How XSS attacks exploit unescaped output in PHP - Visual Walkthrough

Concept Flow - How XSS attacks exploit unescaped output

User Input

↓

Input Stored

↓

Output Without Escaping

↓

Browser Executes Malicious Script

↓

XSS Attack Successful

↓

User Data Compromised

User input is stored and then output without escaping, causing the browser to run malicious scripts, leading to an XSS attack.

Execution Sample

PHP

<?php
$user_input = "<script>alert('XSS');</script>";
echo "User says: $user_input";
?>

This PHP code outputs user input directly without escaping, allowing script tags to run in the browser.

Execution Table

Step	Variable	Value	Action	Output
1	$user_input	"<script>alert('XSS');</script>"	Assign user input	No output yet
2	echo	User says: <script>alert('XSS');</script>	Output user input directly	Browser renders and executes alert popup
3	Browser	Executes <script>alert('XSS');</script>	Runs JavaScript from output	Alert box with 'XSS' appears
4	-	-	XSS attack successful	User data compromised

💡 Execution stops after browser runs the malicious script embedded in output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
$user_input	undefined	"<script>alert('XSS');</script>"	"<script>alert('XSS');</script>"	"<script>alert('XSS');</script>"

Key Moments - 2 Insights

Why does the browser run the script inside the user input?

Because the PHP code outputs the user input directly without escaping, the browser treats the <script> tags as executable code (see execution_table step 2 and 3).

What would happen if the output was escaped properly?

If escaped, the <script> tags would be shown as text, not run as code, preventing the alert popup and XSS attack (not shown in this trace but implied).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of $user_input after step 1?

A "<script>alert('XSS');</script>"

B "alert('XSS');"

C "User says: <script>alert('XSS');</script>"

D undefined

💡 Hint

Check the 'Value' column for $user_input at step 1 in the execution_table.

At which step does the browser execute the malicious script?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in execution_table for when the browser runs the script.

If the PHP code escaped the output, what would change in the execution table?

A The $user_input value would change.

B The echo statement would not run.

C The browser would not execute the script in step 3.

D The alert popup would appear twice.

💡 Hint

Escaping output changes how the browser treats the script tags, see key_moments explanation.

Concept Snapshot

XSS attacks happen when user input with scripts is output without escaping.
PHP echo outputs raw input, so browsers run scripts inside.
Always escape output to show scripts as text, not code.
Unescaped output = security risk.
Escaping stops XSS by neutralizing scripts.

Full Transcript

This example shows how a PHP script outputs user input directly without escaping. The input contains a script tag that triggers an alert in the browser. The execution table traces the variable assignment, output, and browser execution steps. The variable tracker shows the user input remains unchanged. Key moments explain why the browser runs the script and how escaping output prevents this. The quiz tests understanding of variable values and execution steps. The snapshot summarizes the risk of unescaped output and the need to escape to prevent XSS attacks.