XSS prevention in templates in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is XSS and why is it a security risk in web applications?

XSS (Cross-Site Scripting) is when attackers inject harmful scripts into web pages viewed by others. It can steal data, hijack user sessions, or change page content, making it a serious security risk.

beginner

How does Vue.js prevent XSS attacks by default in templates?

Vue automatically escapes HTML special characters in template expressions, so user input is shown as text, not code. This stops scripts from running inside the page.

intermediate

What is the risk of using the v-html directive in Vue templates?

v-html inserts raw HTML into the page without escaping. If the HTML comes from untrusted sources, it can run malicious scripts and cause XSS attacks.

intermediate

Name two best practices to avoid XSS when using Vue templates.

1. Avoid using v-html with untrusted content. 2. Always sanitize any HTML before inserting it. 3. Use Vue's default template syntax for binding text.

advanced

How can you sanitize HTML content before using it with v-html in Vue?

Use a trusted library like DOMPurify to clean the HTML string by removing unsafe tags and attributes before binding it with v-html.

What does Vue do by default to prevent XSS in templates?

A Automatically sanitizes all HTML content

B Escapes HTML special characters in template expressions

C Disables JavaScript execution in the browser

D Blocks all user input

Vue escapes HTML special characters in template expressions to show user input as text, preventing scripts from running.

Which Vue directive can cause XSS if used with untrusted content?

A v-html

B v-if

C v-bind

D v-text

v-html inserts raw HTML without escaping, so untrusted content can run scripts and cause XSS.

What is a safe way to display user input in Vue templates?

A Insert input into innerHTML manually

B Use v-html directly

C Use template expressions with double curly braces {{ }}

D Disable Vue's escaping feature

Using {{ }} escapes HTML characters, showing input as text safely.

Which tool can help sanitize HTML before using it with v-html?

A DOMPurify

B jQuery

C Axios

D Lodash

DOMPurify cleans HTML strings by removing unsafe code, preventing XSS.

Why should you avoid using v-html with user-generated content?

A It breaks CSS styles

B It slows down the app

C It disables Vue reactivity

D It can cause XSS attacks if content is not sanitized

Untrusted HTML can contain scripts that run when inserted with v-html, causing XSS.

Explain how Vue's template syntax helps prevent XSS attacks.

Think about how Vue treats {{ userInput }} in templates.

Describe the risks of using v-html and how to safely use it in Vue applications.

Consider what happens if untrusted HTML is inserted directly.

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