information or change the page. The key to stopping XSS is to sanitize or escape user input before showing it on the page. The execution table shows each step from input to script running, helping visualize how XSS happens and why security checks are needed.