Email verification pattern in Flask

Introduction

Email verification helps confirm that a user owns the email they provide. It improves security and trust by preventing fake or mistyped emails.

When users sign up and you want to confirm their email address.

When resetting a password to ensure the request is from the real owner.

When sending important notifications that require a verified contact.

When you want to reduce spam or fake accounts in your app.

Syntax

Flask

from flask import Flask, request, redirect, url_for, render_template_string
from itsdangerous import URLSafeTimedSerializer

app = Flask(__name__)
app.config['SECRET_KEY'] = 'your-secret-key'

serializer = URLSafeTimedSerializer(app.config['SECRET_KEY'])

def generate_verification_token(email):
    return serializer.dumps(email, salt='email-confirm-salt')

def confirm_verification_token(token, expiration=3600):
    try:
        email = serializer.loads(token, salt='email-confirm-salt', max_age=expiration)
    except Exception:
        return False
    return email

Use URLSafeTimedSerializer to create tokens that expire after some time.

Keep your SECRET_KEY safe and unique for your app.

Examples

This creates a token string for the given email.

Flask

token = generate_verification_token('[e-mail]')
print(token)

This checks if the token is valid and not expired, then returns the email.

Flask

email = confirm_verification_token(token)
if email:
    print(f'Email {email} is verified')
else:
    print('Invalid or expired token')

Sample Program

This Flask app lets a user enter their email to register. It creates a verification link with a token. Visiting the link verifies the email if the token is valid and not expired.

Flask

from flask import Flask, request, redirect, url_for, render_template_string
from itsdangerous import URLSafeTimedSerializer

app = Flask(__name__)
app.config['SECRET_KEY'] = 'your-secret-key'

serializer = URLSafeTimedSerializer(app.config['SECRET_KEY'])

def generate_verification_token(email):
    return serializer.dumps(email, salt='email-confirm-salt')

def confirm_verification_token(token, expiration=3600):
    try:
        email = serializer.loads(token, salt='email-confirm-salt', max_age=expiration)
    except Exception:
        return False
    return email

@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        email = request.form['email']
        token = generate_verification_token(email)
        verify_url = url_for('verify_email', token=token, _external=True)
        return f'Verification link (send by email): {verify_url}'
    return '''<form method="post">
Email: <input name="email" type="email" required>
<input type="submit" value="Register">
</form>'''

@app.route('/verify/<token>')
def verify_email(token):
    email = confirm_verification_token(token)
    if email:
        return f'Email {email} has been verified successfully!'
    else:
        return 'Verification link is invalid or expired.'

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

In real apps, send the verification link by email instead of showing it on screen.

Tokens expire after 1 hour by default; adjust expiration as needed.

Always validate and sanitize user input to avoid security issues.

Summary

Email verification confirms user ownership of an email address.

Use itsdangerous.URLSafeTimedSerializer to create and check tokens.

Send users a link with a token to verify their email safely and simply.

Practice

1. What is the main purpose of email verification in a Flask application?

easy

A. To store user data securely in the database

B. To confirm that the user owns the provided email address

C. To speed up the login process

D. To encrypt the user's password

Solution

1. Step 1: Understand email verification purpose

   Email verification is used to confirm that the user actually owns the email they provided during signup.

2. Step 2: Differentiate from other security tasks

   Encrypting passwords and speeding login are different tasks unrelated to email verification.

3. Final Answer:

   To confirm that the user owns the provided email address -> Option B

4. Quick Check:

   Email verification = Confirm ownership [OK]

Hint: Email verification confirms user email ownership [OK]

Common Mistakes:

• Confusing email verification with password encryption
• Thinking it speeds up login
• Assuming it stores data securely

2. Which Flask library is commonly used to create timed tokens for email verification?

easy

A. flask_sqlalchemy

B. flask_login

C. itsdangerous

D. flask_mail

Solution

1. Step 1: Identify token generation library

   The itsdangerous library provides URLSafeTimedSerializer to create timed tokens.

2. Step 2: Differentiate from other Flask extensions

   flask_sqlalchemy manages databases, flask_login handles user sessions, and flask_mail sends emails but does not create tokens.

3. Final Answer:

   itsdangerous -> Option C

4. Quick Check:

   Token creation = itsdangerous [OK]

Hint: Use itsdangerous for timed token creation [OK]

Common Mistakes:

• Choosing flask_mail for token creation
• Confusing flask_login with token generation
• Selecting flask_sqlalchemy incorrectly

3. Given this Flask code snippet, what will verify_token(token) return if the token is expired?

from itsdangerous import URLSafeTimedSerializer, SignatureExpired

serializer = URLSafeTimedSerializer('secret-key')

def verify_token(token):
    try:
        email = serializer.loads(token, max_age=3600)
        return email
    except SignatureExpired:
        return None

medium

A. Returns None

B. Raises a SignatureExpired exception

C. The email string inside the token

D. Returns an empty string

Solution

1. Step 1: Understand token verification logic

   The function tries to load the token with a max age of 3600 seconds (1 hour).

2. Step 2: Handle expired token exception

   If the token is expired, it raises SignatureExpired, which is caught and returns None.

3. Final Answer:

   Returns None -> Option A

4. Quick Check:

   Expired token returns None [OK]

Hint: Expired tokens return None in try-except block [OK]

Common Mistakes:

• Expecting exception to propagate
• Assuming it returns empty string
• Thinking it returns the email anyway

4. Identify the error in this Flask email verification token generation code:

from itsdangerous import URLSafeTimedSerializer

serializer = URLSafeTimedSerializer('secret-key')

def generate_token(email):
    token = serializer.dumps(email)
    return token

# Later verification
email = serializer.loads(token, max_age=3600)

medium

A. Token variable is undefined at verification

B. Missing secret key in serializer initialization

C. max_age parameter is invalid in loads()

D. Using dumps() instead of loads() to verify

Solution

1. Step 1: Check token generation and usage

   The token is generated inside generate_token but not stored or passed outside.

2. Step 2: Verify token usage scope

   At verification, token is used but not defined in that scope, causing a NameError.

3. Final Answer:

   Token variable is undefined at verification -> Option A

4. Quick Check:

   Undefined token variable causes error [OK]

Hint: Ensure token variable is defined before verification [OK]

Common Mistakes:

• Ignoring variable scope for token
• Thinking secret key is missing
• Misusing max_age parameter

5. You want to implement email verification in Flask so that users receive a link with a token that expires after 24 hours. Which approach correctly combines token creation, sending email, and verification?

hard

A. Generate token without secret key, send email, verify token ignoring expiration

B. Store plain email in database, send email without token, verify by matching email only

C. Send token as plain text in email body, verify token with max_age=3600

D. Generate token with URLSafeTimedSerializer, send email with token link, verify token with max_age=86400

Solution

1. Step 1: Create token with expiration

   Use URLSafeTimedSerializer with a secret key to generate a token that expires after 86400 seconds (24 hours).

2. Step 2: Send email with token link and verify properly

   Send the token inside a URL link in the email. Verify the token using loads() with max_age=86400 to enforce expiration.

3. Final Answer:

   Generate token with URLSafeTimedSerializer, send email with token link, verify token with max_age=86400 -> Option D

4. Quick Check:

   Token creation + email + 24h expiry = Generate token with URLSafeTimedSerializer, send email with token link, verify token with max_age=86400 [OK]

Hint: Use max_age=86400 for 24h token expiry [OK]

Common Mistakes:

• Not using tokens in email links
• Ignoring token expiration
• Sending tokens without secret key