Secure headers configuration in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of setting secure HTTP headers in a Flask application?

Secure HTTP headers help protect your web app from common attacks like cross-site scripting (XSS), clickjacking, and data sniffing by instructing browsers how to behave securely.

beginner

Name three common secure headers you can set in Flask.

Content-Security-Policy (CSP), X-Frame-Options, and Strict-Transport-Security (HSTS) are common secure headers to improve security.

intermediate

How can you add secure headers to all responses in a Flask app?

Use the @app.after_request decorator to modify the response object and add headers before sending it to the client.

intermediate

What does the 'Content-Security-Policy' header do?

It controls which resources (like scripts, images) the browser is allowed to load, helping prevent malicious code injection.

intermediate

Why is the 'Strict-Transport-Security' header important?

It forces browsers to use HTTPS only, preventing attackers from intercepting data over insecure HTTP connections.

Which Flask decorator is best to add secure headers to every response?

A @app.after_request

B @app.route

C @app.before_request

D @app.errorhandler

The @app.after_request decorator lets you modify the response after the view function runs, perfect for adding headers.

What does the 'X-Frame-Options' header protect against?

A Cross-site scripting

B Clickjacking

C SQL injection

D Data encryption

X-Frame-Options prevents your site from being embedded in frames or iframes, protecting against clickjacking attacks.

Which header tells browsers to only use HTTPS for your site?

A Content-Security-Policy

B X-Content-Type-Options

C Strict-Transport-Security

D Referrer-Policy

Strict-Transport-Security (HSTS) forces browsers to use HTTPS, improving security.

What is a simple way to add a Content-Security-Policy header in Flask?

A Add it in the response headers using @app.after_request

B Set it in the HTML meta tags

C Use Flask's built-in CSP function

D Modify the request headers

You add CSP by modifying the response headers, typically in an @app.after_request function.

Which header helps prevent browsers from MIME-sniffing a response away from the declared content-type?

A Strict-Transport-Security

B X-Frame-Options

C Content-Security-Policy

D X-Content-Type-Options

X-Content-Type-Options with the value 'nosniff' stops browsers from guessing the content type, reducing some attacks.

Explain how to add secure headers to all responses in a Flask app and why it is important.

Think about how Flask lets you change the response before sending it.

Describe the role of Content-Security-Policy and Strict-Transport-Security headers in web security.

One controls what the browser loads, the other controls how it connects.

Practice

1. Which Flask feature is commonly used to add secure headers to every response?

easy

A. The @app.before_first_request decorator

B. The @app.route decorator

C. The @app.after_request decorator

D. The @app.teardown_request decorator

Solution

1. Step 1: Understand Flask request lifecycle

   Flask allows modifying responses after the view function runs using @app.after_request.

2. Step 2: Identify where headers are added

   Secure headers are added to the response object, which is accessible in @app.after_request.

3. Final Answer:

   The @app.after_request decorator -> Option C

4. Quick Check:

   Use @app.after_request to add headers [OK]

Hint: Add headers in @app.after_request for all responses [OK]

Common Mistakes:

• Using @app.route which only defines routes
• Trying to add headers in @app.before_first_request which runs once
• Using @app.teardown_request which is for cleanup, not response modification

2. Which of the following is the correct syntax to add a Content-Security-Policy header in Flask?

easy

A. response.set_header('Content-Security-Policy', "default-src 'self'")

B. response.header['Content-Security-Policy'] = "default-src 'self'"

C. response.headers.add('Content-Security-Policy', "default-src 'self'")

D. response.headers['Content-Security-Policy'] = "default-src 'self'"

Solution

1. Step 1: Recall Flask response header syntax

   Flask response headers are a dictionary-like object accessed via response.headers.

2. Step 2: Assign header value correctly

   Use response.headers['Header-Name'] = value to set or overwrite headers.

3. Final Answer:

   response.headers['Content-Security-Policy'] = "default-src 'self'" -> Option D

4. Quick Check:

   Headers set by assignment to response.headers [OK]

Hint: Use response.headers['Name'] = value to set headers [OK]

Common Mistakes:

• Using header instead of headers
• Trying to call set_header which Flask response lacks
• Using headers.add which is not a Flask method

3. Given this Flask code snippet, what will be the value of the X-Frame-Options header in the response?

from flask import Flask, Response
app = Flask(__name__)

@app.after_request
def add_headers(response: Response):
    response.headers['X-Frame-Options'] = 'DENY'
    return response

@app.route('/')
def home():
    return 'Hello'

medium

A. 'DENY'

B. 'SAMEORIGIN'

C. No X-Frame-Options header

D. 'ALLOW-FROM https://example.com'

Solution

1. Step 1: Check header assignment in @app.after_request

   The function sets X-Frame-Options to 'DENY' on every response.

2. Step 2: Confirm header presence in response

   Since the header is set unconditionally, the response will have X-Frame-Options: DENY.

3. Final Answer:

   'DENY' -> Option A

4. Quick Check:

   X-Frame-Options set to 'DENY' in after_request [OK]

Hint: Headers set in @app.after_request apply to all responses [OK]

Common Mistakes:

• Assuming default header values without setting them
• Confusing SAMEORIGIN with DENY
• Thinking header is missing because not set in route

4. Identify the error in this Flask code that tries to add a Strict-Transport-Security header:

from flask import Flask
app = Flask(__name__)

@app.after_request
def secure_headers(response):
    response.headers['Strict-Transport-Security'] = 'max-age=31536000; includeSubDomains'
    return response

@app.route('/')
def index():
    return 'Welcome'

medium

A. The header name should be lowercase

B. No error; the code is correct

C. The header value string is missing quotes

D. The function must accept two parameters

Solution

1. Step 1: Review header name and value syntax

   Header names are case-insensitive; using 'Strict-Transport-Security' is valid. The value is a valid string with quotes.

2. Step 2: Check function signature and usage

   @app.after_request functions take one parameter (response) and return it. This matches the code.

3. Final Answer:

   No error; the code is correct -> Option B

4. Quick Check:

   Header added correctly in @app.after_request [OK]

Hint: Check function signature and header syntax carefully [OK]

Common Mistakes:

• Thinking header names must be lowercase
• Expecting two parameters in after_request function
• Missing quotes around header value string

5. You want to add multiple secure headers in Flask, but only for HTTPS requests. Which is the best way to do this securely and efficiently?

hard

A. Use @app.after_request to check request.is_secure and add headers conditionally

B. Add headers in each route function only if request.scheme == 'https'

C. Set headers globally without checking protocol, assuming HTTPS everywhere

D. Use @app.before_request to add headers before response is created

Solution

1. Step 1: Understand when headers should be added

   Secure headers like Strict-Transport-Security should only be sent over HTTPS to avoid security risks.

2. Step 2: Choose the best place to add headers conditionally

   @app.after_request runs after response creation and can access request.is_secure to conditionally add headers efficiently for all routes.

3. Step 3: Evaluate other options

   Adding headers in each route duplicates code; setting headers globally without checking protocol risks sending them over HTTP; @app.before_request cannot modify response headers.

4. Final Answer:

   Use @app.after_request to check request.is_secure and add headers conditionally -> Option A

5. Quick Check:

   Conditional header addition in @app.after_request is best [OK]

Hint: Check request.is_secure in @app.after_request [OK]

Common Mistakes:

• Adding headers in each route causing repetition
• Sending secure headers over HTTP by default
• Trying to set headers in @app.before_request which can't modify response