XSS prevention in templates in Vue

Introduction

XSS prevention stops bad code from running in your app. It keeps users safe from attacks that steal data or break your site.

When showing user comments or messages on your website

When displaying data from external sources like APIs

When accepting and showing user input in forms

When rendering dynamic content that might include HTML

When building any public-facing web page with user content

Syntax

Vue

<template>
  <div>{{ userInput }}</div>
</template>

Vue automatically escapes content inside double curly braces {{ }}.

Use v-html only if you trust the content, because it inserts raw HTML.

Examples

Vue shows the script tags as text, not as code to run. This prevents XSS.

Vue

<template>
  <p>{{ message }}</p>
</template>

<script setup>
const message = '<script>alert("XSS")</script>'
</script>

Using v-html inserts HTML directly. Only do this with safe content.

Vue

<template>
  <p v-html="trustedHtml"></p>
</template>

<script setup>
const trustedHtml = '<strong>Safe bold text</strong>'
</script>

Sample Program

This example shows how Vue escapes the comment inside {{ }} so the script does not run. But using v-html inserts the raw HTML, which can run harmful code.

Vue

<template>
  <div>
    <h2>User Comment:</h2>
    <p>{{ userComment }}</p>
    <h2>Unsafe HTML with v-html (dangerous):</h2>
    <p v-html="userComment"></p>
  </div>
</template>

<script setup>
import { ref } from 'vue'

const userComment = ref('<img src=x onerror=alert("XSS Attack")>')
</script>

Important Notes

Always prefer using {{ }} for user content to keep it safe.

Only use v-html with content you fully trust or have sanitized.

Consider using libraries to clean HTML if you must display user HTML safely.

Summary

Vue escapes content inside {{ }} to prevent XSS automatically.

Using v-html inserts raw HTML and can cause XSS if content is unsafe.

Always validate or sanitize user input before showing it as HTML.

Practice

1. In Vue templates, which syntax automatically escapes content to prevent XSS attacks?

easy

A. Directly manipulating the DOM with JavaScript

B. Using v-html directive to insert HTML

C. Using triple curly braces like {{{ userInput }}}

D. Using double curly braces like {{ userInput }}

Solution

1. Step 1: Understand Vue's default escaping

   Vue automatically escapes content inside {{ }} to prevent XSS by converting special characters to safe HTML entities.

2. Step 2: Compare with other methods

   Using v-html inserts raw HTML and can cause XSS if content is unsafe. Direct DOM manipulation bypasses Vue's protections.

3. Final Answer:

   Using double curly braces like {{ userInput }} -> Option D

4. Quick Check:

   Vue escapes {{ }} content = A [OK]

Hint: Remember: {{ }} escapes content, v-html does not [OK]

Common Mistakes:

• Thinking v-html escapes content automatically
• Assuming v-text sanitizes HTML
• Believing direct DOM manipulation is safe

2. Which of the following Vue template usages is syntactically correct and safe from XSS by default?

easy

A. <div v-html="userInput"></div>

B. <div>{{ userInput }}</div>

C. <div v-text=userInput></div>

D. <div>{{ userInput</div> }}

Solution

1. Step 1: Check syntax correctness

   <div>{{ userInput }}</div> uses correct Vue syntax with double curly braces and quotes are properly placed.

2. Step 2: Check XSS safety

   <div>{{ userInput }}</div> escapes content automatically, making it safe. <div v-html="userInput"></div> uses v-html which inserts raw HTML and can be unsafe. <div v-text=userInput></div> misses quotes around the binding value, causing syntax error. <div>{{ userInput</div> }} has mismatched braces causing syntax error.

3. Final Answer:

   <div>{{ userInput }}</div> -> Option B

4. Quick Check:

   Correct syntax + auto-escape = D [OK]

Hint: Use {{ }} with quotes for safe, correct syntax [OK]

Common Mistakes:

• Using v-html without sanitizing input
• Omitting quotes in directives
• Mismatching curly braces

3. Given the Vue template code:
<pre><template> <div>{{ userInput }}</div> <div v-html="userInput"></div> </template> And userInput is '<img src=x onerror=alert(1)>', what will be rendered in the browser?

medium

A. The first div shows the raw HTML tag as text; the second div executes the alert popup.

B. Both divs show the raw HTML tag as text without executing alert.

C. Both divs execute the alert popup.

D. The first div executes alert; the second div shows raw HTML tag as text.

Solution

1. Step 1: Analyze the first div with {{ userInput }}

   Vue escapes the content, so the HTML tag is shown as plain text, no alert runs.

2. Step 2: Analyze the second div with v-html

   v-html inserts raw HTML, so the <img> tag runs and triggers the alert.

3. Final Answer:

   The first div shows the raw HTML tag as text; the second div executes the alert popup. -> Option A

4. Quick Check:

   v-html inserts raw HTML = A [OK]

Hint: v-html runs HTML, {{ }} escapes it [OK]

Common Mistakes:

• Assuming both divs escape content
• Thinking v-html escapes content
• Confusing which div triggers alert

4. You have this Vue template:

<div v-html="userContent"></div>

User input is not sanitized. What is the best fix to prevent XSS?

medium

A. Replace v-html with {{ userContent }} to auto-escape

B. Keep v-html but add v-text on the same element

C. Use v-html but trust all user input as safe

D. Wrap v-html inside a <script> tag

Solution

1. Step 1: Identify the risk of v-html with unsanitized input

   v-html inserts raw HTML, so unsanitized user input can cause XSS attacks.

2. Step 2: Choose a safe alternative

   Replacing v-html with {{ userContent }} escapes content automatically, preventing XSS.

3. Final Answer:

   Replace v-html with {{ userContent }} to auto-escape -> Option A

4. Quick Check:

   Use {{ }} to escape user input safely = C [OK]

Hint: Avoid v-html with unsafe input; use {{ }} instead [OK]

Common Mistakes:

• Adding v-text does not sanitize v-html content
• Trusting all user input without sanitization
• Using script tags incorrectly

5. You want to display user comments that may contain safe HTML tags like <b> and <i> but prevent XSS in Vue. Which approach is best?

hard

A. Use {{ comment }} to escape all HTML, showing tags as text

B. Use v-html directly without sanitizing user comments

C. Use v-html with a sanitizer function that removes unsafe tags before binding

D. Replace <b> and <i> tags with <script> tags

Solution

1. Step 1: Understand the need to allow some HTML but block unsafe tags

   Using v-html allows HTML rendering but can cause XSS if input is unsafe.

2. Step 2: Use a sanitizer to clean user input

   Sanitizing removes dangerous tags/scripts but keeps safe tags like <b> and <i>.

3. Step 3: Bind sanitized content with v-html

   This approach safely renders allowed HTML while preventing XSS.

4. Final Answer:

   Use v-html with a sanitizer function that removes unsafe tags before binding -> Option C

5. Quick Check:

   Sanitize input before v-html = B [OK]

Hint: Sanitize user HTML before v-html to allow safe tags [OK]

Common Mistakes:

• Using v-html without sanitizing
• Escaping all HTML losing formatting
• Replacing safe tags with script tags