Sanitizing user input in Vue - Performance & Optimization

Performance: Sanitizing user input

HIGH IMPACT

Sanitizing user input affects page security and rendering stability by preventing injection attacks and layout shifts caused by malicious or malformed input.

Displaying user-generated HTML content safely in a Vue app

Vue

import DOMPurify from 'dompurify';
const userInput = '<img src=x onerror=alert(1)>Hello';
const safeInput = DOMPurify.sanitize(userInput);
<template>
  <div v-html="safeInput"></div>
</template>

Sanitizing removes harmful scripts and malformed tags, preventing layout shifts and security issues.

📈 Performance GainPrevents CLS and reflows caused by injected scripts or broken elements; improves user experience and security.

Displaying user-generated HTML content safely in a Vue app

Vue

const userInput = '<img src=x onerror=alert(1)>Hello';
// Directly binding with v-html
<template>
  <div v-html="userInput"></div>
</template>

Directly injecting user input with v-html allows malicious scripts and causes layout shifts if images fail to load or scripts run.

📉 Performance CostTriggers layout shifts (CLS) and potential reflows due to unexpected DOM changes; security risks can cause browser slowdowns.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Direct v-html with unsanitized input	Adds unpredictable nodes and event handlers	Multiple reflows if images/scripts load or fail	High paint cost due to layout shifts	[X] Bad
v-html with sanitized input (e.g., DOMPurify)	Adds only safe nodes, no scripts	Single reflow on initial render	Low paint cost, stable layout	[OK] Good

Rendering Pipeline

User input is sanitized before being inserted into the DOM, preventing unexpected layout recalculations and repaints caused by malicious or malformed content.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckLayout due to unexpected DOM changes from unsanitized input

Core Web Vital Affected

CLS

Sanitizing user input affects page security and rendering stability by preventing injection attacks and layout shifts caused by malicious or malformed input.

Optimization Tips

1Never bind raw user input directly with v-html without sanitization.

2Use a trusted sanitization library like DOMPurify to clean HTML input.

3Sanitizing user input reduces layout shifts and improves visual stability (CLS).

Performance Quiz - 3 Questions

What is the main performance risk of inserting unsanitized user input with v-html in Vue?

A Causes layout shifts and security vulnerabilities

B Increases JavaScript bundle size

C Improves rendering speed

D Reduces paint cost

DevTools: Performance

How to check: Record a performance profile while loading user content; look for layout shifts and long tasks caused by script execution or image loading.

What to look for: Check for CLS score spikes and layout recalculations in the flame chart indicating unstable or heavy DOM changes.

Practice

1. What is the main purpose of sanitizing user input in a Vue application?

easy

A. To store user input without any changes

B. To speed up the rendering of components

C. To change the style of input fields

D. To remove harmful or unwanted content from user input

Solution

1. Step 1: Understand what sanitizing means

   Sanitizing means cleaning input to remove harmful or unwanted parts.

2. Step 2: Identify the purpose in Vue apps

   In Vue, sanitizing user input helps keep the app safe and prevents issues like code injection.

3. Final Answer:

   To remove harmful or unwanted content from user input -> Option D

4. Quick Check:

   Sanitizing input = Remove harmful content [OK]

Hint: Sanitizing cleans input to keep apps safe [OK]

Common Mistakes:

• Thinking sanitizing changes styles
• Believing sanitizing speeds up rendering
• Assuming sanitizing stores input unchanged

2. Which Vue directive is best used to bind user input for sanitizing as the user types?

easy

A. v-if

B. v-model

C. v-for

D. v-bind

Solution

1. Step 1: Recall Vue directives for input binding

   The v-model directive binds input values to data reactively.

2. Step 2: Understand why v-model fits sanitizing

   Using v-model, you can watch and sanitize input as it changes.

3. Final Answer:

   v-model -> Option B

4. Quick Check:

   Bind input with v-model [OK]

Hint: Use v-model to bind and sanitize input live [OK]

Common Mistakes:

• Confusing v-if with input binding
• Using v-for which is for lists
• Using v-bind which binds attributes, not input values

3. Given this Vue code snippet, what will be the value of cleanInput after typing ""?

  <template>
    <input v-model="userInput" @input="sanitizeInput" />
  </template>

  <script setup>
  import { ref } from 'vue'
  const userInput = ref('')
  const cleanInput = ref('')

  function sanitizeInput() {
    cleanInput.value = userInput.value.replace(/<.*?>/g, '')
  }
  </script>

medium

A. "alert(1)"

B. ""

C. "" (empty string)

D. "<script>alert(1)</script>"

Solution

1. Step 1: Understand the regex in sanitizeInput

   The regex /<.*?>/g matches any text between < and > including the tags.

2. Step 2: Apply regex to input string

   Removing all tags from "<script>alert(1)</script>" leaves "alert(1)".

3. Final Answer:

   "alert(1)" -> Option A

4. Quick Check:

   Tags removed, text remains [OK]

Hint: Regex removes tags, leaving inner text [OK]

Common Mistakes:

• Thinking tags remain in output
• Assuming entire string is removed
• Confusing escaped HTML with raw input

4. Identify the error in this Vue sanitizing method:

function sanitizeInput() {
  cleanInput.value = userInput.value.replace(/<.*>/g, '')
}

medium

A. The function is missing a return statement

B. The replace method is used incorrectly without a callback

C. The regex is greedy and removes too much content

D. cleanInput.value should be assigned userInput.value directly

Solution

1. Step 1: Analyze the regex pattern

   The regex /<.*>/g is greedy and matches from the first < to the last > in the string.

2. Step 2: Understand the impact on input

   This causes removal of large parts of the string, not just individual tags, which is incorrect for sanitizing.

3. Final Answer:

   The regex is greedy and removes too much content -> Option C

4. Quick Check:

   Greedy regex causes over-removal [OK]

Hint: Use non-greedy regex to avoid removing too much [OK]

Common Mistakes:

• Thinking replace needs a return
• Assigning input without sanitizing
• Ignoring regex greediness effects

5. You want to sanitize user input in Vue to allow only letters and spaces, removing all other characters. Which method correctly sanitizes the input?

function sanitizeInput() {
  cleanInput.value = userInput.value.replace(_____, '')
}

hard

A. /[^a-zA-Z ]/g

B. /[a-zA-Z ]/g

C. /[^0-9]/g

D. /[0-9]/g

Solution

1. Step 1: Understand the goal

   We want to remove all characters except letters (a-z, A-Z) and spaces.

2. Step 2: Choose the correct regex

   The regex /[^a-zA-Z ]/g matches any character NOT a letter or space, so replacing these removes unwanted characters.

3. Final Answer:

   /[^a-zA-Z ]/g -> Option A

4. Quick Check:

   Negated regex removes unwanted chars [OK]

Hint: Use negated regex to keep only allowed chars [OK]

Common Mistakes:

• Using regex that matches allowed chars to remove
• Removing digits instead of letters
• Not including space in allowed characters